Short-Circuit Study

Revision 9 · Oct 17, 2026 +733 −686

SynC Standards Team — Specifier (Platform Standards) ✓ OfficialDiscloses employment SynC (SynC Platform Team)

Rewritten so the study owns the system model and the equipment duty evaluation, the study engineer is a decision rather than a default, and the coordination and arc-flash studies build on its results.

Showing changes from Rev 8 to Rev 9 in Short-Circuit Study.

−---

−title: Short-Circuit Study

−category: Electrical / Power Studies

−toc_depth: 3

−description: >

− When to use: Calculation of available three-phase and line-to-ground short-circuit currents at every bus, transformer secondary, switchboard, panelboard, motor control center, and significant motor connection in a low- or medium-voltage power distribution system. The study output is used to specify equipment interrupting ratings, withstand ratings, and bus bracing, and to verify that installed and proposed equipment complies with NFPA 70 Article 110.9 and 110.10.

− Not intended for: Protective device coordination (see [[sync/protective-coordination-study]]), incident energy / arc flash hazard analysis (see [[sync/arc-flash-study]]), load flow or voltage drop studies, transient stability studies, harmonic analysis, or grounding system resistance design (see [[sync/grounding-and-bonding]]).

−---

−

−# Scope {toc}

−

−## This specification covers the performance and reporting of a short-circuit study for the project's electrical power distribution system. {note}

−

−## The study shall calculate the available short-circuit current at every point in the distribution system at which an interrupting device, a passive current-carrying component, or an equipment short-circuit current rating must be verified.

−

−## Calculated values shall be used to confirm that every installed and proposed device has an interrupting rating equal to or greater than the available fault current at its line terminals.

−

−## Calculated values shall be used to confirm that every bus, cable, and bracing system is rated to withstand the available fault current for the duration required by the upstream protective devices.

−

−## The short-circuit study is one of three power studies that are commonly performed together for the same distribution system. {note}

−

−## The short-circuit study establishes the magnitudes of fault current; the protective coordination study (see [[sync/protective-coordination-study]]) establishes how those fault currents are cleared by upstream and downstream devices; and the arc flash study (see [[sync/arc-flash-study]]) translates the cleared fault current and clearing time into incident energy for personnel protection. {note}

−

−## The three studies share input data and shall be performed by the same engineering organization where practical to avoid inconsistent system modeling between studies.

−

−## A short-circuit study is a deliverable of the design and construction process; it is not a substitute for the contractor's responsibility to obtain and apply the available fault current value from the serving utility, nor for the electrical engineer of record's responsibility to specify equipment with adequate interrupting ratings on the contract documents. {note}

−

−# Referenced Standards {toc}

−

−## The study, its inputs, and its deliverables shall comply with the latest adopted edition of the following standards and codes.

−

−## Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.

−

−## Referenced Standards List {toc}

−

−| Standard | Title |

−|----------|-------|

−| NFPA 70 | National Electrical Code (Articles 110.9, 110.10, 110.16, 110.24, 240.86) |

−| NFPA 70E | Standard for Electrical Safety in the Workplace |

−| OSHA 29 CFR 1910.303 | General Requirements for Electric Utilization Equipment |

−| IEEE 141 | Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book) |

−| IEEE 241 | Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book) |

−| IEEE 399 | Recommended Practice for Industrial and Commercial Power Systems Analysis (Brown Book) |

−| IEEE 551 | Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems (Violet Book) |

−| IEEE 3002.3 | Recommended Practice for Conducting Short-Circuit Studies and Analysis of Industrial and Commercial Power Systems |

−| ANSI C37.010 | Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis |

−| ANSI C37.13 | Low-Voltage AC Power Circuit Breakers Used in Enclosures |

−| ANSI C37.06 | AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis — Preferred Ratings |

−| UL 489 | Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures |

−| UL 248 | Low-Voltage Fuses |

−| IEC 60909 | Short-Circuit Currents in Three-Phase AC Systems (where IEC methodology is specified) |

−| ANSI/NETA ATS | Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems |

−

−## The relevant NEC sections shall be understood as the legal driver of the study's purpose: NEC 110.9 requires that equipment intended to interrupt fault current have an interrupting rating sufficient for the available fault current at its line terminals; NEC 110.10 requires that the entire circuit — overcurrent device, conductors, and equipment — be coordinated so that components are not damaged by a fault; and OSHA 29 CFR 1910.303(b)(4) and (b)(5) impose substantively the same requirements as federal workplace safety rules. {note}

−

−## The short-circuit study is the calculation that proves compliance with NEC 110.9, NEC 110.10, and OSHA 29 CFR 1910.303. {note}

−

−# Study Performer Qualifications {toc}

−

−```datasheet

−label: Study Performer

−type: radio

−options:

− - "Independent registered Professional Engineer (PE), licensed in the project state"

− - "Equipment manufacturer's engineering services (PE-stamped)"

− - "Testing agency engineering services (PE-stamped)"

− - "Contractor's engineering subcontractor (PE-stamped)"

−default: "Independent registered Professional Engineer (PE), licensed in the project state"

−```

−

−```datasheet

−label: Independent Review of Study

−type: radio

−options:

− - "Required — review by the Engineer of Record before acceptance"

− - "Not required — engineer of record relies on performer's PE seal"

−default: "Required — review by the Engineer of Record before acceptance"

−```

−

−## The short-circuit study shall be performed under the responsible charge of a Professional Engineer licensed in the state where the project is located.

−

−## The final study report, all single-line diagrams, and all calculation summaries shall be sealed and signed by that engineer.

−

−## The performer shall demonstrate a minimum of five years of documented experience performing short-circuit studies on power systems of comparable size and complexity.

−

−## An independent engineering firm is preferred over a manufacturer-affiliated performer where the study results may influence equipment procurement decisions, because a manufacturer-performed study has an inherent conflict of interest in concluding that its own equipment is adequately rated. {note}

−

−## Where the manufacturer performs the study under a furnished-equipment scope, the study shall be subject to independent review.

−

−## The Contractor shall not self-perform the short-circuit study without engaging a qualified engineer.

−

−## A study performed by a non-engineer using the output of a software tool without engineering judgment is not acceptable, regardless of the tool used, because the inputs, assumptions, and interpretation of results require professional engineering review. {note}

−

−# Submittals {toc}

−

−## Action Submittals {toc}

−

−### The Contractor shall submit the following for the Engineer's review before any equipment subject to study findings is procured or fabricated:

−

−- Study performer's qualifications, including PE license verification and project experience

−- Description of the calculation methodology, software, and standard (ANSI/IEEE or IEC) to be used

−- Proposed list of system data sources, including utility short-circuit data request status, equipment nameplate data sources, and any assumptions for missing data

−- Preliminary single-line diagram with all buses, branches, and devices labeled with the identifiers that will be used in the study report

−- Study schedule showing key milestones and final delivery date relative to equipment procurement deadlines

−

−```datasheet

−label: Pre-Study Submittals Required

−type: checkbox

−options:

− - "Performer qualifications and PE license"

− - "Calculation methodology description"

− - "Data source list and assumption log"

− - "Preliminary single-line diagram with bus identifiers"

− - "Study schedule"

−default: "Performer qualifications and PE license"

−```

−

−### Equipment procurement shall not proceed for any item whose interrupting rating, withstand rating, or short-circuit current rating depends on the study results until the study is reviewed and accepted.

−

−## Study Deliverables {toc}

−

−### The completed study shall be submitted as a single, paginated, sealed report containing the items required by the Deliverables section of this standard.

−

−### Draft versions of the study shall be submitted for review before equipment is released for manufacture.

−

−## Closeout Submittals {toc}

−

−### At substantial completion the Contractor shall submit:

−

−- The final, sealed, stamped study report incorporating all field-verified data

−- Native-format study files (importable into manufacturer-agnostic study software) for future re-studies

−- A summary table of any equipment ratings that were field-verified and that differed from the design-basis ratings

−- A re-study trigger list identifying the conditions under which the Owner should commission an updated study

−

−```datasheet

−label: Required Closeout Submittals

−type: checkbox

−options:

− - "Final sealed, stamped study report with field-verified data"

− - "Native-format study files for future re-studies"

− - "Summary table of field-verified rating differences"

− - "Re-study trigger list"

−default:

− - "Final sealed, stamped study report with field-verified data"

− - "Native-format study files for future re-studies"

− - "Summary table of field-verified rating differences"

− - "Re-study trigger list"

−```

−

−# Quality Assurance {toc}

−

−## Software Tools {toc}

−

−```datasheet

−label: Calculation Software

−type: radio

−options:

− - "Commercial power system analysis software (manufacturer-agnostic)"

− - "Hand calculations per IEEE 551 (small systems only)"

− - "Combination — software with hand-calculation verification at key buses"

−default: "Commercial power system analysis software (manufacturer-agnostic)"

−```

−

−### The study shall be performed using a commercial power system analysis software package that implements the selected calculation standard (ANSI/IEEE or IEC) and that is in current commercial use by the industry.

−

−### The Contractor shall not specify or accept a software package by name; the requirement is that the package implement the selected standard correctly and that its results be verifiable by hand calculation at representative buses.

−

−### Hand-calculation-only studies may be used for very small systems where the number of buses is small enough that a software model adds no value.

−

−### For all other projects, software is required because it produces an auditable, re-runnable model that can be updated for future re-studies. {note}

−

−## Standard Identification {toc}

−

−```datasheet

−label: Calculation Methodology

−type: radio

−options:

− - "ANSI/IEEE C37 (per IEEE 551)"

− - "IEC 60909"

−default: "ANSI/IEEE C37 (per IEEE 551)"

−```

−

−### The ANSI/IEEE methodology is the default for projects in the United States and is required for any project where the installed equipment is rated by ANSI/IEEE standards (substantially all US-market low- and medium-voltage equipment).

−

−### IEC 60909 shall be used only where the project equipment is rated to IEC standards, where the project is located outside the United States in a jurisdiction that requires IEC, or where the Owner specifically directs.

−

−### The two methodologies produce different numerical results for the same physical system and are not interchangeable; the methodology shall be stated explicitly in the report.

−

−## Acceptance Criteria {toc}

−

−### The study shall be accepted by the Engineer of Record only when all of the following are documented:

−

−- Every device intended to interrupt fault current has a documented interrupting rating equal to or greater than the calculated available fault current at its line terminals

−- Every bus, busway, and switchgear assembly has a documented short-circuit withstand or short-circuit current rating equal to or greater than the calculated available fault current at its location, for the duration corresponding to the upstream protective device clearing time

−- Every cable subject to fault current has been checked for short-circuit thermal capacity for the upstream device clearing time

−- All deficiencies have been identified and a remediation plan submitted

−

−### Where the study identifies any device or bus that is under-rated for the available fault current, the Contractor shall submit a remediation proposal — current-limiting fuses, series-rated combinations, equipment replacement, or upstream impedance addition — for the Engineer's review before any affected equipment is energized.

−

−# Study Methodology {toc}

−

−## Calculation Points {toc}

−

−### The study shall calculate available short-circuit current at every bus, every transformer secondary, and at the line terminals of every device that is required to interrupt fault current or to withstand fault current without damage.

