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SynC · Editorial revision

Medium-Voltage Distribution Transformers

Revision2
EditedSep 14, 2026
StatusCurrent
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Current revision. This is editorial revision 2, the current text of this standard. Read it on the standard's page.

SynC Standards Team — Specifier (Platform Standards) ✓ OfficialDiscloses employment SynC (SynC Platform Team)
Remake for template neutrality (from-scratch field inventory, five-state defaults, parameters and derived fields, schedule deferrals)

1 Scope

NOTE This standard governs three-phase distribution transformers with a primary winding rated above 600 V through 34.5 kV and a self-cooled rating of 10 MVA and smaller, purchased and installed by the Owner's contractor to step a medium-voltage supply down to the voltage the downstream distribution equipment uses. (1.1)
NOTE Two decisions fix the product family, and they are made independently of each other: the insulating medium (liquid-immersed, or dry-type with vacuum-pressure-impregnated, vacuum-pressure-encapsulated, or cast-resin windings) and the enclosure construction (pad-mounted compartmental, or substation type with bushings or terminal chambers on the tank or enclosure). All four combinations are manufactured, and this standard states requirements per medium and per construction rather than assuming that a liquid-immersed unit is pad-mounted or that a dry-type unit sits in an electrical room. (1.2)
NOTE A transformer ties together the upstream supply, the downstream distribution gear, the pad, vault, or room it occupies, and the grounding system its secondary is bonded into, so most of the coordination in this standard runs outward to the standards that govern those interfaces. (1.3)
NOTE The following are governed elsewhere and are outside this standard: (1.4)
  • transformers rated 600 V and below on both windings (Low Voltage Dry Type TransformersLow-Voltage Dry-Type TransformersResolves to the current adopted revision.sync/low-voltage-dry-type-transformers)
  • unit substations in which the transformer, the primary switch, and the secondary switchgear are one close-coupled assembly (Secondary Unit SubstationsSecondary Unit SubstationsResolves to the current adopted revision.sync/secondary-unit-substations)
  • the medium-voltage switchgear, primary switch, or service equipment upstream of the transformer (Medium Voltage SwitchgearMedium Voltage SwitchgearResolves to the current adopted revision.sync/medium-voltage-switchgear, Electrical Service EntranceElectrical Service EntranceResolves to the current adopted revision.sync/electrical-service-entrance)
  • the switchboards and panelboards downstream of the transformer (Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards, PanelboardsPanelboardsResolves to the current adopted revision.sync/panelboards)
  • the primary and secondary cables and their raceways (Medium Voltage CablesMedium-Voltage CablesResolves to the current adopted revision.sync/medium-voltage-cables, Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables, Raceways And ConduitRaceways and ConduitResolves to the current adopted revision.sync/raceways-and-conduit)
  • the concrete pad as a structure (Concrete PadsConcrete Equipment PadsResolves to the current adopted revision.sync/concrete-pads)
  • the grounding electrode system the transformer is bonded to (Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding)
  • single-phase pad-mounted units, power transformers larger than 10 MVA, regulating transformers, and utility-owned transformers
1.5 The transformer location shall be as indicated on the electrical plans.
1.6 The Contractor shall coordinate the transformer's primary and secondary terminations, its pad or room, and its grounding with the standards named in this section before the transformer is released for manufacture.
1.7 The transformer secondary is a separately derived system unless it constitutes the service, and its system bonding jumper, grounding electrode conductor, and grounding electrode connection shall be installed in accordance with NEC 250.30 or NEC 250.24 as applicable and with Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding.

2 Referenced Standards

2.1 Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2 Where the contract documents, the serving utility's requirements, the Authority Having Jurisdiction, 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 (Article 450, Transformers and Transformer Vaults)
NFPA 70E Standard for Electrical Safety in the Workplace
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
IEEE C57.12.28 Pad-Mounted Equipment, Enclosure Integrity
IEEE C57.12.29 Pad-Mounted Equipment, Enclosure Integrity for Coastal Environments
IEEE C57.12.34 Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers, 10 MVA and Smaller
IEEE C57.12.36 Requirements for Liquid-Immersed Distribution Substation Transformers
IEEE C57.12.51 Ventilated Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase, with High-Voltage 601 to 34 500 Volts
IEEE C57.12.70 Terminal Markings and Connections for Distribution and Power Transformers
IEEE C57.12.80 Terminology for Power and Distribution Transformers
IEEE C57.12.90 Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEEE C57.12.91 Test Code for Dry-Type Distribution and Power Transformers
IEEE C57.91 Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators
IEEE C57.96 Guide for Loading Dry-Type Distribution and Power Transformers
IEEE C57.104 Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers
IEEE C57.106 Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment
IEEE C57.110 Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents
IEEE C57.124 Recommended Practice for the Detection of Partial Discharge and the Measurement of Apparent Charge in Dry-Type Transformers
IEEE C57.147 Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers
IEEE C62.11 Metal-Oxide Surge Arresters for AC Power Circuits
IEEE C62.22 Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems
IEEE 386 Separable Insulated Connector Systems for Power Distribution Systems Rated 2.5 kV through 35 kV
IEEE C37.47 High-Voltage Distribution Class Current-Limiting Type Fuses and Fuse Disconnecting Switches
UL 1562 Transformers, Distribution, Dry-Type, Over 600 Volts
ASTM D3487 Mineral Insulating Oil Used in Electrical Apparatus
ASTM D6871 Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus
ASTM D4652 Silicone Fluid Used for Electrical Insulation
ASTM D5222 High Fire-Point Mineral Electrical Insulating Oils
ASTM D877 Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes
ASTM D1816 Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes
ASTM D1533 Water in Insulating Liquids by Coulometric Karl Fischer Titration
ASTM D3612 Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography
10 CFR Part 431 Subpart K Energy Conservation Standards for Distribution Transformers
40 CFR Part 112 Oil Pollution Prevention (Spill Prevention, Control, and Countermeasure)
NEMA TR 1 Transformers, Step Voltage Regulators, and Reactors (audible sound levels)
NEMA ST 20 Dry-Type Transformers for General Applications
NEMA 260 Safety Labels for Padmounted Switchgear and Transformers Sited in Public Areas
ANSI Z535.4 Product Safety Signs and Labels
NETA ATS Acceptance Testing Specifications for Electrical Power Equipment and Systems
ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (Chapter 13)

3 Submittals

3.1 Action Submittals for Every Transformer

3.1.1 The Contractor shall submit the following for review before any transformer is released for manufacture:
  • shop drawings giving overall dimensions, weight with and without liquid, center of gravity, lifting and jacking provisions, anchor bolt pattern, the location and size of every primary and secondary termination, and the clearances the manufacturer requires around the unit
  • nameplate data giving kVA rating at each cooling stage, primary and secondary voltages, winding connection and vector group, tap voltages, BIL of each winding, percent impedance at the rated tap, insulation system temperature class or liquid type, average winding rise, frequency, and sound level
  • the certified no-load loss, the load loss at rated load, the load loss at 50 percent of rated load, and the efficiency at 50 percent of rated load, together with the manufacturer's written certification of compliance with 10 CFR Part 431 for the unit category
  • the impedance the unit is designed to, stated at the rated tap, for use in the project's short-circuit and coordination studies
  • surge arrester data giving the arrester class, duty-cycle rating, maximum continuous operating voltage, and protective characteristics, where arresters are furnished
  • the seismic certification for the transformer as an active component, where the datasheet requires one
  • the finish system data and the color chip for the exterior finish
Action Submittals for Every Transformercheckbox
☑ Shop drawings with dimensions, weights, terminations, and clearances
☑ Nameplate data
☑ Certified losses and efficiency with 10 CFR Part 431 compliance certification
☑ Design impedance at the rated tap
☐ Surge arrester data
☐ Seismic certification
☐ Finish system data and color chip
3.1.2 A transformer shall not be released for manufacture until the action submittals covering it have been reviewed and returned.