−### As a minimum, calculation points shall include:

−

−- Utility service point and main service equipment line terminals

−- Secondary of every power transformer (including dry-type distribution transformers)

−- Line terminals of every switchboard, switchgear assembly, panelboard, and motor control center

−- Line terminals of every motor starter, variable frequency drive, and other equipment whose short-circuit current rating must be verified

−- Each significant feeder splice or tap point

−- Each remote distribution panel and downstream lighting/appliance panel served from the system

−

−```datasheet

−label: Calculation Point Coverage

−type: checkbox

−options:

− - "Utility service point"

− - "Every transformer secondary"

− - "Every switchboard, switchgear, MCC line terminals"

− - "Every panelboard line terminals"

− - "Every motor starter and VFD line terminals"

− - "Significant feeder splices and tap points"

− - "Branch-circuit panels"

−default: "Utility service point"

−```

−

−## Fault Types {toc}

−

−```datasheet

−label: Fault Types Calculated

−type: checkbox

−options:

− - "Three-phase bolted fault (symmetrical, maximum)"

− - "Line-to-line bolted fault"

− - "Single line-to-ground bolted fault"

− - "Three-phase fault — momentary (½ cycle)"

− - "Three-phase fault — interrupting (1.5 to 4 cycles)"

− - "Three-phase fault — time-delayed (30 cycles, where applicable)"

−default:

− - "Three-phase bolted fault (symmetrical, maximum)"

−```

−

−### A three-phase bolted fault produces the maximum symmetrical current at most points in a typical industrial or commercial system and is the controlling case for equipment interrupting rating and bus bracing. {note}

−

−### Line-to-ground faults shall be calculated and reported for systems with grounded sources, because in solidly grounded systems the single line-to-ground fault current can exceed the three-phase fault current at certain points.

−

−### Line-to-line faults shall be reported where required by the protective coordination study.

−

−### Per ANSI/IEEE methodology, three-phase fault currents shall be calculated at three time intervals: the first-cycle (momentary) value for bus bracing and instantaneous relay response, the interrupting (contact-parting) value at 1.5 to 4 cycles for medium- and high-voltage breaker interrupting ratings, and the time-delayed (30-cycle) value for time-delayed relay coordination.

−

−### For low-voltage systems the first-cycle value is the controlling value for both bus bracing and breaker interrupting rating per ANSI C37.13.

−

−## Impedance Modeling {toc}

−

−```datasheet

−label: Cable Impedance Source

−type: radio

−options:

− - "Manufacturer published impedances by conductor size and configuration"

− - "Typical values per IEEE 141 / IEEE 241"

− - "Field-measured cable lengths with calculated impedance"

−default: "Manufacturer published impedances by conductor size and configuration"

−```

−

−### The system shall be modeled using positive, negative, and zero sequence impedances for every source, transformer, cable, and rotating machine.

−

−### Sequence impedances shall be obtained from equipment nameplates, manufacturer test reports, or — where measured data is unavailable — from the typical values published in IEEE 141, IEEE 241, or IEEE 399.

−

−### The use of typical values in lieu of measured data shall be explicitly noted in the assumption log for each affected component.

−

−### Cable impedances shall be calculated using the as-installed cable size, conductor material, raceway type (steel or non-magnetic), and route length.

−

−### The raceway material shall be modeled correctly per [[sync/raceways-and-conduit]] and [[sync/conductors-and-cables]] selections, because steel raceway produces higher reactance than non-magnetic raceway and the difference is significant for parallel-feeder systems.

−

−## Motor Contribution {toc}

−

−```datasheet

−label: Motor Contribution Modeling

−type: radio

−options:

− - "All motors 50 HP and larger modeled individually"

− - "All motors 50 HP and larger modeled individually; smaller motors lumped per bus"

− - "All motors lumped per bus by total connected HP"

−default: "All motors 50 HP and larger modeled individually; smaller motors lumped per bus"

−```

−

−### Motors contribute to fault current for the first several cycles after a fault because the inertia of the rotor acts as a temporary source. {note}

−

−### Motor contributions shall be modeled per IEEE 551 with subtransient reactance values from the motor nameplate or, where not available, from the typical values published in IEEE 551 by motor type and size.

−

−### Per the ANSI/IEEE methodology, induction motor contribution decays over the first several cycles and is represented by different impedance multipliers for the first-cycle, interrupting, and time-delayed networks. {note}

−

−### Motors smaller than 50 HP that are remote from the bus may be aggregated as a lumped motor contribution at the panel level, scaled by total connected horsepower.

−

−### Motors 50 HP and larger, all synchronous motors, and all motors served by adjustable-speed drives that can contribute regenerative current shall be modeled individually because their contribution to a nearby bus fault may be controlling for equipment selection.

−

−### Motor loads served entirely through current-limiting power electronics (most modern VFDs without regenerative front ends) do not contribute meaningful fault current and may be excluded from the contribution calculation.

−

−### The performer shall identify which motor loads are served entirely through current-limiting power electronics and shall document the basis for exclusion.

−

−## Utility Source Modeling {toc}

−

−```datasheet

−label: Utility Available Fault Current Source

−type: radio

−options:

− - "Written confirmation from serving utility (preferred)"

− - "Worst-case estimate per utility published data"

− - "Infinite bus assumption (worst case where utility data unavailable)"

−default: "Written confirmation from serving utility (preferred)"

−```

−

−```datasheet

−label: Future Expansion Margin Applied to Utility Contribution

−type: select

−unit: '%'

−drawing_ref: true

−options:

− - "0% — model exactly as provided by utility"

− - "10% — modest growth margin"

− - "25% — significant growth or interconnection planned"

−default: "10% — modest growth margin"

−```

−

−### The utility's available short-circuit current at the service point is the single most important input to the study because the entire downstream system's fault duties scale with it. {note}

−

−### The Contractor shall request the available three-phase and single-line-to-ground short-circuit currents (with associated X/R ratios) from the serving utility in writing.

−

−### The Contractor shall include the utility's response, including any future expansion or interconnection allowances stated by the utility, as an appendix to the study report.

−

−### Where written utility data cannot be obtained before equipment procurement, the study shall use a conservative estimate documented as such.

−

−### An "infinite bus" assumption — modeling the utility as a source of unlimited current at its rated voltage — is the most conservative assumption and is acceptable as a worst-case basis.

−

−### The study report shall identify clearly which utility-source case was used and shall flag the study for re-execution if the actual utility value, when received, differs materially from the assumed value.

−

−### A future-expansion margin on the utility contribution is recommended because utility short-circuit values rise over time as the utility upgrades its system, and equipment installed today is expected to remain in service for thirty years or more. {note}

−

−### The future-expansion margin shall be a documented design decision, not a hidden assumption.

−

−## Generator and Standby Source Modeling {toc}

−

−```datasheet

−label: Standby Source Operating Cases

−type: checkbox

−options:

− - "Utility source only"

− - "Standby source only"

− - "Both sources paralleled (where momentarily paralleled during transfer)"

− - "Generator paralleled with utility (cogeneration / peak shaving)"

−default: "Utility source only"

−```

−

−### On-site sources — emergency generators, standby generators, and uninterruptible power supplies that can supply fault current — shall be modeled per IEEE 551.

−

−### The study shall calculate fault currents both with and without the standby source connected, because equipment downstream of an automatic transfer switch may see different available fault currents depending on which source is energized.

−

−### The higher of the with-source and without-source cases controls the equipment rating.

−

−### Where the system permits paralleled operation of utility and on-site generation, the parallel case shall be calculated explicitly and is normally the worst-case basis for downstream equipment.

−

−### Transfer switch and parallel-source configurations shall be coordinated with [[sync/emergency-and-standby-power]].

−

−# Input Data Requirements {toc}

−

−## The accuracy of the study output depends entirely on the accuracy of its inputs. {note}

−## The Contractor shall furnish, and the study performer shall document, every input value used.

−

−## Required Data {toc}

−

−### The following input data shall be obtained for each system component and shall be tabulated in the study report:

−

−| Component | Required Data |

−|-----------|---------------|

−| Utility service | Three-phase and SLG available short-circuit currents and X/R ratios |

−| Power transformers | kVA rating, voltage ratings, impedance (%Z), X/R ratio, connection type |

−| Distribution transformers | Same as power transformers; manufacturer test report preferred |

−| Generators | kVA rating, subtransient/transient/synchronous reactances, X/R, time constants |

−| Motors > 50 HP | HP, voltage, locked-rotor or subtransient reactance, type (induction/synchronous) |

−| Aggregated motors < 50 HP | Total connected HP per bus, assumed average characteristics |

−| Cables | Size, material, length, raceway type, configuration (single/parallel) |

−| Busway | Manufacturer published impedance per foot, length |

−| Circuit breakers | Frame size, interrupting rating, voltage rating, manufacturer trip-unit data |

−| Fuses | Class, ampere rating, interrupting rating, current-limiting (I²t) data |

−

−```datasheet

−label: Equipment Data Source

−type: radio

−options:

− - "Manufacturer nameplate / test reports (preferred)"

− - "Shop drawings"

− - "Field-measured / surveyed"

− - "Typical values per IEEE color books"

−default: "Manufacturer nameplate / test reports (preferred)"

−```

−

−### Wherever a typical value is used in place of a measured or nameplate value, the substitution shall be flagged in the assumption log so that the Engineer of Record can assess the sensitivity of the study results to that assumption.

−

−## Existing Facility Surveys {toc}

−

−```datasheet

−label: Field Survey of Existing Equipment Required

−type: radio

−options:

− - "Yes — survey of all equipment within study scope"

− - "Yes — survey limited to equipment whose nameplate data is missing from drawings"

− - "No — drawings and submittals are complete"

−default: "Yes — survey limited to equipment whose nameplate data is missing from drawings"

−```

−

−### For studies that include existing equipment, the as-built configuration shall not be assumed from the original construction drawings, because equipment is routinely modified, replaced, and re-tapped over a facility's life without revisions to the drawings.

−

−### The Contractor shall perform a field survey of all existing equipment within the study scope, recording nameplate data, breaker frame sizes and trip settings, and cable sizes where visible.

−

−### Discrepancies between the field-surveyed condition and the existing drawings shall be reported to the Engineer.

−

−# Calculation Requirements {toc}

−

−## ANSI/IEEE Calculation Networks {toc}

−

−### For ANSI/IEEE methodology, the study shall calculate fault currents using the impedance multipliers and network adjustments of ANSI C37.010 and IEEE 551 for the following cases:

−

−- **First-cycle (momentary) network** — for bus bracing, instantaneous protective device response, and low-voltage breaker interrupting rating per ANSI C37.13

−- **Interrupting (contact-parting) network** — for medium-voltage breaker interrupting rating per ANSI C37.010, with contact parting time per the breaker rating

−- **Time-delayed (30-cycle) network** — for time-delayed relay coordination per [[sync/protective-coordination-study]]

−

−### The same model shall be used for all three networks; the differences are in the impedance multipliers applied to rotating machines, not in the network topology.

−

−## IEC 60909 Calculation {toc}

−

−### Where IEC 60909 methodology is specified, the study shall calculate the initial symmetrical short-circuit current (I"k), peak short-circuit current (ip), symmetrical short-circuit breaking current (Ib), and steady-state short-circuit current (Ik) per IEC 60909-0.