3.2 Additional Action Submittals for Liquid-Immersed Units

3.2.1 Where the datasheet selects a liquid-immersed unit, the Contractor shall in addition submit the following:
  • the insulating liquid designation, its product standard, its safety data sheet, and for a less-flammable liquid the listing that establishes its fire point
  • the fuse arrangement, the fuse element ratings, and the time-current characteristic curves of every internal fuse, for use in the coordination study
  • the primary switch arrangement, its continuous and loadbreak ratings, and its operating sequence
  • the liquid preservation system description and the pressure-relief device rating
  • the tank leak test and vacuum withstand certification
Additional Action Submittals for Liquid-Immersed Unitscheckbox
☑ Insulating liquid designation, product standard, safety data sheet, and fire-point listing
☑ Internal fuse arrangement, ratings, and time-current curves
☑ Primary switch arrangement and ratings
☐ Liquid preservation system and pressure-relief device data
☐ Tank leak test and vacuum withstand certification

3.3 Additional Action Submittals for Pad-Mounted Units

3.3.1 Where the datasheet selects pad-mounted compartmental construction, the Contractor shall in addition submit the following:
  • the compartment arrangement drawing showing the primary and secondary compartments, the door and sill arrangement, the bushing positions, the parking stands, and the fuse and switch handle positions
  • the pad drawing showing the cable opening, the conduit stub-up window, the ground connection points, the anchor locations, and the outline the pad must clear
  • the enclosure integrity certification to IEEE C57.12.28 or IEEE C57.12.29 as selected
Additional Action Submittals for Pad-Mounted Unitscheckbox
☑ Compartment arrangement drawing
☑ Pad drawing with cable opening, conduit window, and ground points
☑ Enclosure integrity certification

3.4 Informational Submittals

3.4.1 The Contractor shall submit the following with or before the action submittals:
  • the manufacturer's qualification statement required under Quality Assurance
  • the factory test procedure, where witnessed testing is selected in the datasheet
  • the field acceptance testing firm's qualification statement and the proposed field test procedure
  • the loss evaluation calculation, where the datasheet selects a loss-evaluated design
  • the seismic anchorage calculation and anchor detail for the pad, vault, or room installation
Informational Submittalscheckbox
☑ Manufacturer's qualification statement
☐ Factory test procedure
☑ Field testing firm qualification and test procedure
☐ Loss evaluation calculation
☑ Seismic anchorage calculation and detail

3.5 Closeout Submittals

3.5.1 The Contractor shall submit the following before the transformer is accepted:
  • the certified factory test report for each transformer, identified by serial number, giving every measured value
  • the field acceptance test report for each transformer, giving every measured value, the acceptance criterion applied, and the tester's determination
  • for a liquid-immersed unit, the liquid quality test report on the sample drawn at the site and the baseline dissolved-gas analysis report
  • the infrared scan report from the initial in-service inspection
  • operation and maintenance data giving the recommended inspection intervals, the torque values for every accessible connection, the tap changer operating procedure, and, for a liquid-immersed unit, the liquid sampling and testing program
  • the as-built tap position and the as-built winding connection for each transformer
  • the warranty documentation for the transformer and for the installation
  • a signed receipt from the Owner for the spare parts delivered
Closeout Submittalscheckbox
☑ Certified factory test reports
☑ Field acceptance test reports
☐ Site liquid quality and baseline dissolved-gas analysis reports
☑ Infrared scan report
☑ Operation and maintenance data
☑ As-built tap position and winding connection
☑ Warranty documentation
☐ Spare parts receipt

4 Quality Assurance

4.1 Manufacturer Qualifications

4.1.1 The transformer shall be produced by a manufacturer that has manufactured transformers of the same medium, construction, voltage class, and rating range for not less than the period indicated in the datasheet.
Minimum Manufacturer Production Experiencerange
years
351020
4.1.2 The manufacturer shall operate a quality management system certified to ISO 9001 or shall demonstrate an equivalent documented system to the Engineer of Record's satisfaction.

4.2 Listing and Certification

4.2.1 A dry-type transformer shall be listed and labeled to UL 1562 by a Nationally Recognized Testing Laboratory.
4.2.2 A liquid-immersed transformer shall be designed, manufactured, and tested to IEEE C57.12.00 and to IEEE C57.12.34 or IEEE C57.12.36 as applicable to the construction selected, and shall bear the manufacturer's certification of compliance.
4.2.3 A less-flammable insulating liquid shall be listed by a Nationally Recognized Testing Laboratory as a less-flammable liquid, and the transformer shall be listed or labeled for use with that liquid where NEC 450.23 requires it.

4.3 Energy Conservation Compliance

4.3.1 Every transformer within the scope of 10 CFR Part 431 Subpart K shall meet the efficiency required by that regulation for its category, kVA rating, and BIL class, established at 50 percent of nameplate load by the test method the regulation references.
4.3.2 The efficiency basis for the transformer shall be as indicated in the datasheet.
Efficiency Basisselect
Federal minimum efficiency for the unit category per 10 CFR Part 431
Efficiency above the federal minimum as stated in the submittal
Loss-evaluated design using the Owner's no-load and load loss valuation
4.3.3 Where the datasheet selects a loss-evaluated design, the no-load and load loss valuation factors shall be as indicated in the contract documents.
NOTE The federal efficiency levels are set per kVA rating and per category, and a liquid-immersed unit and a dry-type unit of the same rating carry different minimum efficiencies, so the certification identifies the category the unit was evaluated under. (4.3.4)
NOTE A loss evaluation prices each watt of no-load loss and each watt of load loss over the transformer's service life and selects the design with the lowest total of first cost plus capitalized losses. Where the unit runs continuously at moderate load factor, the evaluation usually favors a lower-loss design than the federal minimum; where the unit sees light or intermittent load, the capitalized losses are small and the federal-minimum design is usually the result. (4.3.5)

4.4 Field Testing Firm Qualifications

4.4.1 Field acceptance testing shall be performed by an independent testing firm that is not the manufacturer, the installing contractor, or a supplier of the equipment, and that is accredited to perform the tests in NETA ATS.
4.4.2 The field testing firm shall be qualified in accordance with Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing.

5 Environmental and Service Conditions

5.1 Usual Service Conditions

5.1.1 The transformer shall be rated for continuous operation under the usual service conditions of IEEE C57.12.00 or IEEE C57.12.01 as applicable to its medium, and every departure from those conditions at the installation shall be stated on the order.
5.1.2 Where Site Ambient Temperature MaximumSite Ambient Temperature MaximumParameterEach project supplies its own value.site-ambient-temperature-maximum exceeds 40°C, or where the 24-hour average ambient at the site exceeds 30°C, an outdoor transformer shall be rated for the actual site ambient in accordance with the applicable general requirements standard.
5.1.3 The design ambient temperature for an indoor transformer shall be as indicated in the datasheet.
Indoor Design Ambient Temperaturerange
°C
30404550
NOTE The rating basis of the general requirements standards is a 40°C maximum ambient with a 30°C 24-hour average, so a 40°C indoor design ambient is the value at which the nameplate rating applies without correction; a higher value buys margin for a room whose ventilation cannot hold the standard ambient at design load, at the cost of a larger unit for the same nameplate. (5.1.4)
5.1.5 The transformer shall be de-rated for AltitudeAltitudeParameterEach project supplies its own value.altitude in accordance with the altitude correction factors of the applicable general requirements standard, for both dielectric strength and temperature rise.
NOTE Air thins with altitude, which reduces both the dielectric strength of the air clearances and the convective cooling of the surfaces the losses leave through, so a unit rated at sea level carries less kVA and less insulation margin at elevation than its nameplate states. (5.1.6)

5.2 Load Character

5.2.1 The transformer's capability for nonsinusoidal load shall be as indicated in the datasheet.
K-Factor Ratingselect
K-1
K-4
K-13
K-20
K-30
NOTE Harmonic currents raise the eddy-current and stray losses in the windings and structural parts faster than they raise the fundamental load, so a unit carrying a rich harmonic spectrum at its full nameplate kVA runs hotter than its rise rating. A K-factor rated unit is built to carry the stated spectrum at nameplate without that penalty; a K-1 unit applied to the same load must be de-rated in accordance with IEEE C57.110. (5.2.2)
5.2.3 Where the datasheet selects a K-factor rating above K-1, the manufacturer shall state in the submittal the winding, shielding, and neutral provisions by which the rating is achieved.

5.3 Loading Beyond Nameplate

5.3.1 The transformer shall be capable of the short-time and cyclic overloads permitted by IEEE C57.91 or IEEE C57.96 as applicable to its medium, without loss of life beyond that the loading guide associates with the loading cycle.
NOTE The loading guides trade insulation life for capacity on a known curve, so an Owner who intends to operate above nameplate for part of each day is making a life-expectancy decision and not merely a capacity one. (5.3.2)

6 Medium and Construction

6.1 Insulating Medium

6.1.1 The insulating medium and winding encapsulation shall be as indicated in the datasheet.
Insulating Medium and Winding Encapsulationradio
○ Liquid-immersed
○ Dry-type, open-wound, vacuum-pressure-impregnated
○ Dry-type, vacuum-pressure-encapsulated
○ Dry-type, cast-resin
NOTE A liquid-immersed unit puts the core and coils in a sealed tank of dielectric liquid that both insulates and carries heat to the tank wall, which yields the smallest and lowest-loss unit for a given rating and a sealed assembly that tolerates outdoor exposure. The liquid brings a fire-point and spill-containment obligation that the rest of this standard addresses per liquid type. (6.1.2)
NOTE A dry-type unit has no liquid to contain or to burn, which is what recommends it where the transformer sits inside an occupied building without a vault, on an upper floor, or where a spill would reach a waterway. It is larger, heavier, and higher in loss than a liquid-immersed unit of the same rating, and it depends on the room's ventilation to carry away the heat a liquid unit sheds through its tank. (6.1.3)
NOTE Vacuum-pressure-impregnated windings are open-wound coils sealed with a varnish under vacuum and pressure; vacuum-pressure-encapsulated windings carry a thicker resin coating that seals the coil surface against moisture and contaminants; cast-resin windings are cast solid in epoxy under vacuum, which yields a winding that is impervious to moisture, tolerant of a contaminated atmosphere, and the strongest of the three against short-circuit forces, at the highest first cost. (6.1.4)
NOTE Where the transformer will be energized after a period de-energized in an unconditioned space, or where the atmosphere carries conductive dust, salt, or chemical contamination, a sealed or cast winding does not need the dry-out that an open-wound winding needs before it can be energized safely. (6.1.5)

6.2 Enclosure Construction

6.2.1 The enclosure construction shall be as indicated in the datasheet.
Enclosure Constructionradio
○ Pad-mounted compartmental
○ Substation type
NOTE Pad-mounted compartmental construction encloses the primary and secondary terminations in tamper-resistant compartments that form part of the transformer, so the unit can stand unattended on a pad accessible to the public with no fence and no separate enclosure, and its cable connections are made inside the compartments. (6.2.2)
NOTE Substation-type construction brings the terminations out through bushings on the tank or enclosure, through air-filled terminal chambers, or through a throat to adjacent equipment, and depends on a fenced yard, a vault, or an electrical room for the protection the pad-mounted cabinet provides on its own. (6.2.3)
6.2.4 A pad-mounted liquid-immersed unit shall comply with IEEE C57.12.34, and a substation-type liquid-immersed unit shall comply with IEEE C57.12.36.
6.2.5 A pad-mounted dry-type unit shall be furnished in a tamper-resistant compartmental enclosure that provides the same restriction of access to energized parts as IEEE C57.12.34 requires of a liquid-immersed unit.
6.2.6 A substation-type dry-type unit 501 kVA and larger shall comply with IEEE C57.12.51.

7 Ratings

7.1 Rated Capacity

7.1.1 The self-cooled kVA rating shall be as indicated in the datasheet.
Self-Cooled kVA Ratingrange
kVA
4575112.51502253005007501000150020002500300037505000750010000
Per drawings — the one-line diagram (deferred by default)
NOTE The setpoints follow the preferred kVA ratings of IEEE C57.12.00 and IEEE C57.12.01 within the scope of this standard; a manufacturer can offer an intermediate rating, and the one-line diagram governs the rating for each unit. (7.1.2)
7.1.3 The cooling stages for a liquid-immersed unit shall be as indicated in the datasheet.
Liquid-Immersed Cooling Stagesselect
Self-cooled only
Self-cooled with one forced-air stage
Self-cooled with provisions for a future forced-air stage
7.1.4 The cooling stages for a dry-type unit shall be as indicated in the datasheet.
Dry-Type Cooling Stagesselect
Self-cooled only
Self-cooled with one forced-air stage
Self-cooled with provisions for a future forced-air stage
7.1.5 Where a forced-air stage is selected, the forced-air rating shall be the manufacturer's published forced-air rating for the design and shall be not less than 115 percent of the self-cooled rating, and the fans, their controls, their power supply, and the winding temperature device that starts them shall be furnished with the transformer.
NOTE A forced-air stage adds capacity without adding core, so it is the least expensive kVA a transformer can carry, but the fans are the least reliable component on the unit and the added capacity is unavailable whenever they are. Where the forced-air rating is needed to carry the normal load rather than a contingency, the unit is undersized for its duty. (7.1.6)
7.1.7 The rated frequency shall be as indicated in the datasheet.
Rated Frequencyradio
● 60 Hz
○ 50 Hz

7.2 Voltage Ratings

7.2.1 The primary voltage rating shall be as indicated in the datasheet.
Primary Voltage Ratingrange
kV
2.44.164.86.97.21212.4713.213.814.420.822.92324.934.5
Per drawings — the one-line diagram (deferred by default)
7.2.2 The secondary voltage rating shall be as indicated in the datasheet.
Secondary Voltage Ratingselect
208Y/120 V
240 V delta
240/120 V delta, four-wire
480 V delta
480Y/277 V
600 V delta
600Y/347 V
2.4 kV delta
4.16 kV delta
4.16Y/2.4 kV
Per drawings — the one-line diagram (deferred by default)
7.2.3 The Contractor shall confirm the serving utility's nominal primary voltage, its system grounding, and the available fault current at the service point with the utility before the transformer is released for manufacture, and shall report any difference from the one-line diagram to the Engineer of Record.
NOTE The primary rating must match the system it connects to, not the nominal class name: a 12.47 kV grounded-wye system and a 13.8 kV delta system are both 15 kV class, and a unit ordered for one does not deliver rated secondary voltage on the other. (7.2.4)

7.3 Winding Connection and Vector Group

7.3.1 The winding connection and vector group shall be as indicated in the datasheet.
Winding Connection and Vector Groupselect
Delta primary, grounded-wye secondary, Dyn1
Delta primary, grounded-wye secondary, Dyn11
Grounded-wye primary, grounded-wye secondary, YNyn0
Ungrounded-wye primary, grounded-wye secondary, Yyn0
Delta primary, delta secondary, Dd0
Grounded-wye primary, delta secondary, YNd1
Ungrounded-wye primary, delta secondary, Yd1
NOTE A delta primary with a grounded-wye secondary isolates the secondary neutral from the primary system, provides a path for triplen harmonic currents to circulate in the delta rather than flow into the primary, and yields a four-wire secondary for line-to-neutral loads; the standard North American phase relationship is the secondary lagging the primary by 30 degrees, designated Dyn1. (7.3.2)
NOTE A grounded-wye primary with a grounded-wye secondary is common on utility loop-fed underground systems because it does not produce the overvoltages from ferroresonance that a delta primary can produce during single-phase switching of a lightly loaded cable-fed unit; it passes primary zero-sequence voltage unbalance through to the secondary, and on a three-legged core it drives tank heating under unbalanced load unless the core is built to carry the resulting flux. (7.3.3)
NOTE A grounded-wye primary with a delta secondary acts as a grounding source on the primary system and contributes to primary ground-fault current, which many utilities do not permit on a customer transformer connected to their distribution system. (7.3.4)
7.3.5 Where the datasheet selects a grounded-wye primary and grounded-wye secondary connection, the core shall be of five-legged, triplex, or shell-form design so that the zero-sequence flux under unbalanced load has a return path other than the tank.
7.3.6 Where the datasheet selects a grounded-wye primary, the Contractor shall obtain the serving utility's written acceptance of the primary connection before the transformer is released for manufacture.
7.3.7 The vector group of a transformer that will be paralleled with another transformer shall match the vector group of that transformer, and the two shall not be paralleled where the vector groups differ.
7.3.8 Terminal markings and connections shall comply with IEEE C57.12.70.