−

−### Voltage factor c shall be applied per IEC 60909-0 Table 1 for the appropriate system voltage and case (maximum or minimum fault).

−

−## X/R Ratio and Asymmetry {toc}

−

−```datasheet

−label: X/R Adjustment to Equipment Ratings

−type: radio

−options:

− - "Applied where calculated X/R exceeds tested X/R per ANSI C37.010"

− - "Not applied — equipment selected with margin to envelope any X/R"

−default: "Applied where calculated X/R exceeds tested X/R per ANSI C37.010"

−```

−

−### The X/R ratio at each fault point shall be calculated separately from the magnitude of the symmetrical fault current.

−

−### The X/R ratio determines the dc component of the fault current and therefore the asymmetrical peak that equipment must withstand and interrupt. {note}

−

−### Where the calculated system X/R exceeds the tested X/R for a given device, an adjustment multiplier shall be applied per ANSI C37.010 or per the manufacturer's published derating data.

−

−## Pre-Fault Voltage {toc}

−

−```datasheet

−label: Pre-Fault Voltage Assumption

−type: select

−options:

− - "1.00 pu (rated voltage)"

− - "1.05 pu (high-voltage condition for maximum fault)"

− - "Per IEC 60909 voltage factor c (IEC methodology)"

−default: "1.00 pu (rated voltage)"

−```

−

−### For ANSI/IEEE methodology, a pre-fault voltage of 1.00 per unit at the source is the standard assumption and produces results that are consistent with how equipment is tested. {note}

−

−### A higher pre-fault voltage (commonly 1.05 pu) may be specified where the system is known to operate above nominal voltage and a more conservative result is required.

−

−### The selected pre-fault voltage value shall be stated in the study report.

−

−# Equipment Rating Verification {toc}

−

−## The principal purpose of the short-circuit study is to verify that every device required to interrupt or withstand fault current has a rating equal to or greater than the calculated available fault current at its location. {note}

−

−## Interrupting Rating Verification {toc}

−

−### For every overcurrent protective device — main breakers, feeder breakers, branch breakers, fuses, and any other device whose function includes interrupting fault current — the study shall tabulate:

−

−- Device identifier and location

−- Manufacturer's interrupting rating at the applicable voltage

−- Calculated available fault current at the device's line terminals

−- Margin (manufacturer's rating minus calculated current), or deficiency where negative

−- Pass / Fail / Pass-with-series-rating status

−

−```datasheet

−label: Interrupting Rating Margin (Target)

−type: select

−unit: '%'

−options:

− - "0% — exact equality is acceptable"

− - "10% — modest safety margin"

− - "25% — conservative margin for future growth"

−default: "10% — modest safety margin"

−```

−

−### A device whose interrupting rating exactly equals the calculated available fault current technically complies with NEC 110.9 but provides no margin for utility-source growth, modeling uncertainty, or future system changes. {note}

−

−### A target interrupting-rating margin of at least 10 percent is recommended for new construction.

−

−## Series-Rated Combinations {toc}

−

−```datasheet

−label: Series-Rated Combinations

−type: radio

−options:

− - "Not permitted — every device must be fully rated"

− - "Permitted only where tested combinations are explicitly listed by manufacturer"

− - "Permitted per NEC 240.86"

−default: "Permitted only where tested combinations are explicitly listed by manufacturer"

−```

−

−### A series-rated combination relies on the operation of an upstream device to limit the let-through current to a level the downstream device can interrupt. {note}

−

−### Series ratings are permitted by NEC 240.86 only where the combination has been tested and listed by the manufacturer and where labeled on the downstream equipment.

−

−### Series-rated combinations shall be reviewed in conjunction with the [[sync/protective-coordination-study]] before being accepted, because they introduce a coordination penalty in which the upstream device often must trip on faults that an independently rated downstream device would clear alone.

−

−### For service entrance equipment and for any system where selective coordination is required (life safety, healthcare, data centers), series ratings shall not be used.

−

−## Short-Circuit Withstand Rating {toc}

−

−```datasheet

−label: Bus Bracing Verification

−type: checkbox

−options:

− - "Main switchgear / switchboard"

− - "Distribution panelboards"

− - "Motor control centers"

− - "Busway runs"

− - "Branch-circuit panelboards"

−default: "Main switchgear / switchboard"

−```

−

−### For every bus, switchboard, switchgear assembly, motor control center, busway, and panelboard, the study shall verify that the assembly's short-circuit current rating or short-time withstand rating equals or exceeds the calculated available fault current.

−

−### For equipment with a short-time withstand rating (typically medium-voltage switchgear and low-voltage power circuit breaker switchgear per [[sync/low-voltage-switchgear]]), the withstand duration shall be at least as long as the upstream protective device's clearing time at the calculated fault current.

−

−## Cable Short-Circuit Withstand {toc}

−

−### Cables subject to high available fault current shall be checked for thermal short-circuit withstand using the I²t capacity of the conductor and the upstream device's let-through I²t.

−

−### The conductor shall be able to absorb the fault energy without exceeding the insulation's short-circuit temperature limit (typically 250 °C for thermoset insulations such as XHHW and XLPE).

−

−### For feeders protected by current-limiting fuses, the let-through I²t is significantly reduced and is published by the fuse manufacturer; for feeders protected by non-current-limiting breakers, the let-through energy is approximately the available current squared times the breaker's clearing time. {note}

−

−## Equipment Short-Circuit Current Rating (SCCR) {toc}

−

−```datasheet

−label: SCCR Verification for Industrial Control Panels

−type: radio

−options:

− - "Required — every industrial control panel verified per UL 508A"

− - "Required only for panels in scope of equipment furnished under this project"

− - "Not applicable — no industrial control panels in scope"

−default: "Required only for panels in scope of equipment furnished under this project"

−```

−

−### NEC 409.110 requires industrial control panels to be marked with a Short-Circuit Current Rating (SCCR), which is the maximum available fault current the panel can withstand without damage. {note}

−

−### The study shall verify that every industrial control panel within the project scope has a labeled SCCR equal to or greater than the calculated available fault current at its line terminals.

−

−### Where a panel's SCCR is inadequate, the panel manufacturer shall be requested to provide a higher SCCR (typically achievable by changing branch components or adding upstream current-limiting protection) before the panel is installed.

−

−# Deliverables {toc}

−

−## Study Report Contents {toc}

−

−### The final study report shall be a single bound or paginated PDF document containing, in this order:

−

−1. Cover page with project identification, performer identification, date, revision history, and PE seal

−2. Executive summary stating the study scope, methodology, key findings, and any deficiencies identified

−3. System description with single-line diagram of the modeled system, with every bus and device identified

−4. Input data tables for all sources, transformers, motors, cables, and protective devices

−5. Assumption log enumerating every assumption made and the basis for it

−6. Calculation results — fault current and X/R at every bus, for every fault type calculated

−7. Equipment rating comparison tables (interrupting rating, withstand rating, SCCR)

−8. Deficiency findings with proposed remediation for each

−9. Recommendations for re-study and re-verification

−10. Appendices: utility correspondence, manufacturer data sheets, software input/output files

−

−```datasheet

−label: Report Format

−type: checkbox

−options:

− - "Bound printed copy (sealed and signed)"

− - "Sealed PDF (digitally signed by PE)"

− - "Native study software files (.sav / .axd / .olr or equivalent)"

− - "Editable single-line diagram (CAD or equivalent)"

−default: "Sealed PDF (digitally signed by PE)"

−```

−

−## Single-Line Diagram {toc}

−

−### The single-line diagram included in the report shall show every modeled component with consistent identifiers, every transformer's kVA and impedance, every cable's size and length, and every bus's calculated three-phase and SLG fault current values shown adjacent to the bus.

−

−### The single-line diagram shall be legible at the printed page size; for large systems, the diagram may be subdivided across multiple sheets with cross-references.

−

−### The single-line diagram from the study shall be reconciled with the contract one-line diagram, and discrepancies between the study one-line and the construction documents shall be identified and resolved before the study is accepted.

−

−## Deficiency Reporting {toc}

−

−```datasheet

−label: Deficiency Reporting Format

−type: radio

−options:

− - "Tabular summary with per-deficiency remediation proposal"

− - "Tabular summary only (remediation handled separately)"

−default: "Tabular summary with per-deficiency remediation proposal"

−```

−

−### Every device or assembly found to be under-rated for the calculated available fault current shall be reported individually, with location, calculated fault current, rated capability, and a proposed remediation.

−

−### The Contractor and Engineer shall jointly resolve each deficiency finding before the affected equipment is energized.

−

−# Re-Study Triggers {toc}

−

−## A short-circuit study reflects the system as it existed when the study was performed. {note}

−## The Owner shall commission an updated study whenever any of the following conditions occur, because each can change the calculated available fault current at one or more points in the system.

−

−```datasheet

−label: Re-Study Triggers Documented in O&M Manual

−type: checkbox

−options:

− - "Utility upgrade or change in available fault current at service"

− - "Addition or removal of on-site generation or UPS"

− - "Replacement or re-tapping of any power transformer"

− - "Change in service or feeder conductor size, type, or length"

− - "Addition of large motor loads (>50 HP) at any bus"

− - "Major facility expansion or significant load addition"

− - "Five-year periodic re-study (NFPA 70E recommendation)"

−default: "Utility upgrade or change in available fault current at service"

−```

−

−## The five-year periodic re-study aligns with the NFPA 70E recommended re-evaluation interval for arc flash analyses, which depend on short-circuit results; even where no facility change has occurred, utility source impedance commonly decreases over time as the utility upgrades its transmission and distribution system. {note}

−

−## The re-study trigger list shall be included in the closeout O&M manual so that future owners and facility managers know when to commission an updated study.

−

−## The Contractor shall provide field-marking of service equipment with the maximum available fault current and the date the marking was applied or verified, per NEC 110.24 and [[sync/equipment-labeling]], using the study's service-point result.

−

−# Coordination with Other Studies {toc}

−

−## Protective Coordination Study {toc}

−

−### The protective coordination study (see [[sync/protective-coordination-study]]) uses the short-circuit study's calculated fault currents and X/R ratios at every bus to verify that protective devices operate in the correct sequence for faults at every point in the system. {note}

−

−### The short-circuit study and the protective coordination study shall be performed by the same engineering organization where practical, and any change to the short-circuit model shall trigger a corresponding update to the coordination study.

−

−## Arc Flash Study {toc}

−

−### The arc flash study (see [[sync/arc-flash-study]]) uses the calculated bolted-fault currents from the short-circuit study and the device clearing times from the coordination study to calculate incident energy per IEEE 1584. {note}

−

−### Arc flash results are derivative of short-circuit and coordination results; a change to either of those studies invalidates the arc flash labels and requires an arc flash re-study.

−

−### The three studies form a single, coupled analysis package and shall be managed as such.

−

−## Grounding and Bonding {toc}

−

−### The short-circuit study assumes that the grounding and bonding system establishes an effective ground-fault current path per [[sync/grounding-and-bonding]], and the line-to-ground fault current calculated in the study is meaningful only to the extent that the installed grounding and bonding system actually provides the assumed return path. {note}

−

−### Field verification of bonding continuity per [[sync/grounding-and-bonding]] is a prerequisite to relying on the line-to-ground fault current results.