7.4 Basic Impulse Insulation Level

7.4.1 The primary winding BIL shall be as indicated in the datasheet.
Primary Winding BILrange
kV
3045607595110125150200
Derived — the primary voltage rating, the insulating medium, and AltitudeAltitudeParameterEach project supplies its own value.altitude, per the insulation level tables of IEEE C57.12.00 for a liquid-immersed unit or IEEE C57.12.01 for a dry-type unit (by default)
7.4.2 The secondary winding BIL shall be as indicated in the datasheet.
Secondary Winding BILrange
kV
102030456075
Derived — the secondary voltage rating, the insulating medium, and AltitudeAltitudeParameterEach project supplies its own value.altitude, per the insulation level tables of IEEE C57.12.00 for a liquid-immersed unit or IEEE C57.12.01 for a dry-type unit (by default)
NOTE The general requirements standards assign each voltage class a standard BIL and, for several classes, one or more optional higher levels; a dry-type unit carries a lower standard BIL than a liquid-immersed unit of the same class, so the class alone does not fix the value. (7.4.3)
NOTE A BIL above the standard level for the class buys protective margin between the arrester's discharge voltage and the winding's withstand, and is used where the primary is exposed to lightning through an overhead line, where switching transients from vacuum interrupters reach the winding, or where the arrester lead length erodes the margin the arrester would otherwise provide. (7.4.4)
7.4.5 The BIL selected shall be coordinated with the surge arrester protective characteristics so that the protective margin required by IEEE C62.22 is maintained at each winding.

7.5 Taps

7.5.1 The transformer shall be furnished with full-capacity primary taps in the arrangement indicated in the datasheet.
Primary Tap Arrangementselect
Two 2.5 percent taps above and two 2.5 percent taps below rated voltage
Four 2.5 percent taps above and four 2.5 percent taps below rated voltage
Four 2.5 percent taps below rated voltage
Two 5 percent taps below rated voltage
No taps
NOTE The two-above, two-below arrangement gives a 10 percent adjustment range in 2.5 percent steps and is the arrangement the general requirements standards describe as standard, so it applies without knowing anything about the project. Taps below rated voltage only serve a supply that is expected to run low and never high, and a wider range serves a supply that varies more than 5 percent from nominal or a unit that feeds long secondary feeders whose voltage drop must be compensated at the source. (7.5.2)
7.5.3 The tap changer operation shall be as indicated in the datasheet.
Tap Changer Operationradio
○ Externally operable de-energized tap changer
○ Internal tap links reconnected with the cover or panel removed
7.5.5 The tap changer shall be operable only with the transformer de-energized, and an externally operable tap changer shall carry a warning label stating that condition and shall be provided with a padlocking provision.
NOTE Load tap changing is outside the scope of this standard, and every tap changer under it is a de-energized device. (7.5.6)

7.6 Impedance

7.6.1 The percent impedance at the rated tap shall be as indicated in the datasheet.
Percent Impedance at Rated Taprange
%
22.533.544.555.55.7566.577.588.5910
7.6.2 Where no impedance is selected in the datasheet, the manufacturer's standard impedance for the rating and construction shall apply, and the manufacturer shall report the design impedance in the action submittals for use in the project's short-circuit and coordination studies.
7.6.3 The impedance tolerance shall be the tolerance of the applicable general requirements standard, and the manufacturer shall state the design value rather than a range.
NOTE Impedance sets the secondary fault current and the voltage regulation together: a lower impedance delivers a stiffer secondary and a higher fault current that the downstream equipment must interrupt, and a higher impedance limits the fault current at the cost of more voltage drop under load and a larger core. The general requirements standards assign a standard impedance to each rating, and a value away from it is a special design. (7.6.4)
7.6.5 Where a specific impedance is selected, it shall be coordinated with the project's short-circuit study so that the available fault current at the secondary terminals does not exceed the interrupting rating of the downstream equipment.
7.6.6 Where the transformer will be paralleled with another transformer, the impedances of the two shall be within 7.5 percent of each other on a common kVA base.

7.7 Sound Level

7.7.1 The audible sound level shall not exceed the NEMA TR 1 value for a liquid-immersed unit or the NEMA ST 20 value for a dry-type unit, for the unit's rating and cooling class, reduced by the amount indicated in the datasheet.
Sound Level Reduction Below the NEMA Valuerange
dB
35810
NOTE The NEMA tables give the maximum average sound level as a function of rating and cooling class, so the requirement is stated as a reduction below the table value rather than as an absolute level that would be right for one rating and wrong for every other. (7.7.2)
NOTE Each 3 dB of reduction roughly halves the acoustic energy the core radiates and is bought with a lower core flux density, which means more core steel, a larger and heavier unit, and a higher first cost. A reduction is used where the transformer is installed against an occupied space, inside a building with a low background level, or near a property line with a noise ordinance whose limit the table value would exceed. (7.7.3)
7.7.4 Sound level shall be measured in accordance with IEEE C57.12.90 or IEEE C57.12.91 as applicable to the medium.

8 Core and Windings

8.1 The winding conductor material shall be as indicated in the datasheet.
Winding Conductor Materialradio
○ Copper
○ Aluminum
NOTE Copper carries more current per unit of cross-section, which yields a smaller and lighter winding and a smaller unit for the same rating, and its terminations are less sensitive to connection practice; aluminum yields a lower first cost at a larger winding and a larger unit, and its terminations depend on the connector, the preparation, and the torque being right. Both are manufactured across the full scope of this standard. (8.2)
8.3 The core material shall be as indicated in the datasheet.
Core Materialradio
● Grain-oriented silicon steel
○ Amorphous metal
NOTE An amorphous metal core carries a no-load loss on the order of one-third that of a silicon steel core of the same rating, at a higher first cost, a larger core, and a higher sound level for the same flux density; it repays its premium where the transformer is energized continuously at light average load, which is the condition under which no-load loss dominates the total. (8.4)
8.5 The core shall be built from laminations stacked and clamped so that the core losses and the sound level stated in the submittal are achieved on every unit, and the core shall be grounded to the tank or frame at one point.
8.6 The windings shall be braced to withstand the short-circuit forces corresponding to the short-circuit current the applicable general requirements standard requires the unit to withstand, without displacement or damage.
8.7 Dry-type windings shall be insulated with a system rated for the temperature class selected under this standard, and the resin or varnish shall be applied under vacuum so that no void remains in the winding insulation.
8.8 Where the datasheet selects an electrostatic shield, a grounded shield shall be placed between the primary and secondary windings and brought out to a grounding terminal.

9 Insulation System and Temperature Rise

9.1 Liquid-Immersed Temperature Rise

9.1.1 The average winding temperature rise of a liquid-immersed unit at rated load shall be as indicated in the datasheet.
Liquid-Immersed Average Winding Riserange
°C
5565
NOTE The 65°C rise is the rating basis of IEEE C57.12.00 for a liquid-immersed unit, so it applies without knowing anything about the project; a 55°C rise unit of the same nameplate runs cooler, and where furnished with a dual 55/65°C rating carries 112 percent of its 55°C rating at the 65°C rise, which is capacity bought with additional winding conductor rather than with insulation life. (9.1.2)
9.1.3 Where a 55°C rise is selected, the nameplate shall carry the 55°C rating and the corresponding 65°C rating.