−

−## Field Acceptance Testing {toc}

−

−### The available fault current at the service shall be verified, where practical, during NETA acceptance testing per [[sync/low-voltage-switchgear]] and per ANSI/NETA ATS.

−

−### Verification methods include re-confirmation of the utility's stated available fault current at the time of energization and review of as-installed transformer and cable parameters against the study's modeled values.

−

−### Material discrepancies discovered during acceptance testing shall trigger a study revision before the equipment is placed in normal service.

−

−# Warranty {toc}

−

−```datasheet

−label: Engineering Services Warranty / Errata Period

−type: select

−options:

− - "1 year — correction of errors at no additional cost"

− - "2 years — correction of errors at no additional cost"

− - "Per the engineer's standard professional services agreement"

−default: "1 year — correction of errors at no additional cost"

−```

−

−## The study performer's professional engineering services shall be performed in accordance with generally accepted engineering practice and shall be warranted as such under the performer's professional liability coverage.

−

−## The Contractor shall provide the Owner with the engineer's contact information so that any future re-study, errata correction, or interpretation of the original study report can be addressed by the engineer of record for the study.

−

−## The errata period covers correction of calculation errors and reporting errors identified after delivery; it does not cover updates required by system changes, utility changes, or other re-study triggers, which are separately commissioned re-studies. {note}

+---

+title: Short-Circuit Study

+category: Electrical / Power Studies

+description: >

+ When to use: The short-circuit study for a project's electrical power distribution system, from the utility point of delivery through every point at which an interrupting rating, a close-and-latch rating, a short-time withstand rating, a bus bracing rating, or a marked short-circuit current rating is confirmed, on AC systems rated 1000 V and below through 38 kV with utility, generator, inverter-based, and paralleled sources. Covers who performs the study and when it is delivered, the calculation method and the duty networks, the fault types and the maximum and minimum cases, the system model and the data it is built from (the utility's available fault current and X/R, transformer impedances, conductor impedances and lengths, motor and generator contribution, inverter-based sources, and the survey of existing equipment on a renovation), the source operating cases, the equipment duty evaluation, the treatment of series-rated combinations, the available fault current and date that NFPA 70 110.24 requires at the service equipment, the report and the model handed to the coordination and arc-flash studies, and the events that call for a re-study.

+ Not intended for: the time-current coordination of the protective devices and their settings ([[sync/protective-coordination-study]]); the incident energy and arc-flash boundary at each piece of equipment ([[sync/arc-flash-study]]); the field acceptance program applied to the installed equipment before energization ([[sync/electrical-acceptance-testing]]); the label on which the available fault current is marked, which the project's identification standard governs ([[sync/electrical-identification]]); the grounding electrode system and the bonding that forms the ground-fault return path ([[sync/grounding-and-bonding]]); the short-circuit current rating determination of an industrial control panel under UL 508A Supplement SB ([[sync/industrial-control-panels]]); load-flow, voltage-drop, motor-starting, harmonic, and transient-stability studies; DC system fault calculations; and fault calculations on the utility's side of the point of delivery.

+---

+# Scope {toc}

+## This standard governs the short-circuit study as an engineering deliverable: the engineer who performs it and when it is delivered, the calculation method, the system model and the data it is built from, the cases studied, the evaluation of every rating the calculated duty is compared against, the available fault current data for the service equipment, the report, and the model that the coordination and arc-flash studies take over from it. {note}

+## NFPA 70 110.9 requires every device that interrupts fault current to have an interrupting rating not less than the current available at its line terminals, and 110.10 requires the conductors, the overcurrent devices, and the equipment of every circuit to withstand the fault current the overcurrent device lets through without extensive damage. OSHA 29 CFR 1910.303(b)(4) and (b)(5) state the same two requirements as a condition of the workplace. Neither can be shown from a nameplate; both rest on a calculation of the current available at every point in the system, and this study is that calculation. {note}

+## The short-circuit study is the first of three studies performed on the same system model. It establishes the magnitude of the fault current at every bus and the X/R ratio behind it; the coordination study under [[sync/protective-coordination-study]] uses those currents to set the protective devices so that the nearest one clears; and the arc-flash study under [[sync/arc-flash-study]] turns the bolted fault current and the clearing time at each bus into incident energy. A change to this study's model moves both of the others, which is why the model and the results are delivered in a form the other two can take over without rebuilding. {note}

+## The following are governed elsewhere and are outside this standard: {note}

+- the protective device settings and the time-current coordination they achieve, under [[sync/protective-coordination-study]], which takes the accepted short-circuit study as its input and does not start without it

+- the incident energy, the arc-flash boundary, and the arc-flash labels, under [[sync/arc-flash-study]], which takes the bolted fault currents from this study and the clearing times from the coordination study

+- the field tests that verify the installed equipment and its settings before energization, under [[sync/electrical-acceptance-testing]]

+- the ratings the equipment is ordered to, which each equipment standard carries in its own datasheet and this study confirms, under [[sync/low-voltage-switchgear]], [[sync/medium-voltage-switchgear]], [[sync/low-voltage-switchboards]], [[sync/panelboards]], [[sync/motor-control-centers]], [[sync/busway]], [[sync/automatic-transfer-switches]], [[sync/generator-paralleling-switchgear]], [[sync/secondary-unit-substations]], and the other equipment standards that name this study

+- the utility point of delivery and the rules that apply at the service, under [[sync/electrical-service-entrance]], the service conductors, under [[sync/distribution-feeder-cables]], and the service equipment as products, under [[sync/low-voltage-switchboards]], [[sync/low-voltage-switchgear]], [[sync/enclosed-switches-and-disconnects]], and [[sync/panelboards]]

+- the available fault current label at the service equipment, as a product, under the project's identification standard (see [[sync/electrical-identification]]); this study supplies the value and the date that go on it

+- the grounding electrode system, the bonding, and the system grounding method of each source, under [[sync/grounding-and-bonding]]; this study models the grounding method the one-line diagram shows and calculates the ground-fault current that results

+- the short-circuit current rating determination of an industrial control panel, under [[sync/industrial-control-panels]]; this study compares the marked rating with the available fault current at the panel

+- the facility's electrical safety program, under [[sync/electrical-safety-program]], which a field survey of energized existing equipment is performed under

+- load-flow, voltage-drop, motor-starting, harmonic, and transient-stability studies, DC system fault calculations, and fault calculations on the utility's side of the point of delivery

+## The study shall extend from the utility point of delivery through every point within the study boundary described under Study Boundary and Calculation Points, on every source and in every operating case this standard requires.

+# Referenced Standards {toc}

+## The study, its model, and its report shall comply with the latest adopted edition of each of the following unless a specific edition is cited.

+## Where the contract documents, the adopted edition of NFPA 70, the serving utility's requirements, or two referenced standards impose conflicting requirements, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.

+| Standard | Title |

+|----------|-------|

+| NFPA 70 | National Electrical Code (Articles 110, 240, 409, 440, 700, 701, 705, and 708) |

+| NFPA 70E | Standard for Electrical Safety in the Workplace |

+| OSHA 29 CFR 1910.303 | General Requirements (Subpart S, Electrical) |

+| IEEE 551 | Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems (Violet Book) |

+| IEEE 3002.3 | Recommended Practice for Conducting Short-Circuit Studies and Analysis of Industrial and Commercial Power Systems |

+| IEEE 141 | Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book) |

+| IEEE 241 | Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book) |

+| IEEE 242 | Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book) |

+| IEEE C37.010 | Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis |

+| IEEE C37.04 | Rating Structure for AC High-Voltage Circuit Breakers |

+| IEEE C37.06 | AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis, Preferred Ratings and Related Required Capabilities |

+| IEEE C37.5 | Guide for Calculation of Fault Currents for Application of AC High-Voltage Circuit Breakers Rated on a Total Current Basis |

+| IEEE C37.13 | Low-Voltage AC Power Circuit Breakers Used in Enclosures |

+| IEEE C37.20.1 | Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear |

+| IEEE C37.20.2 | Metal-Clad Switchgear |

+| IEEE C57.12.00 | General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers |

+| IEEE C57.12.01 | General Requirements for Dry-Type Distribution and Power Transformers |

+| UL 489 | Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures |

+| UL 248 | Low-Voltage Fuses |

+| UL 508A | Industrial Control Panels (Supplement SB) |

+| UL 1008 | Transfer Switch Equipment |

+| UL 1449 | Surge Protective Devices |

+| ICEA P-32-382 | Short-Circuit Characteristics of Insulated Cable |

+| IEC 60909-0 | Short-Circuit Currents in Three-Phase AC Systems, Calculation of Currents |

+# Submittals {toc}

+## Action Submittals {toc}

+### The Contractor shall submit the following for review by the Engineer of Record:

+- the qualifications of the study engineer, giving the license, the firm, and the comparable studies described under Study Engineer

+- the study plan, giving the calculation method, the analysis software and its version, the duty networks, fault types, and operating cases to be calculated, the study boundary as a preliminary one-line diagram carrying the bus and device identifiers the report will use, the source of each class of input data, and the schedule of the study against the release dates of the equipment whose ratings it confirms

+- the study report described under Study Report, at the point in the project the datasheet selects under Study Delivery

+- where the study is delivered before equipment release, a confirmation table listing each item of equipment whose rating the study confirms, by its designation on the one-line diagram, with the calculated duty, the rating proposed in the equipment submittal, and the result

+```datasheet

+label: Action Submittals

+type: checkbox

+options:

+ - "Study engineer qualifications"

+ - "Study plan with preliminary one-line diagram"

+ - "Study report"

+ - "Equipment rating confirmation table by designation"

+default:

+ - "Study engineer qualifications"

+ - "Study plan with preliminary one-line diagram"

+ - "Study report"

+ - "Equipment rating confirmation table by designation"

+constraint: all_of

+```

+### The study plan shall be reviewed and returned before the model is built, and the study report shall not be submitted until the plan covering it has been returned.

+### The study plan settles, before any data is entered, the questions that otherwise surface when the report is rejected: which buses are in the model and which are not, which cases are run, whose data the model is built from, and whether the study will be back before the switchgear order has to go. A plan returned with the boundary drawn and the schedule tied to the equipment release dates is what makes the study a gate the equipment order waits for rather than a document that arrives after it. {note}

+## Closeout Submittals {toc}

+### The Contractor shall submit the following before the equipment within the study boundary is accepted:

+- the final study report, revised to the as-built system where the datasheet selects an as-built update under Study Delivery, and otherwise re-issued with a reconciliation of the submittal-phase model against the equipment furnished and the conductor lengths installed

+- the native model files of the analysis software, with the software version and device library revision that produced the report

+- the available fault current and the date of the calculation for each item of service equipment within the study boundary, as the record that NFPA 70 110.24 requires to be documented and kept available

+- the re-study trigger list described under Study Revisions and Re-Study Triggers, written for the Owner's operations staff

+- the utility's written statement of available fault current and the manufacturer data sheets and test reports the model is built from

+```datasheet

+label: Closeout Submittals

+type: checkbox

+options:

+ - "Final study report with as-built reconciliation"

+ - "Native model files with software version and library revision"

+ - "Available fault current record for each item of service equipment"

+ - "Re-study trigger list"

+ - "Utility statement and source data"

+default:

+ - "Final study report with as-built reconciliation"

+ - "Native model files with software version and library revision"

+ - "Available fault current record for each item of service equipment"

+ - "Re-study trigger list"

+ - "Utility statement and source data"

+constraint: all_of

+```

+### The native model files shall be delivered in a form the Owner can open and edit with a licensed copy of the software, without a password, a locked library, or a dependency on the study engineer's account.

+### A short-circuit model is rebuilt from scratch whenever the native file is lost, and rebuilt wrong whenever the one-line and the data tables in the PDF disagree with what was actually modeled. The native file is the study; the report is a printout of it. {note}

+# Quality Assurance {toc}

+## Study Engineer {toc}

+### The study shall be performed by the study engineer indicated in the datasheet.

+```datasheet

+label: Study Engineer

+type: select

+options:

+ - "Engineer of Record"

+ - "Independent engineer engaged by the Contractor"

+ - "Equipment manufacturer's engineering services"

+ - "Testing organization's engineering group"

+```

+### The Engineer of Record holds the design model and the utility correspondence and has no stake in the equipment, and performs the study where the fault currents are needed in design to size the equipment, with the result that the study is built on design data and is reconciled to the equipment furnished later; an independent engineer engaged by the Contractor is the arrangement where the study is a construction deliverable priced in the bid, built from the submittals of the equipment actually furnished, and reviewed by the Engineer of Record as a submittal; the equipment manufacturer's engineering services know their own equipment's ratings and series listings best and deliver the study with the switchgear order at the lowest added cost, and their model of the other makers' equipment, the conductors, and any existing system is only as good as the data handed to them; the testing organization's engineering group puts the study, the coordination settings, and the acceptance tests in one firm's hands, so that the party who sets the trip units built the model they were set from. No one arrangement is the norm across projects, and the choice turns on when the ratings are needed and who holds the data. {note}

+### The study shall be performed under the responsible charge of a Professional Engineer licensed in the jurisdiction of the project, and the report, the one-line diagram, and the results tables shall bear that engineer's seal and signature.

+### The study engineer shall have performed not fewer than five short-circuit studies on systems of the same voltage class and of comparable size within the five years before the study, and shall name them in the qualifications.

+### Where the equipment manufacturer's engineering services perform the study, the model shall include every source, bus, conductor, and device within the study boundary, whether or not the manufacturer furnished it, and a study limited to the manufacturer's own equipment shall not be accepted.

+### Where the study engineer is not the Engineer of Record, the Engineer of Record shall review the study report and return it before the ratings it confirms are relied on.

+### Where the study engineer and the Engineer of Record disagree on an input value, an assumption, or the evaluation of a result, the Engineer of Record shall make the determination and the study shall record it in the assumption register.

+## Study Delivery {toc}

+### The study shall be delivered at the point in the project indicated in the datasheet.

+```datasheet

+label: Study Delivery

+type: select

+options:

+ - "Before equipment release, on submittal data"

+ - "After installation, on as-built data"

+ - "Before equipment release, with an as-built update after installation"

+default: "Before equipment release, on submittal data"

+```

+### A study delivered before equipment release confirms every interrupting, withstand, and short-circuit current rating while the rating can still be changed on paper, which is why the equipment standards hold the release of switchgear, switchboards, panelboards, transfer switches, and drives for it, and it is built on submittal data and design conductor lengths that the installation departs from; a study delivered after installation is built on the nameplates and the lengths that exist, so its results are the ones the arc-flash labels and the record carry, and a rating it finds short is corrected by replacing equipment that is already installed; a study delivered before release and updated after installation gives both, at the cost of a second pass through the model and a second review, and is the arrangement a project chooses where the system is large, where the conductor routing is uncertain at the time of the submittals, or where the coordination and arc-flash studies will be run on the as-built model. {note}

+### Where the study is delivered before equipment release, no item of equipment whose interrupting rating, close-and-latch rating, short-time withstand rating, bus bracing, or marked short-circuit current rating the study confirms shall be released for manufacture until the study has been reviewed and returned without a deficiency against that item.

+### Where the study is delivered after installation, the equipment shall be ordered to the ratings the contract documents state, and the Contractor shall furnish the study before the equipment within the study boundary is energized.

+### Where an as-built update is selected, the update shall be issued after the equipment is installed and the conductors are pulled and before the equipment within the study boundary is energized, and the coordination study and the arc-flash study shall be run on the updated model.

+### The equipment standards that name this study hold the release of equipment for it, and a Contractor who releases equipment ahead of the study carries the cost of any rating the study then finds short. {note}

+## Calculation Tool {toc}

+### The study shall be performed with the calculation tool indicated in the datasheet.

+```datasheet

+label: Calculation Tool

+type: radio

+options:

+ - "Commercial power system analysis software"

+ - "Hand calculation by the method of IEEE 551"

+default: "Commercial power system analysis software"

+```

+### Commercial analysis software holds the model as a file that the coordination and arc-flash studies open next and that a re-study opens years later, carries the device and conductor libraries the model draws on, and applies the duty multipliers of the ANSI/IEEE method without transcription, which is why it is the tool on nearly every study; a hand calculation by the per-unit or ohmic method of IEEE 551 is reproducible by any engineer with the report in hand and is the tool a project accepts where the system is a single service, one or two transformers, and a handful of buses, with the result that there is no model file to hand to the next study. {note}

+### Where software is selected, the report shall identify the software, its version, and the revision of the device library used, and the Contractor shall not name the software in the study plan as a condition of the work.

+### Where software is selected, the study engineer shall verify the software result by a hand calculation at the service equipment and at one bus on the secondary of each transformer class in the system, and shall include the verification in the report.

+### Where hand calculation is selected, the report shall show every impedance in the calculation in per-unit on a stated base or in ohms at a stated voltage, the reduction of the network for each fault location, and the multipliers applied, so that every result can be reproduced from the report alone.

+# Calculation Basis {toc}

+## Calculation Method {toc}

+### The study shall be calculated by the method indicated in the datasheet.

+```datasheet

+label: Calculation Method

+type: radio

+options:

+ - "ANSI/IEEE method per IEEE 551"

+ - "IEC 60909 method"

+default: "ANSI/IEEE method per IEEE 551"

+```

+### The ANSI/IEEE method calculates the duty on each device in the terms its rating standard tests it in, with the first-cycle, interrupting, and time-delayed networks and the X/R multipliers of IEEE 551, IEEE C37.010, and IEEE C37.13, and it is the method for every system whose equipment is rated under IEEE and UL standards, which is every system this standard is written for in its home market; the IEC 60909 method calculates the initial symmetrical current, the peak current, the breaking current, and the steady-state current with the voltage factor c and the impedance correction factors of IEC 60909-0, and is the method for a system whose equipment is rated under IEC standards or whose owner's engineering practice is written to IEC. The two methods give different numbers for the same system and a rating tested under one is not evaluated against a current calculated under the other. {note}

+### Under the ANSI/IEEE method, the duty on each device shall be calculated from the network and with the multipliers that the standard the device is rated under assigns: IEEE C37.010 for a medium-voltage circuit breaker rated on a symmetrical current basis, IEEE C37.5 for an existing medium-voltage circuit breaker rated on a total current basis, IEEE C37.13 for a low-voltage power circuit breaker, and UL 489 and UL 248 for a molded-case circuit breaker and a fuse.

+### Under the IEC 60909 method, the study shall calculate the initial symmetrical short-circuit current, the peak short-circuit current, the symmetrical breaking current at the minimum time delay of each device, and the steady-state short-circuit current, with the voltage factor c of IEC 60909-0 for the maximum and the minimum case, and shall evaluate each device against the rating its standard states in those terms.

+### The report shall state the method in its executive summary and on the title block of every results table.

+## Duty Networks {toc}

+### Under the ANSI/IEEE method, the study shall calculate the duty networks indicated in the datasheet at every calculation point.

+```datasheet

+label: Duty Networks Calculated

+type: checkbox

+options:

+ - "First-cycle network"

+ - "Interrupting network"

+ - "Time-delayed network"

+default:

+ - "First-cycle network"

+ - "Interrupting network"

+ - "Time-delayed network"

+constraint: all_of

+```

+### The three networks are one network with three sets of rotating-machine impedances. The first-cycle network, at one half cycle, carries every motor and generator at subtransient reactance with the IEEE 551 multipliers for motor size, and gives the current that a fuse, a molded-case breaker, a low-voltage power breaker, a bus brace, and the close-and-latch capability of a medium-voltage breaker are evaluated against; the interrupting network, at the contact-parting time of the breaker, lets the induction motor contribution decay by the IEEE 551 multipliers and gives the current a medium-voltage breaker interrupts; the time-delayed network, at about thirty cycles, drops the induction motors and places the generators at transient reactance, and gives the current a time-delayed relay sees and the current the minimum case is built from. On a system with no equipment above 1000 V the interrupting network adds nothing the first-cycle network does not already decide, and a project that leaves it out is leaving out a calculation the software performs anyway. {note}

+### The first-cycle network shall be used to evaluate every low-voltage interrupting rating, every bus bracing and short-time withstand rating, every marked short-circuit current rating, and the close-and-latch rating of every medium-voltage circuit breaker.

+### The interrupting network shall be used to evaluate the interrupting rating of every medium-voltage circuit breaker, at the contact-parting time IEEE C37.010 assigns to the breaker's rated interrupting time.

+### The time-delayed network shall be used for the current a time-delayed relay element sees and for the minimum case delivered to the coordination study.

+### Where the calculated X/R at a device exceeds the X/R the device's rating standard tests it at, the symmetrical current shall be multiplied by the factor that standard or the manufacturer's published application data assigns before it is compared with the rating.

+## Fault Types {toc}

+### The study shall calculate the fault types indicated in the datasheet at every calculation point.

+```datasheet

+label: Fault Types Calculated

+type: checkbox

+options:

+ - "Three-phase bolted fault"

+ - "Single line-to-ground bolted fault"

+ - "Line-to-line bolted fault"

+ - "Double line-to-ground bolted fault"

+default:

+ - "Three-phase bolted fault"

+ - "Single line-to-ground bolted fault"

+constraint: all_of

+```

+### The three-phase bolted fault is the highest current at nearly every point of a system and is the case every interrupting and withstand rating is evaluated against; the single line-to-ground fault on a solidly grounded system can exceed the three-phase fault close to a wye-connected transformer or generator and is the current the ground-fault protection and the equipment grounding conductors are sized against; the line-to-line fault is about 87 percent of the three-phase value and is the lowest phase fault the coordination study uses as its minimum at the end of a feeder; the double line-to-ground fault is calculated where the ground-fault current through a grounded neutral is needed on a system whose grounding method makes it the larger case. On a resistance-grounded or ungrounded system the ground-fault current is set by the grounding resistor or the system capacitance rather than by the fault calculation, and the study reports it from the grounding design. {note}

+### The system grounding method of every source and separately derived system shall be modeled as shown on [[drawing: the one-line diagram]], and the neutral grounding resistor or reactor of a resistance-grounded or reactance-grounded source shall be included in the zero-sequence network.

+### Every fault type calculated shall be reported with its symmetrical RMS current and its X/R at the fault location, and the three-phase fault shall also be reported with the asymmetrical current or the peak current that the device rating standard at that location evaluates against.