9.2 Dry-Type Insulation System and Temperature Rise

9.2.1 The insulation system temperature class of a dry-type unit shall be as indicated in the datasheet.
Dry-Type Insulation System Temperature Classrange
°C
130155180200220
9.2.2 The average winding temperature rise of a dry-type unit at rated load shall be as indicated in the datasheet.
Dry-Type Average Winding Riserange
°C
80115150
9.2.3 The sum of the 40°C maximum ambient, the average winding rise, and the hot-spot allowance shall not exceed the insulation system temperature class, and a 150°C rise shall be furnished only on a 220°C system.
NOTE The temperature class is the insulation's continuous thermal limit and the rise is how much of that limit the design consumes at rated load; a lower rise on the same class runs cooler, carries a continuous overload margin, and loses less energy, at a larger and costlier winding. Open-wound impregnated units are commonly built on a 220°C system, and cast-resin units on a 155°C or 180°C system, so no single class is the norm across the dry-type scope. (9.2.4)
9.2.5 Insulation systems shall be designated by their numeric temperature class in accordance with IEEE C57.12.01 and UL 1562, and a letter class designation shall not be used on the nameplate or in the submittals.
NOTE Where a dry-type unit will carry a nonsinusoidal load or a recurring overload, a rise lower than the maximum the class permits gives the unit thermal margin that a K-factor rating alone does not, because the K-factor addresses the harmonic loss and not the fundamental load above nameplate. (9.2.6)

10 Insulating Liquid, Tank, and Enclosure Finish

10.1 Insulating Liquid

10.1.1 The insulating liquid shall be as indicated in the datasheet.
Insulating Liquidselect
Mineral oil to ASTM D3487
Natural ester to ASTM D6871
Synthetic ester, listed less-flammable
Silicone fluid to ASTM D4652
High fire-point hydrocarbon to ASTM D5222
NOTE Mineral oil is the lowest-cost liquid, has the longest service history, and has a fire point near 165°C, which classifies it as a flammable liquid for the purposes of NEC Article 450 and drives the vault requirement indoors and the separation requirements outdoors. (10.1.2)
NOTE A natural ester has a fire point above 300°C, is listed as a less-flammable liquid, is derived from vegetable oil and biodegrades in a spill, and tolerates a higher winding temperature for the same insulation life; it absorbs more moisture than mineral oil and its pour point is higher, so a unit that will stand de-energized in severe cold or sit open to the atmosphere needs its liquid handling planned for that. (10.1.3)
NOTE A synthetic ester and a silicone fluid are also listed less-flammable liquids with fire points above 300°C; the synthetic ester biodegrades and the silicone fluid does not, and both cost more than a natural ester. A high fire-point hydrocarbon is a mineral-oil-derived liquid with a fire point above 300°C and mineral-oil handling characteristics. (10.1.4)
NOTE A less-flammable liquid is used where the transformer is installed indoors without a vault under NEC 450.23, where the outdoor separation from combustible construction under NEC 450.27 cannot be achieved with mineral oil, or where the Owner's insurer requires it. (10.1.5)
10.1.6 Where the datasheet selects a less-flammable liquid, the unit shall be furnished with the pressure-relief and tank withstand provisions the liquid's listing requires for the installation, and the nameplate shall identify the liquid.
10.1.7 The liquid as shipped shall meet the acceptance limits of IEEE C57.106 for mineral oil, IEEE C57.147 for a natural ester, or the liquid manufacturer's published acceptance limits for another liquid, and the manufacturer shall furnish the certificate of analysis for the liquid in the unit.

10.2 Liquid Preservation and Pressure Relief

10.2.1 The liquid preservation system shall be as indicated in the datasheet.
Liquid Preservation Systemradio
● Sealed tank with gas space
○ Sealed tank with automatic pressure-vacuum bleeder
○ Conservator with air-cell bladder
NOTE A sealed tank is the construction of every pad-mounted unit and of substation-type units through the scope of this standard: the gas space above the liquid expands and contracts with temperature within the tank's pressure design, and the liquid never contacts the atmosphere. A bleeder vents the gas space automatically when the pressure leaves the design band, which limits the pressure excursion on a unit with a wide temperature swing at the cost of admitting air on the vacuum side; a conservator keeps the tank full and moves the gas volume to a separate vessel, which is uncommon below 10 MVA and is furnished where the Owner's fleet standard calls for it. (10.2.2)
10.2.3 The tank shall be furnished with a pressure-relief device rated for the tank and the liquid, arranged to relieve without spraying liquid onto a person at the compartment or at the operating position, and with a visual indicator of operation.
10.2.4 The tank shall withstand, without permanent deformation, the internal pressure and the vacuum stated in the applicable general requirements standard for its construction.
10.2.5 The tank shall be leak-tested at the factory at a positive internal pressure, and the test pressure and duration shall be stated in the submittal.

10.3 Tank, Cabinet, and Finish

10.3.1 The enclosure coating and corrosion protection system shall be as indicated in the datasheet.
Enclosure Coating and Corrosion Protection Systemradio
● Coated mild steel to IEEE C57.12.28
○ Coated stainless steel to IEEE C57.12.29
NOTE IEEE C57.12.28 is the enclosure integrity standard for pad-mounted equipment in ordinary exposures and sets the coating's adhesion, impact, salt-spray, and ultraviolet performance; IEEE C57.12.29 raises the salt-spray requirement and requires a stainless steel enclosure for an installation within reach of salt air, chemical fallout, or de-icing spray, where a mild steel cabinet corrodes from the sill upward within a few years. (10.3.2)
10.3.3 The exterior finish color shall be as indicated in the datasheet.
Exterior Finish Colorselect
Munsell 7GY 3.29/1.5 green
ANSI 61 light gray
ANSI 70 sky gray
ANSI 49 medium gray
NOTE Munsell 7GY 3.29/1.5 is the color IEEE C57.12.34 assigns to pad-mounted equipment and is the color utility-owned pad-mounted units carry, so a pad-mounted unit in that color reads as part of the surrounding utility plant; the ANSI grays are the colors of indoor and substation electrical equipment. Which color a project wants follows from where the unit stands and what it stands beside, so this field carries no default. (10.3.4)
10.3.5 A pad-mounted cabinet shall be tamper-resistant in accordance with IEEE C57.12.34, with a hood and sill arrangement that prevents access to the compartment interiors without opening the doors, a pentahead captive bolt and a padlocking provision on each door, and a low-voltage compartment door that must be opened before the high-voltage compartment door can be opened.
10.3.6 The high-voltage and low-voltage compartments of a pad-mounted unit shall be separated by a steel barrier, and each compartment shall be accessible from the front without reaching across the other.
10.3.7 A substation-type unit shall be furnished with the guards, terminal chambers, or throat flanges the datasheet terminations require so that no energized part is exposed outside a chamber or a fenced enclosure.

11 Terminations

11.1 Primary Feed Arrangement

11.1.1 The primary feed arrangement shall be as indicated in the datasheet.
Primary Feed Arrangementradio
○ Radial feed, one set of primary terminations
○ Loop feed, two sets of primary terminations
Per drawings — the one-line diagram (deferred by default)
NOTE A loop-fed unit carries two sets of primary terminations so that the primary cable passes through the transformer and on to the next unit, and the loop can be sectionalized at the transformer; a radial-fed unit terminates one cable and is the end of its primary circuit. (11.1.2)

11.2 Pad-Mounted High-Voltage Terminations

11.2.1 The high-voltage terminations of a pad-mounted unit shall be as indicated in the datasheet.
Pad-Mounted High-Voltage Terminationsselect
Dead-front, 200 A loadbreak bushing wells with removable inserts
Dead-front, 200 A loadbreak integral bushings
Dead-front, 600 A deadbreak apparatus bushings
Live-front, porcelain bushings with exposed terminals in the compartment
NOTE Dead-front construction mates the primary cable to the transformer through separable insulated connectors so that no energized primary part is exposed inside the compartment once the connectors are made up, and it is the construction the general requirements standards describe as standard for a pad-mounted unit; bushing wells with removable inserts allow the loadbreak insert to be replaced or a different accessory to be fitted without draining the tank, where an integral bushing does not. (11.2.2)
NOTE Six-hundred-ampere deadbreak apparatus bushings are used where the primary loop current exceeds the 200 A rating of a loadbreak elbow, where the primary cable is larger than a 200 A elbow accepts, or where the serving utility's underground standard is built around 600 A connectors. (11.2.3)
NOTE Live-front construction leaves the primary terminals exposed within the compartment and is used where the primary arrives as bare conductor from an overhead riser, or where the serving utility's practice requires it. (11.2.4)
11.2.5 Separable insulated connectors, bushing wells, inserts, and apparatus bushings shall comply with IEEE 386 and shall be rated for the primary voltage class and BIL of the winding.
11.2.6 A dead-front pad-mounted unit shall be furnished with a parking stand adjacent to each primary bushing so that a loadbreak elbow can be parked on an insulated standoff, and with a ground connection point in the high-voltage compartment for a grounding elbow.
11.2.7 A live-front pad-mounted unit shall provide the clearances between energized terminals and between terminals and the cabinet that IEEE C57.12.34 requires for its voltage class, and the compartment door shall carry a warning of exposed energized parts.