+## Maximum and Minimum Cases {toc}

+### The study shall calculate the cases indicated in the datasheet at every calculation point.

+```datasheet

+label: Cases Calculated

+type: radio

+options:

+ - "Maximum fault current"

+ - "Maximum and minimum fault current"

+default: "Maximum and minimum fault current"

+```

+### The maximum case decides every rating; the minimum case decides whether the protective devices see enough current to operate at the far end of a feeder, which is the coordination study's question, and sets the lower bound the arc-flash study applies its arcing-current reduction to. A study that reports the maximum only serves the equipment order and leaves the two studies that follow it to build their own minimum case from the same model, and a project accepts that where no coordination or arc-flash study will be run on the model. {note}

+### The maximum case shall be calculated with the utility at its stated maximum contribution and the growth margin applied, every on-site source that can operate in parallel with the utility connected, every motor contributing, every transformer at its minimum impedance, and every conductor at 20 °C.

+### The minimum case shall be calculated with the utility at the minimum contribution it states or, where it states none, at its maximum contribution, the on-site sources at the configuration that gives the lowest current at the bus, no motor contribution, every transformer at its maximum impedance, and every conductor at the conductor temperature its insulation is rated for.

+## Pre-Fault Voltage {toc}

+### Under the ANSI/IEEE method, the pre-fault voltage at every bus shall be the per-unit value indicated in the datasheet.

+```datasheet

+label: Pre-Fault Voltage

+type: range

+unit: pu

+options:

+ min: 1.0

+ max: 1.1

+ setpoints: [1.0, 1.05, 1.1]

+default: 1.0

+constraint: exact

+```

+### A pre-fault voltage of 1.0 per unit at the bus nominal voltage is the value equipment is tested at and the value nearly every study uses; 1.05 per unit raises every result by 5 percent and is used where the system is operated above nominal voltage on a regulating transformer or a utility whose delivered voltage sits at the top of its range; 1.1 per unit matches the maximum voltage factor of IEC 60909-0 for a low-voltage system and is used where a study is to be compared with one calculated under IEC. {note}

+### Under the IEC 60909 method, the voltage factor c of IEC 60909-0 for the system voltage and the case shall apply in place of the pre-fault voltage of this article.

+# System Model {toc}

+## Study Boundary and Calculation Points {toc}

+### The study shall calculate the available fault current at every calculation point indicated in the datasheet within the study boundary.

+```datasheet

+label: Calculation Points

+type: checkbox

+options:

+ - "Utility point of delivery and the service equipment line terminals"

+ - "Secondary terminals of every transformer"

+ - "Line terminals of every switchgear, switchboard, and motor control center"

+ - "Line terminals of every panelboard"

+ - "Line terminals of every motor controller, starter, and drive"

+ - "Each end of every busway run and each plug-in unit"

+ - "Both source terminals and the load terminals of every transfer switch"

+ - "Terminals of every generator, UPS, and energy storage source"

+ - "Line terminals of every control panel and other utilization equipment with a marked short-circuit current rating"

+ - "Line terminals of every surge protective device"

+ - "Points of common coupling designated for a harmonic evaluation"

+ - "Other points designated by the Engineer of Record"

+default:

+ - "Utility point of delivery and the service equipment line terminals"

+ - "Secondary terminals of every transformer"

+ - "Line terminals of every switchgear, switchboard, and motor control center"

+ - "Line terminals of every panelboard"

+ - "Line terminals of every motor controller, starter, and drive"

+ - "Each end of every busway run and each plug-in unit"

+ - "Both source terminals and the load terminals of every transfer switch"

+ - "Terminals of every generator, UPS, and energy storage source"

+ - "Line terminals of every control panel and other utilization equipment with a marked short-circuit current rating"

+ - "Line terminals of every surge protective device"

+constraint: all_of

+```

+### Every point at which a rating is confirmed is a calculation point, and the list is long because the ratings are many: the service equipment and every distribution assembly for their interrupting ratings and bus bracing, every transfer switch for its withstand and closing rating from each source, every busway run for its bracing at the source end and every plug-in unit for its rating at the opening, every controller, drive, control panel, and surge protective device for its marked short-circuit current rating, and every source terminal for the breaker that protects the source. A point of common coupling is added where a drive or a harmonic filter is evaluated under IEEE 519, because the ratio of the short-circuit current to the demand current at that point sets the harmonic limit. The points a project leaves out are the ones whose equipment has no rating to confirm. {note}

+### Every calculation point shall be identified in the model, the report, and the one-line diagram by the designation the contract documents give the equipment at that point.

+### Where the model terminates at a bus and the equipment below it is evaluated from that bus, the report shall state the termination and shall evaluate the equipment below it against the current at the bus.

+## Utility Source {toc}

+### The utility's available fault current at the point of delivery shall be taken from the basis indicated in the datasheet.

+```datasheet

+label: Utility Available Fault Current Basis

+type: select

+options:

+ - "Written statement from the serving utility for the point of delivery"

+ - "Utility's published standard value for the service class"

+ - "Infinite bus at the primary terminals of the service transformer"

+default: "Written statement from the serving utility for the point of delivery"

+```

+### A written statement from the utility gives the three-phase and line-to-ground contribution and the X/R at the point of delivery for the system as it exists or as the utility plans it, is the basis on nearly every study, and takes a written request and a lead time the study plan has to allow for; a published standard value is what some utilities state for every service of a class in place of a site value, is already a planning maximum, and gives no X/R and no minimum, which the study then assumes; an infinite bus at the primary terminals of the service transformer takes the transformer impedance as the only limit on the current, gives the highest value that transformer can pass, is the basis where the utility provides nothing in time or where the service transformer is the project's and the utility's contribution behind it is large relative to it, and cannot be used where the utility delivers at the equipment voltage with no transformer between. {note}

+### The Contractor shall request the available three-phase and line-to-ground fault current, the X/R ratio of each, and any minimum or future value the utility holds, in writing from the serving utility for the point of delivery, and shall include the request and the response in the report.

+### Where the study is delivered on a basis other than the utility's written statement and the statement is received before the equipment within the study boundary is energized, the study engineer shall re-run the maximum case with the utility's value and shall re-issue every result that changes.

+### The utility contribution used in every maximum case shall be the utility's value increased by the growth margin indicated in the datasheet.

+```datasheet

+label: Utility Contribution Growth Margin

+type: range

+unit: '%'

+options:

+ min: 0

+ max: 50

+ setpoints: [0, 5, 10, 15, 20, 25, 50]

+constraint: exact

+```

+### Utility fault current rises over the life of a building as the utility reinforces its system, and equipment ordered today is rated for the contribution stated today. A margin of zero takes the utility's value as given, which is the practice where the utility states a planning or ultimate value rather than a present one; a margin of 10 to 25 percent buys headroom against a reinforcement the utility has not announced, at the cost of the next higher interrupting rating at the service and sometimes one level down; a margin of 50 percent approaches the infinite-bus result on a stiff system and is the figure a project reaches for where the utility has told it a substation upgrade is planned and has not quantified it. {note}

+### The growth margin shall be recorded in the assumption register as a design decision of the study, and the available fault current record for the service equipment shall state the utility's value and the margin separately.

+## Transformers {toc}

+### Every transformer in the model shall be entered with its rated kVA, its primary and secondary voltages and connection, its percent impedance on its own base, and its X/R ratio, taken from the nameplate or the certified test report of the unit furnished where available.

+### Where the tested impedance of a transformer furnished under the project is not yet available, the study shall use the design impedance stated in the submittal, reduced by the manufacturing tolerance IEEE C57.12.00 or IEEE C57.12.01 permits for the maximum case and increased by it for the minimum case, and the as-built reconciliation shall replace both with the tested value.

+### Where no impedance is available for an existing transformer, the study shall use the standard impedance that IEEE C57.12.00 or IEEE C57.12.01 assigns to a unit of its kVA and voltage class, reduced by the tolerance for the maximum case, and shall record the substitution in the assumption register.

+### Where a transformer's X/R is not published, the study shall use the typical X/R for its kVA from IEEE 551 or IEEE 242 and shall record the substitution.

+### A transformer's impedance is the single value that most decides the fault current on its secondary, and a tolerance of 7.5 percent on the impedance is about 8 percent on the secondary fault current. A transformer whose test report shows a lower impedance than its design value is the common way a secondary switchboard ordered to a submittal-phase study ends up with no margin, which is why the tolerance is taken in the maximum case until the tested value is in hand. {note}

+## Conductors and Busway {toc}

+### Every conductor and busway run between two calculation points shall be modeled with its resistance and reactance taken from the source indicated in the datasheet, for its conductor material, size, number of conductors per phase, insulation, and raceway material.

+```datasheet

+label: Conductor Impedance Source

+type: radio

+options:

+ - "Published typical values for the conductor size, material, and raceway type"

+ - "Manufacturer's published values for the conductor furnished"

+default: "Published typical values for the conductor size, material, and raceway type"

+```

+### Published typical values, the tables of IEEE 141 and IEEE 241 or the conductor library of the analysis software built from them, are available at the time the study is run and differ from the furnished conductor by less than the uncertainty in its length, which is why they are the source on nearly every study; the manufacturer's published values for the conductor furnished are the source where the conductor is a cable type with an impedance the tables do not carry, such as a shielded medium-voltage cable, a large interlocked-armor cable, or a busway, or where the study is an as-built update and the submittals are in hand. Busway impedance is the manufacturer's value in every case, because no table generalizes across busway designs. {note}

+### The reactance of a conductor shall be modeled for the raceway material it is installed in, magnetic or non-magnetic, and the model shall carry the raceway material of each run.

+### Conductor lengths shall be taken from the routing on the contract documents for a study delivered before equipment release and from the installed routing for an as-built study or an as-built update, and the length used for each run shall be listed in the input data tables.

+### A conductor whose length is not known shall be modeled at the shortest length consistent with the equipment locations for the maximum case, and the assumption register shall record it.

+### Conductor impedance is what separates the fault current at a panelboard from the fault current at the switchboard that feeds it, and the length of the run matters more than the source of the impedance values. A short run or a run of parallel conductors in steel conduit modeled as if in aluminum conduit carries the fault current of the switchboard down to equipment rated for less, which is why the raceway material and the length are inputs the report has to show. {note}

+## Rotating Machine Contribution {toc}

+### Every induction motor rated at or above the horsepower indicated in the datasheet, every synchronous motor, and every generator shall be modeled individually, at the subtransient reactance and the X/R the manufacturer publishes for the machine furnished, and at the typical values of IEEE 551 for its type and size where none is published.

+```datasheet

+label: Individually Modeled Motor Threshold

+type: range

+unit: hp

+options:

+ min: 0

+ max: 250

+ setpoints: [0, 25, 50, 75, 100, 150, 200, 250]

+default: 50

+constraint: exact

+```

+### Fifty horsepower is the line IEEE 551 draws between motors modeled one by one and motors grouped at their bus, and the line most studies follow without discussion; a threshold of zero models every motor individually, which a small system or a project with a complete motor list supports and which changes no result at the buses that decide a rating; a threshold above 50 horsepower is the arrangement of a large plant with hundreds of motors, where the individual data below the threshold is effort without effect at the medium-voltage buses and the motors are grouped at each motor control center instead. {note}

+### Induction motors below the threshold shall be treated as indicated in the datasheet.