11.3 Substation-Type High-Voltage Terminations

11.3.1 The high-voltage terminations of a substation-type unit shall be as indicated in the datasheet.
Substation-Type High-Voltage Terminationsselect
Cover-mounted porcelain bushings for open-air connection
Air-filled terminal chamber for cable termination
Throat flange to adjacent primary switch or switchgear
NOTE Cover-mounted bushings receive an overhead or bus connection in a fenced yard and leave the terminals exposed; a terminal chamber encloses the cable terminations in an air-filled box bolted to the tank or enclosure with cable entry from below; a throat flange bolts to the adjacent primary switch so that the connection is made inside the two enclosures with no exposed bus. (11.3.2)
11.3.3 A terminal chamber shall be sized to receive the number and size of primary cables and their terminations as indicated on the one-line diagram and the conduit and cable schedule, with space to make up a stress cone or termination on each cable without bending the cable below its minimum radius.

11.4 Low-Voltage Terminations

11.4.1 The low-voltage terminations shall be as indicated in the datasheet.
Low-Voltage Terminationsselect
Molded spade bushings with NEMA hole pattern
Air-filled terminal chamber with cable lugs
Bus duct throat flange
Throat flange to adjacent switchboard or switchgear
11.4.2 The low-voltage terminals shall accept the number and size of secondary conductors as indicated on the one-line diagram and the conduit and cable schedule, and where the conductors are aluminum the terminals or lugs shall be listed for aluminum conductors.
11.4.3 The secondary neutral provision shall be as indicated in the datasheet.
Secondary Neutral Bushingradio
● X0 bushing with a removable strap to the tank or enclosure ground
○ X0 bushing insulated from the tank or enclosure with no strap
NOTE The removable strap lets the installer bond the neutral at the transformer or remove the strap and bond it at the first disconnecting means, which is the choice NEC 250.30 gives for the location of the system bonding jumper; an insulated X0 with no strap is furnished where the Owner's practice is always to bond at the downstream equipment. (11.4.4)
NOTE Where the datasheet selects a delta secondary, there is no neutral to bring out, and the neutral bushing provisions of this article do not apply. (11.4.5)
11.4.6 Where the datasheet selects a delta secondary, the manufacturer shall furnish secondary terminals for the three phase conductors and no neutral bushing.

11.5 Grounding Provisions on the Unit

11.5.1 The tank or enclosure shall be furnished with NEMA two-hole grounding pads, one in each compartment of a pad-mounted unit and not fewer than two on a substation-type unit at diagonally opposite corners, sized for the grounding electrode conductor and the equipment grounding conductors as indicated on the grounding details.
11.5.2 The core, the tank or frame, the compartment barriers, and every metallic accessory enclosure shall be bonded to the grounding pads.

12 Primary Switching and Fusing

12.1 Primary Switching

12.1.1 The primary switching furnished within the transformer shall be as indicated in the datasheet.
Integral Primary Switchingselect
No integral primary switch
Two-position on-off loadbreak switch
Four-position loop sectionalizing loadbreak switch
Two two-position loadbreak switches, one per loop cable
Per drawings — the one-line diagram (deferred by default)
NOTE A loadbreak switch inside the transformer lets the unit be isolated from its primary without a separate switching device on its own pad; a four-position switch on a loop-fed unit selects which loop cable feeds the transformer and opens the loop on either side, and two two-position switches give the same isolation with independent control of each cable. Where the upstream switchgear provides a lockable disconnect for the transformer, the integral switch duplicates that function and is omitted. (12.1.2)
12.1.3 An integral primary switch shall be rated for the primary voltage class and BIL, for the loop or feeder continuous current, and for loadbreak duty at that current, and its momentary and fault-close ratings shall be not less than the available fault current at the primary terminals as indicated on the one-line diagram.
12.1.4 The switch operating handle shall be operable from the front of the high-voltage compartment with a hot stick, shall be lockable in each position, and shall carry position indication visible from the operating position.
12.1.5 A dry-type unit ordered with integral primary switching shall be furnished with the switch in a separate compartment of the enclosure, isolated from the windings by a grounded barrier.

12.2 Primary Fusing

12.2.1 The primary fusing furnished within a liquid-immersed unit shall be as indicated in the datasheet.
Liquid-Immersed Internal Primary Fusingselect
No internal fuses, protection by the upstream device
Bayonet expulsion fuses
Bayonet expulsion fuses in series with partial-range current-limiting fuses
Full-range current-limiting fuses in dry-well canisters
Drawout expulsion fuses in dry-well canisters
NOTE A bayonet fuse is an expulsion fuse under the liquid in a holder that withdraws from the front of the high-voltage compartment with a hot stick, so a blown element is replaced at the pad; it clears overload and low-magnitude fault current and depends on a series current-limiting fuse to clear a high-magnitude internal fault before the arc energy ruptures the tank. A full-range current-limiting fuse clears the whole range in one element at a higher element cost and a longer replacement outage, and an unfused unit relies on the upstream device to clear an internal fault and on that device's characteristic reaching far enough into the transformer to do so. (12.2.2)
12.2.3 Where the datasheet selects a series current-limiting fuse, the current-limiting fuse shall be rated to interrupt the available fault current at the primary terminals, shall be coordinated with the expulsion fuse so that the expulsion fuse clears every current within its interrupting rating, and shall comply with IEEE C37.47.
12.2.4 The bayonet fuse element sensing shall be as indicated in the datasheet.
Bayonet Fuse Element Sensingradio
○ Current-sensing
○ Dual-sensing, current and liquid temperature
NOTE A dual-sensing element opens on a sustained overload that raises the liquid temperature as well as on fault current, so it protects the transformer's insulation against slow overload; a current-sensing element opens on current alone, so it is applied where the transformer is deliberately loaded above nameplate under a loading guide and a temperature-sensing element would open on the intended overload. (12.2.5)
12.2.6 Where internal fuses are furnished, the fuse ratings shall be coordinated with the upstream protective device and with the transformer's inrush and damage characteristics in the project's coordination study, and the Contractor shall not install fuse elements of a rating other than the rating the study establishes.
12.2.7 A dry-type unit shall be protected by the primary overcurrent device upstream of it in accordance with NEC 450.3, and internal primary fusing shall not be furnished in a dry-type unit unless the datasheet selects integral primary switching with fuses.
12.2.8 The fuse drip shield, the fuse handling instructions, and the fuse ratings shall be posted inside the high-voltage compartment door of a fused pad-mounted unit.

13 Surge Arresters

13.1 The surge arrester provision at the transformer shall be as indicated in the datasheet.
Surge Arrester Provisionselect
No arresters at the transformer, protection by arresters at the upstream equipment
Elbow-type metal-oxide arresters on the dead-front primary bushings
Metal-oxide arresters on parking stands or bushing-well accessories in the primary compartment
Distribution-class metal-oxide arresters mounted in the terminal chamber or on the tank
Intermediate-class metal-oxide arresters mounted in the terminal chamber or on the tank
Station-class metal-oxide arresters mounted in the terminal chamber or on the tank
NOTE An arrester at the transformer protects the winding against a surge arriving on the primary cable, and its protective margin depends on the distance and the lead length between the arrester and the winding it protects, so an arrester at the upstream switchgear protects a transformer at the end of a long cable less well than the same arrester at the transformer terminals. A cable-fed unit whose primary originates on an overhead line is exposed to lightning that the cable's surge impedance does not attenuate, and a unit fed from vacuum switchgear is exposed to switching transients that reach the winding through the cable. (13.2)
NOTE Distribution-class arresters carry the lowest energy rating and the highest discharge voltage of the three classes; intermediate- and station-class arresters lower the discharge voltage and raise the energy capability, which buys protective margin where the BIL is at the standard level and the exposure is severe. (13.3)
13.4 Arresters shall be metal-oxide arresters complying with IEEE C62.11, applied in accordance with IEEE C62.22, and rated as indicated in the datasheet.
Arrester Duty-Cycle Voltage Ratingrange
kV
369101215182124273036
Derived — the primary voltage rating and the primary system grounding method, applied per IEEE C62.22 so that the maximum continuous operating voltage of the arrester is not less than the maximum line-to-ground voltage the system can sustain (by default)
NOTE An arrester on a solidly grounded system is rated for the line-to-ground voltage; an arrester on a delta, ungrounded, or impedance-grounded primary must hold the full line-to-line voltage during a ground fault on the other phases, so its rating rises accordingly, and an under-rated arrester on such a system fails during the first sustained ground fault. (13.5)
13.6 Arrester ground leads shall be as short and as straight as the installation allows, and each arrester ground shall be bonded to the tank or enclosure grounding pad.

14 Accessories

14.1 Standard Accessories on Liquid-Immersed Units

14.1.1 A liquid-immersed unit shall be furnished with the following accessories as a minimum:
  • a nameplate of stainless steel or anodized aluminum, engraved or stamped in accordance with IEEE C57.12.00, mounted in the low-voltage compartment of a pad-mounted unit or on the tank of a substation-type unit
  • lifting lugs on the tank rated for the filled weight of the unit, and jacking provisions at the base
  • a drain valve with a sampling device, located so that a sample can be drawn without opening the tank
  • a fill plug or upper filter-press connection
  • a liquid level indicator
  • a dial-type liquid temperature indicator with a maximum-reading pointer
  • a pressure-relief device with an operation indicator
  • a pressure-vacuum gauge on a unit larger than 2500 kVA
14.1.2 Optional accessories on a liquid-immersed unit shall be as indicated in the datasheet.
Liquid-Immersed Optional Accessoriescheckbox
☐ Liquid temperature indicator with alarm contacts
☐ Liquid level indicator with alarm contacts
☐ Pressure-relief device with alarm contacts
☐ Pressure-vacuum gauge with alarm contacts
☐ Winding temperature indicator with alarm and fan-start contacts
☐ Sudden-pressure relay
☐ On-line dissolved-gas monitor
☐ Alarm contacts wired to a terminal block for remote monitoring
☐ Current transformer provisions on the low-voltage bushings for metering
☐ Utility metering compartment
NOTE Contacts on the liquid temperature, liquid level, and pressure devices convert a scheduled visual inspection into an alarm, and they are furnished where the transformer serves a load whose loss matters more than the cost of the wiring to a monitoring point, or where the unit stands where no one walks past it. A sudden-pressure relay detects the pressure wave of an internal arc faster than any fuse or relay that senses current, and is furnished where the tank stands close enough to a building or to people that a tank rupture is the failure being guarded against. (14.1.3)

14.2 Standard Accessories on Dry-Type Units

14.2.1 A dry-type unit shall be furnished with the following accessories as a minimum:
  • a nameplate of stainless steel or anodized aluminum, engraved or stamped in accordance with IEEE C57.12.01
  • lifting provisions and a base arranged for rolling or skidding in either direction
  • ventilation openings screened against the entry of vermin and debris
  • a winding temperature indicator with a sensor on each phase, with alarm and trip contacts, on a cast-resin unit and on any dry-type unit 1000 kVA and larger
14.2.2 Optional accessories on a dry-type unit shall be as indicated in the datasheet.
Dry-Type Optional Accessoriescheckbox
☐ Winding temperature indicator with a sensor on each phase, where not a standard accessory
☐ Electrostatic shield between primary and secondary windings
☐ Enclosure space heaters with thermostat
☐ Vibration isolation between the core-and-coil assembly and the base
☐ Alarm contacts wired to a terminal block for remote monitoring
☐ Current transformer provisions on the secondary terminals for metering
☐ Surge arresters mounted inside the enclosure
NOTE Space heaters keep the winding above the dew point while the unit is de-energized in an unconditioned outdoor enclosure, which is the condition under which condensation on an open-wound winding leads to a failure at re-energization; an electrostatic shield diverts capacitively coupled transients from the primary to ground rather than into the secondary, which matters where the secondary serves electronic loads. (14.2.3)

15 Testing

15.1 Factory Routine Tests

15.1.1 Every transformer shall receive the routine tests required by IEEE C57.12.90 for a liquid-immersed unit or IEEE C57.12.91 for a dry-type unit, including as a minimum:
  • resistance measurement of every winding at the rated tap
  • ratio measurement at every tap
  • polarity and phase-relation verification
  • no-load loss and excitation current at rated voltage
  • load loss and impedance voltage at rated current
  • applied-voltage dielectric test on each winding
  • induced-voltage dielectric test
  • insulation resistance measurement
  • leak test of the tank and every gasketed joint and bushing, on a liquid-immersed unit
  • partial-discharge measurement in accordance with IEEE C57.124, on a cast-resin unit
15.1.2 The partial-discharge level of a cast-resin unit shall not exceed 10 pC at the test voltage IEEE C57.124 specifies.
15.1.3 Additional factory tests beyond the routine tests shall be performed as indicated in the datasheet.
Additional Factory Testscheckbox
☐ Lightning impulse test on each unit
☐ Temperature rise test on one unit of each design
☐ Audible sound level test on each unit
☐ Short-circuit withstand test on one unit of each design
☐ Zero-sequence impedance measurement
☐ Dissolved-gas analysis of the liquid after the temperature rise test
NOTE The routine tests establish that the unit was built as designed; the design tests establish that the design does what the submittal claims, and they are performed on a representative unit rather than on every unit because a temperature rise test occupies a test bay for a day and a short-circuit test can damage the unit tested. A design test is called for where the unit is the first of a design the manufacturer has not tested before, where the Owner's fleet history justifies it, or where a rating claim in the submittal cannot be verified any other way. (15.1.4)
15.1.5 The certified test report for each unit shall identify the unit by serial number and shall report every measured value together with the acceptance limit it was judged against.

15.2 Witnessed Factory Testing

15.2.1 Factory test witnessing shall be as indicated in the datasheet.
Factory Test Witnessingradio
○ Witnessed by the Owner's representative
● Certified test report without witnessing
15.2.2 Where witnessing is selected, the manufacturer shall give not less than two weeks' notice of test readiness, shall submit the test procedure for review before the test, and shall not ship the unit until the witness has accepted the results or has waived attendance in writing.

15.3 Field Acceptance Tests on Every Unit

15.3.1 Field acceptance tests shall be performed on every transformer in accordance with NETA ATS after installation is complete and before the unit is energized, and shall include the tests indicated in the datasheet.
Field Acceptance Tests on Every Unitcheckbox
☑ Visual and mechanical inspection including compartment cleanliness, shipping brace removal, and anchorage
☑ Bolted connection resistance or torque verification
☑ Insulation resistance, winding-to-winding and winding-to-ground, with polarization index
☑ Turns ratio measurement at every tap
☑ Winding resistance measurement
☐ Polarity and phase-relation verification
☐ Insulation power factor or dissipation factor
☐ Excitation current at test voltage
☑ Verification of the tank or enclosure ground and the neutral bonding
☑ Operational test of every alarm, trip, and fan-start contact
☐ Ratio and polarity verification of metering current transformers
15.3.2 Insulation resistance shall be measured at the test voltage NETA ATS assigns to the winding voltage class, and the result shall meet the NETA ATS acceptance value corrected to 20°C.
15.3.3 The polarization index, where measured, shall be not less than 2.0, and a unit whose index is below that value shall be investigated for moisture before it is energized.
15.3.4 The measured turns ratio at each tap shall be within 0.5 percent of the nameplate ratio, and the measured winding resistances shall be within 1 percent of each other and of the factory values corrected to the same temperature.

15.4 Field Acceptance Tests on Liquid-Immersed Units

15.4.1 In addition to the tests on every unit, a liquid-immersed unit shall receive the tests indicated in the datasheet on a liquid sample drawn at the site after the unit has stood at least 24 hours at the installation.
Field Acceptance Tests on Liquid-Immersed Unitscheckbox
☑ Dielectric breakdown voltage to ASTM D877 or ASTM D1816
☑ Water content to ASTM D1533
☐ Interfacial tension
☐ Neutralization number
☐ Color and visual condition
☐ Power factor of the liquid at 25°C
☑ Baseline dissolved-gas analysis to ASTM D3612
15.4.2 The liquid sample results shall meet the acceptance limits of IEEE C57.106 for mineral oil, IEEE C57.147 for a natural ester, or the liquid manufacturer's published limits for another liquid, and a unit whose liquid fails a limit shall not be energized until the liquid has been processed or replaced and retested.
NOTE A baseline dissolved-gas analysis taken before energization is the reference against which every later sample is interpreted under IEEE C57.104; without it, the first in-service sample cannot distinguish gas generated by a developing fault from gas that was in the liquid when the unit arrived. (15.4.3)

15.5 Field Acceptance Tests on Dry-Type Units

15.5.1 In addition to the tests on every unit, a dry-type unit shall receive the tests indicated in the datasheet.
Field Acceptance Tests on Dry-Type Unitscheckbox
☑ Inspection of the windings for shipping damage, cracked resin, and displaced spacers
☑ Verification of ventilation clearances and screen integrity
☐ Insulation resistance after a dry-out cycle, where the unit was stored in an unconditioned space
☑ Verification of the winding temperature sensor on each phase and its alarm and trip set points
15.5.2 A dry-type unit whose insulation resistance is below the NETA ATS acceptance value shall be dried by circulating heated air or by low-voltage current through the windings with the unit de-energized, under the manufacturer's instructions, until the value is met and stable.