+```datasheet

+label: Motors Below the Threshold

+type: radio

+options:

+ - "Lumped at each bus from the connected motor horsepower"

+ - "Lumped at each bus as a share of the bus or transformer rating"

+ - "Not modeled"

+default: "Lumped at each bus from the connected motor horsepower"

+```

+### Lumping the small motors at each bus from their connected horsepower gives the contribution the real load produces and needs a motor list by bus; lumping them as a share of the transformer or bus rating, commonly half of the transformer kVA at a 480 V bus with mixed load, needs no list and is the practice where the load is not yet known or on an existing system with no load inventory; leaving them out of the model understates the first-cycle duty at the buses the small motors sit on and is accepted where the motor load is a negligible part of the bus load, as on a lighting and receptacle distribution. {note}

+### Lumped motors shall be modeled in the first-cycle network with the IEEE 551 multipliers for motors below 50 horsepower and shall be omitted from the interrupting and time-delayed networks.

+### Every generator shall be modeled with the subtransient reactance, the transient reactance, the X/R, and the decrement curve of the alternator furnished, as the generator standard requires the manufacturer to state them, and the study shall be re-run under Study Revisions and Re-Study Triggers where an alternator of a different frame is furnished after the study is accepted.

+### A motor supplied through a drive, a soft starter in bypass, or a direct connection shall be modeled as contributing, and a motor supplied through a drive with a diode or thyristor front end and no bypass shall be modeled as not contributing, with the drive type recorded in the input data tables.

+### A motor supplied through a drive with an active or regenerative front end shall be modeled with the fault current contribution and duration the drive manufacturer publishes.

+### A motor stays a source for the first few cycles after a fault because its rotor keeps turning and its field does not collapse at once. Its contribution lands on the bus it is connected to and on every bus between it and the fault, and at a motor control center the motors can add a third again to the current the transformer supplies, which is why the motors are modeled at the bus they are connected to and not as a single lump at the service. {note}

+## Inverter-Based Sources {toc}

+### Every photovoltaic inverter, energy storage inverter, and uninterruptible power supply within the study boundary shall be modeled as a source with the fault current contribution and the duration the manufacturer publishes for the unit furnished, and at 1.5 times its rated output current for the first-cycle network where the manufacturer publishes none.

+### An uninterruptible power supply shall be modeled in inverter operation with the current limit of its inverter and in bypass operation with the upstream source passed through, and the higher result at each downstream bus shall govern.

+### The data for each inverter-based source shall be taken from the submittals under [[sync/photovoltaic-systems]], [[sync/battery-energy-storage-systems]], or [[sync/uninterruptible-power-supply-systems]] for the equipment furnished.

+### An inverter is a current-limited source and adds a bounded amount to the fault current at the bus it feeds, usually between one and two times its rated current for a few cycles. On a bus fed by a large utility transformer the addition is small; on the load side of a UPS or on a microgrid bus islanded from the utility it is the whole fault current, and the equipment there is rated against a value that the inverter's current limit, not the system impedance, decides. {note}

+## Source Operating Cases {toc}

+### The study shall calculate every operating case indicated in the datasheet at every calculation point that case supplies, and the highest result at each point across the cases shall govern the maximum case.

+```datasheet

+label: Source Operating Cases

+type: checkbox

+options:

+ - "Utility source"

+ - "Each on-site generator or generator plant alone"

+ - "Utility and on-site generation paralleled"

+ - "Multiple services or transformers operated with ties closed"

+ - "UPS and energy storage sources in bypass and in inverter operation"

+default:

+ - "Utility source"

+ - "Each on-site generator or generator plant alone"

+constraint: all_of

+```

+### Equipment on the load side of a transfer switch sees a different fault current from each source, and the rating has to cover the higher one; a generator plant alone usually gives less than the utility at the service and can give more at a bus near the paralleling switchgear; the utility and the generation paralleled, during a closed transition or in extended parallel operation, gives the sum of both at every bus between them and is the case that decides the paralleling bus rating; a double-ended substation or a pair of services with the tie closed doubles the source behind every bus on the common side and is a case whether or not the operating procedure forbids it, because the tie can be closed; a UPS in bypass passes the upstream source through to loads that in inverter operation see only the inverter's current limit. A case left out is a configuration the system can take up in which no rating has been checked. {note}

+### Where a paralleled case is selected, the contribution of every set shall be summed at the paralleling bus with the reactances and decrement curves of the alternators furnished, and the result shall be delivered to the paralleling switchgear standard for the bus rating it carries.

+### Where a transfer switch is within the study boundary, the study shall report the available fault current at its terminals from each source separately, together with the upstream overcurrent device on each source, for the withstand and closing rating the transfer switch standard carries.

+## Existing Equipment {toc}

+### Where the study boundary includes existing equipment, the data for that equipment shall be taken from the basis indicated in the datasheet.

+```datasheet

+label: Existing Equipment Data Basis

+type: select

+options:

+ - "Field survey of every existing item within the study boundary"

+ - "Field survey of items whose nameplate data is not in the Owner's records"

+ - "Owner's record documents and prior studies without a field survey"

+ - "Not applicable, no existing equipment within the study boundary"

+```

+### A survey of every existing item records what is installed rather than what was drawn, finds the breaker replaced with a lower-rated frame and the transformer retapped since the last study, and costs a survey visit for every item, under the facility's safety program; a survey limited to the items whose nameplate data the Owner's records lack keeps the visit short and trusts the records for the rest, which is the arrangement on a facility with a maintained equipment inventory or a recent prior study; the Owner's records and prior studies alone cost nothing on site and carry forward every change made without a drawing revision, which on an older facility is most of them. On new construction with no existing equipment the question does not arise. {note}

+### Where a field survey is selected, it shall record for each item surveyed the nameplate ratings, the frame size, trip unit, and interrupting rating of each protective device, any series rating label, the transformer impedance, and the conductor size and raceway material where visible, and shall be performed as energized work under the facility's electrical safety program (see [[sync/electrical-safety-program]]).

+### Every discrepancy between the surveyed condition and the Owner's record documents shall be reported to the Engineer of Record and the Owner within five working days of the survey and recorded in the assumption register.

+### An existing device with no legible interrupting rating shall be reported to the Engineer of Record, who shall direct the rating the study is to assume for it, and the device shall be identified in the report as a device of assumed rating.

+### An existing system is routinely modified, re-fed, and re-tapped over its life without a revision to the drawings, and the fault current at an existing panel is decided by what feeds it today. The record drawings are the starting point of the survey, not a substitute for it. {note}

+## Assumption Register {toc}

+### The study shall maintain an assumption register listing every input value that was not taken from a nameplate, a certified test report, the utility's written statement, or a measured length, with the value used, its source, the component it applies to, and the case it affects.

+### Every entry in the assumption register shall be re-examined at the as-built reconciliation or update, and every entry replaced by a measured or nameplate value shall be closed with the value that replaced it.

+### Where the result at a calculation point is within 10 percent of the rating of the equipment at that point and an assumption in the register affects that result, the report shall state the sensitivity of the result to that assumption.

+### A short-circuit result is only as good as its weakest input, and the weakest inputs are the ones that were assumed. The register is what lets the Engineer of Record see which results rest on a typical value and which on a nameplate, and it is the list the as-built pass works down. {note}

+# Equipment Duty Evaluation {toc}

+## Evaluations Performed {toc}

+### The study shall compare the calculated duty at every calculation point with each rating indicated in the datasheet for the equipment at that point, and shall report the rating, the duty, the margin, and the result for each.

+```datasheet

+label: Equipment Duty Evaluations

+type: checkbox

+options:

+ - "Interrupting rating of every overcurrent protective device"

+ - "Close-and-latch rating of every medium-voltage circuit breaker"

+ - "Short-time withstand rating of every switchgear assembly and unfused device"

+ - "Bus bracing of every switchgear, switchboard, motor control center, panelboard, and busway"

+ - "Marked short-circuit current rating of every transfer switch, controller, drive, control panel, and surge protective device"

+ - "Conductor short-circuit withstand for the clearing time of the upstream device"

+ - "Series-rated combinations against their listing and the motor contribution limit"

+default:

+ - "Interrupting rating of every overcurrent protective device"

+ - "Close-and-latch rating of every medium-voltage circuit breaker"

+ - "Short-time withstand rating of every switchgear assembly and unfused device"

+ - "Bus bracing of every switchgear, switchboard, motor control center, panelboard, and busway"

+ - "Marked short-circuit current rating of every transfer switch, controller, drive, control panel, and surge protective device"

+ - "Conductor short-circuit withstand for the clearing time of the upstream device"

+ - "Series-rated combinations against their listing and the motor contribution limit"

+constraint: all_of

+```

+### Every rating in the list is one that NFPA 70 110.9 or 110.10 requires to be at least the available fault current, and each is tested on a different basis: an interrupting rating is a symmetrical current at a tested X/R, a close-and-latch rating is a peak or an asymmetrical current in the first cycle, a short-time withstand rating is a symmetrical current held for a stated time, a bus bracing rating is the first-cycle current the structure holds, a marked short-circuit current rating on a transfer switch, a controller, a drive, a control panel, or a surge protective device is a current the assembly was tested at with a specific class or size of overcurrent device ahead of it, and a conductor withstand is an energy that depends on the clearing time of the device ahead of the conductor. A project leaves an evaluation out where the equipment that carries that rating is not within the study boundary. {note}

+### Every interrupting rating shall be evaluated at the voltage the device is applied at, against the symmetrical current of the network its rating standard assigns, multiplied under Duty Networks where the calculated X/R exceeds the tested X/R.

+### Every close-and-latch rating and every bus bracing rating shall be evaluated against the first-cycle current, as the peak or the asymmetrical RMS value the rating is stated in.

+### Every short-time withstand rating shall be evaluated against the first-cycle symmetrical current for the rated duration of the assembly under IEEE C37.20.1 or IEEE C37.20.2, and the report shall state the rated duration beside the clearing time of the upstream device reported by the coordination study.

+### Every marked short-circuit current rating shall be evaluated together with the overcurrent device ahead of the equipment, and a rating that is conditional on a specific class, type, or maximum size of upstream device shall be reported as met only where the upstream device furnished is of that class, type, and size.

+### Every marked short-circuit current rating of a control panel shall be the rating marked under NFPA 70 409.110 and determined under [[sync/industrial-control-panels]], and the study shall furnish the available fault current at the panel for the record NFPA 70 409.22 requires.

+### Every conductor within the study boundary shall be evaluated for short-circuit withstand by the method of ICEA P-32-382 at the clearing time of the upstream device at the calculated current, taken from the coordination study where it is available and from the maximum clearing time of the device where it is not, and the result shall be re-evaluated when the coordination study is accepted.

+### Every rating evaluated shall be the rating of the equipment furnished or installed, taken from the submittal or the nameplate, and a catalog rating of a device the equipment is listed to accept shall not be used in place of the rating of the device furnished.

+## Series-Rated Combinations {toc}

+### Where an equipment standard selects a series rating basis for an assembly within the study boundary, the study shall identify each series-rated combination, the upstream device each depends on, the combination rating, and the listing under UL 489, UL 891, or UL 67 that covers it.

+### The study shall verify that the available fault current at the line terminals of the upstream device of each combination does not exceed the combination rating, and that the upstream device is fully rated for the current at its own terminals.

+### The study shall verify for each combination that the sum of the full-load currents of the motors connected between the upstream device and the downstream device does not exceed 1 percent of the downstream device's interrupting rating, as NFPA 70 240.86(C) requires, and shall report any combination that exceeds it as a deficiency.

+### Where a series rating is applied to an existing installation under engineering supervision in accordance with NFPA 70 240.86(A), the selection shall be made and documented by the study engineer as the licensed Professional Engineer responsible for it, and the documentation shall be included in the report.

+### A series-rated combination proposed as the remediation of a deficiency shall be referred to the coordination study under [[sync/protective-coordination-study]] before it is accepted, because the upstream device of a series combination opens with the downstream device on a high-magnitude fault.

+### A series rating is a rating of a pair, not of a device. It holds only while the upstream device named in the listing is in place and while the motor contribution between the two devices stays below the limit, which is why the study records the pair and the listing rather than the downstream device's own rating, and why a replacement of either device is a re-study trigger. {note}

+## Deficiencies {toc}

+### Every device, assembly, or conductor whose rating is less than the calculated duty at its location shall be listed in the report as a deficiency, with its designation, its location, the case and network that produced the duty, the calculated duty, the rating, and the remediation options available for it.

+### The study shall consider the following remediation options for each deficiency, as each applies:

+- a device or assembly of a higher rating in the same position

+- a listed series-rated combination with the upstream device, where the equipment standard's rating basis admits one

+- a current-limiting fuse or current-limiting circuit breaker ahead of the deficient item, with the let-through current the manufacturer publishes at the calculated duty

+- added impedance ahead of the deficient item, as a current-limiting reactor or a transformer of higher impedance

+- an operating restriction that removes the case producing the duty, where the case is a tie closed or sources paralleled and the Owner accepts the restriction in writing

+### Where the study is delivered before equipment release, no item listed as a deficiency shall be released for manufacture until the Engineer of Record has directed its remediation in writing.

+### Where a deficiency is found in equipment already installed under the project, the item shall not be energized, and where it is energized, shall be removed from service, until it is remediated or the Engineer of Record directs otherwise in writing.

+### Where a deficiency is found in existing equipment that is to remain, the study engineer shall report it to the Owner and the Engineer of Record within five working days of the finding, and the Owner shall direct its remediation.

+### A deficiency in equipment furnished under the project against a rating the contract documents state shall be remediated by the Contractor at no change in the contract sum, and a deficiency that arises from a utility value, an existing condition, or a design basis that differs from what the contract documents state shall be remediated as the Engineer of Record directs under the change provisions of the contract.

+### A device applied above its interrupting rating does not fail to open; it fails to stop opening, with the arc sustained across contacts that cannot clear it. That is the failure NFPA 70 110.9 exists to prevent, and it is the reason a deficiency is a hold on energization rather than a finding for the record. {note}

+# Study Report {toc}

+## Report Content {toc}

+### The study report shall contain the following, in a single sealed document with numbered pages:

+- a cover page with the project, the system studied, the study engineer, the date of issue, the revision history, and the seal and signature required under Study Engineer

+- an executive summary stating the study boundary, the method, the cases, the highest available fault current in the system and where it occurs, and every deficiency found

+- a system description and the one-line diagram described under One-Line Diagram

+- the method statement, giving the calculation method, the duty networks, the fault types, the cases, the pre-fault voltage, the software and its version and library revision, and the hand verification required under Calculation Tool

+- the input data tables for the utility source, every transformer, every conductor and busway run, every motor and motor group, every generator, every inverter-based source, and every protective device, each with its source

+- the utility's written request and response

+- the assumption register

+- the results tables, giving at every calculation point, for every case, every network, and every fault type calculated, the symmetrical RMS current, the X/R, and the asymmetrical or peak value the rating at that point is evaluated in

+- the equipment duty evaluation tables required under Evaluations Performed, with the series-rated combinations identified

+- the deficiency list and the remediation options for each

+- the available fault current and date for each item of service equipment, as described under Available Fault Current Record

+- the re-study trigger list

+- appendices holding the manufacturer data sheets, test reports, and published tables the model is built from

+### The results tables shall be organized by calculation point in the order of the one-line diagram, so that the reader can follow the fault current from the service to any point in the system without searching the report.

+## One-Line Diagram {toc}

+### The report shall include a one-line diagram of the modeled system showing every source, bus, transformer, conductor and busway run, protective device, and calculation point, with the designation of each as the contract documents give it, the kVA and impedance of every transformer, the size, count, material, length, and raceway material of every conductor run, and the maximum three-phase and line-to-ground fault current and X/R at every calculation point.

+### The one-line diagram shall be reconciled with the contract one-line diagram, and every difference between the two shall be listed in the report and resolved with the Engineer of Record before the study is accepted.

+### Where the system is too large for one sheet, the diagram shall be divided across sheets with cross-references at every cut, and no bus shall appear on two sheets with two different values.

+### The one-line diagram shall be delivered in the format indicated under Report Format alongside the report, with the same identifiers as the results tables and the native model.

+## Available Fault Current Record {toc}

+### The report shall state, for each item of service equipment within the study boundary other than the service equipment of a dwelling unit, the maximum available fault current at its line terminals and the date of the calculation, as the data for the field marking NFPA 70 110.24(A) requires.

+### The value stated for the field marking shall be the maximum-case three-phase symmetrical current calculated under this standard at the line terminals of the service equipment, with the utility's value and the growth margin shown separately in the record.

+### The field marking itself, its material, its format, and its placement on the service equipment, shall be as the project's identification standard requires (see [[sync/electrical-identification]]), and the Contractor shall apply the value and the date this study states and no other.

+### The calculation behind the marking shall be kept available to those who design, install, inspect, maintain, or operate the system, as NFPA 70 110.24(A) requires, and the closeout submittal of the record and the native model is how this standard meets that requirement.

+### Where a modification to the system changes the maximum available fault current at an item of service equipment, the marking shall be updated under NFPA 70 110.24(B), and the modification is a re-study trigger under this standard.

+### The marking at the service is the one place the available fault current is readable by the inspector, the installer of the next piece of equipment, and the electrician replacing a breaker, and a value marked from a utility letter rather than from the study omits the growth margin and the date the study was built on. The study is the source of the number; the identification standard is the source of the label. {note}

+## Report Format {toc}

+### The report shall be delivered in the formats indicated in the datasheet.

+```datasheet

+label: Report Format

+type: checkbox

+options:

+ - "Sealed report in searchable PDF"

+ - "Native model files of the analysis software"

+ - "Editable one-line diagram in CAD format"

+ - "Bound printed copies"

+default:

+ - "Sealed report in searchable PDF"

+ - "Native model files of the analysis software"

+constraint: all_of

+```

+### The sealed PDF is the record, the native model file is what every later study opens, the editable one-line is what the Owner's drafting staff fold into the record drawings, and the printed copy is what sits in the electrical room for the inspector; a project adds the last two where its record-drawing and closeout practice call for them. {note}

+### Where the native model files are selected, they shall be delivered under the condition stated under Closeout Submittals, and the report shall name the software version that opens them.

+## Handoff to the Coordination and Arc-Flash Studies {toc}

+### The accepted model, with its input data, assumption register, and results, shall be furnished to the engineer performing the coordination study under [[sync/protective-coordination-study]] and to the engineer performing the arc-flash study under [[sync/arc-flash-study]], and both studies shall be built on that model or on a model verified against its one-line, input data, and results.

+### The results delivered to the coordination study shall include, at every calculation point, the maximum and minimum three-phase and line-to-ground fault currents and the X/R of each, in every operating case calculated.

+### The results delivered to the arc-flash study shall include, at every calculation point, the maximum and minimum three-phase bolted fault current in every operating case calculated, together with the source configuration that produced each.

+### Where the study is revised after the coordination study or the arc-flash study has been performed on it, the study engineer shall issue the revision to the engineers performing those studies on the day it is issued, and the Contractor shall not energize equipment whose settings or labels rest on the superseded results until both studies have been re-run.

+### Three studies on three models are three answers to one question, and the one that reaches the label on the door is whichever ran last. One model, handed forward with its assumptions, is how the fault current the arc-flash label is calculated from stays the fault current the breaker was ordered to. {note}

+# Study Revisions and Re-Study Triggers {toc}

+## Study Revisions Within the Project {toc}

+### The study engineer shall correct every calculation error and reporting error in the study that is identified before the end of the project's correction period, and shall re-issue the affected pages and the native model, at no cost to the Owner.

+### The study shall be revised whenever, after it is accepted and before the equipment within the study boundary is energized, the equipment furnished departs from the equipment modeled in a transformer impedance, a generator alternator, a protective device frame or type, a series-rated combination, or a conductor size, count, or raceway material, and the revision shall re-run every case the change affects.

+### A revision caused by a substitution or a change in the equipment furnished by the Contractor shall be made at the Contractor's cost, and a revision caused by a change in the utility's value, an existing condition, or the design shall be made as the Engineer of Record directs under the change provisions of the contract.

+### Every revision shall carry a revision number and date on the cover, a list of the results that changed, and a statement of whether the coordination study and the arc-flash study need to be re-run on it.

+## Re-Study Triggers {toc}

+### The report shall include a re-study trigger list, written for the Owner's operations staff, naming the events after which the Owner is to commission an updated short-circuit study before the affected portion of the system is returned to normal operation.

+### The re-study trigger list shall name at least the following events:

+- notice from the utility of a change in the available fault current or the X/R at the point of delivery, or a change in the utility's service transformer

+- the addition, removal, or re-rating of a generator, an inverter-based source, or a UPS, or a change in the alternator of a generator

+- the replacement or re-tapping of a transformer, or a change in its impedance

+- a change in the size, count, length, or raceway material of a service or feeder conductor

+- the addition of a motor at or above the threshold indicated under Rotating Machine Contribution, or of a motor group that changes the lumped load at a bus

+- the replacement of a protective device with one of a different make, frame, or type, where the device is part of a series-rated combination or protects a conductor whose withstand was evaluated at its clearing time

+- a change in the system configuration that creates a new operating case, such as a new tie, a closed-transition transfer, or a parallel connection to the utility

+- a facility expansion or a modification that changes the maximum available fault current at an item of service equipment, which NFPA 70 110.24(B) requires to be re-marked

+- the five-year review of the arc-flash risk assessment under NFPA 70E 130.5(G), which rests on this study's results

+### Where any trigger event occurs, the updated short-circuit study shall be followed by an updated coordination study under [[sync/protective-coordination-study]] and an updated arc-flash study under [[sync/arc-flash-study]] for the affected portion of the system.

+### The re-study trigger list shall be included in the operation and maintenance documentation of the electrical system, and the Owner's change procedures for the electrical system shall name the responsible engineer to be notified when a trigger event occurs.

+### The fault current at a bus is a property of everything upstream of it, and most of the events that change it are ordinary maintenance: a transformer swapped for the one in stock, a breaker replaced with another make, a feeder re-pulled on a different route. None of them look like a reason to commission a study, which is why the list is written for the operations staff in the words of the work they do rather than in the terms of the calculation. {note}

View current revision