15.6 Test Failures and Retesting

15.6.1 A transformer that fails a field acceptance test shall not be energized, and the Contractor shall report the failure to the Engineer of Record within one working day with the measured values.
15.6.2 A unit that fails a factory or field test shall be repaired or replaced by the manufacturer, and the cost of the repair or replacement, of the repeated test, and of any delay to the energization schedule that results shall be borne by the Contractor.

15.7 Initial In-Service Inspection

15.7.1 Within 30 days after energization, and with the transformer carrying not less than 40 percent of its self-cooled rating, the Contractor shall perform an infrared scan of every accessible primary and secondary connection and of the tank or enclosure surfaces, and shall correct any connection whose temperature exceeds that of a comparable connection under the same load by more than 10°C.
15.7.2 Where the load at the 30-day inspection is below 40 percent of the rating, the scan shall be repeated when the load first reaches that level, and the second scan shall be reported to the Engineer of Record.

16 Installation

16.1 Location, Separation, and Vaults

16.1.1 Where a liquid-immersed unit is installed indoors, the installation provision required for it shall be as indicated in the datasheet.
Indoor Installation Provision for Liquid-Immersed Unitsselect
Transformer vault constructed in accordance with NEC 450 Part III
Listed less-flammable liquid installation without a vault in accordance with NEC 450.23
Not applicable, outdoor installation
Derived — the insulating liquid selected, the kVA rating, the primary voltage rating, and whether the transformer is indoors or outdoors, applied per NEC 450.21 through 450.27 (by default)
NOTE NEC 450.26 requires an indoor mineral-oil unit to be installed in a vault, and NEC 450.23 permits an indoor less-flammable-liquid unit outside a vault under stated conditions on the liquid's listing, the room's construction, the liquid confinement area, and the pressure relief; the provision therefore follows from the liquid and the location and is not a project preference. (16.1.2)
16.1.3 Where a vault is required, the vault construction, ventilation, drainage, door, and fire rating shall comply with NEC 450 Part III and shall be as indicated on the vault construction details.
16.1.4 An outdoor liquid-immersed unit shall be separated from combustible building surfaces, from doors, and from windows in accordance with NEC 450.27, and where the liquid is a listed less-flammable liquid the separation reductions its listing permits may be applied.
16.1.5 A dry-type unit installed indoors shall be located in a room or space that satisfies NEC 450.21 for its rating and voltage, and a unit larger than 112.5 kVA shall be installed in a room of fire-resistant construction or shall meet an exception NEC 450.21 provides.
16.1.6 The spill containment provided for a liquid-immersed unit shall be as indicated in the datasheet.
Liquid Spill Containmentselect
No containment beyond the transformer tank
Containment curb or basin around the pad sized for the full liquid volume
Containment basin with an oil-water separator drain
Containment integral to the vault floor and sill
NOTE Federal spill prevention rules under 40 CFR Part 112 apply to a facility whose aggregate oil storage, including transformer liquid, exceeds the regulation's threshold, and a state or local rule can apply below it; a containment basin sized for the full liquid volume is the provision those rules generally require, and a natural ester's biodegradability reduces the cleanup consequence of a spill without removing the containment obligation where a rule imposes one. (16.1.7)
16.1.8 Where containment is provided, its capacity, drainage, and separator shall be as indicated on the pad and containment details.

16.2 Working Space and Physical Protection

16.2.1 Working space around each compartment, terminal chamber, and enclosure opening shall comply with NEC 110.26 for the low-voltage side and with NEC 110.34 for the medium-voltage side, and the door swing of each compartment shall not reduce that space.
16.2.2 The manufacturer's ventilation clearances around a dry-type unit and the clearance to open a pad-mounted cabinet fully shall be maintained in addition to the code working space.
16.2.3 A pad-mounted unit exposed to vehicle traffic shall be protected by bollards or barriers as indicated on the site electrical plan.
16.2.4 Where the site allows, a pad-mounted unit shall be oriented with its compartment doors facing away from building openings, walkways, and property lines.
NOTE A fault inside a compartment vents through its doors, so the door orientation decides where the arc products go. (16.2.5)

16.3 Pad Mounting

16.3.1 A pad-mounted unit shall be installed on a concrete pad constructed under Concrete PadsConcrete Equipment PadsResolves to the current adopted revision.sync/concrete-pads to the outline, cable opening, conduit window, and anchor pattern on the reviewed pad drawing.
16.3.2 The Contractor shall confirm the reviewed pad drawing against the transformer actually delivered before the pad is placed, and shall report a discrepancy to the Engineer of Record before concrete is placed.
16.3.3 The cable opening in the pad shall be sealed after the cables are pulled with a material that excludes vermin and that can be removed to add or replace a cable.
16.3.4 The unit shall be anchored to the pad at the manufacturer's anchor points with anchors sized for the seismic and wind loads applicable to the installation.

16.4 Vault and Room Mounting

16.4.1 A substation-type unit shall be set on a level surface at the location and orientation indicated on the electrical room or vault layout, and shall be anchored at the manufacturer's anchor points.
16.4.2 A dry-type unit shall be set with its ventilation openings unobstructed and with the clearance to walls and to other equipment the manufacturer's listing requires, and shall not be set against a wall on the side that carries its inlet openings.
16.4.3 The room ventilation shall carry away the full-load losses of every transformer in the room so that the room ambient does not exceed the indoor design ambient selected in the datasheet, and the Contractor shall furnish the loss data to the mechanical trade before the ventilation design is fixed.
16.4.4 A dry-type unit that will be lifted into a room shall be lifted by the core-and-coil lifting provisions or the base lifting provisions the manufacturer identifies, and shall not be lifted by the enclosure panels.

16.5 Seismic Qualification and Anchorage

16.5.1 The seismic certification required for the transformer shall be as indicated in the datasheet.
Seismic Certification Requirementselect
No seismic certification required
Anchorage design only, in accordance with ASCE 7 Chapter 13
Special certification of the transformer as an active component in accordance with ASCE 7 Section 13.2.2
Derived — Seismic Design CategorySeismic Design CategoryParameterEach project supplies its own value.seismic-design-category and the component importance factor assigned to the transformer under ASCE 7 Chapter 13 (by default)
NOTE Anchorage keeps the unit on its pad or floor; special certification demonstrates that the unit still functions after the design earthquake, which ASCE 7 requires only for a component whose failure would impair a designated seismic system or whose importance factor is above 1.0. (16.5.2)
16.5.3 Where special certification is required, the certification shall be by shake-table test or by analysis accepted by the Authority Having Jurisdiction, and the certificate shall cover the unit as furnished, including its accessories and the tank or enclosure.

16.6 Grounding and Bonding

16.6.1 The tank or enclosure shall be bonded to the grounding electrode system at each grounding pad with a listed two-hole compression connector and a conductor sized in accordance with NEC 250.122 for the primary overcurrent device, or larger where Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding requires.
16.6.2 A pad-mounted unit shall be provided with a ground ring or ground rods at the pad as indicated on the grounding details and as the serving utility requires.
16.6.3 Where the transformer secondary is a separately derived system, the location of the system bonding jumper shall be as indicated in the datasheet.
System Bonding Jumper Locationradio
● At the transformer secondary neutral
○ At the first downstream disconnecting means
16.6.4 The system bonding jumper and the grounding electrode conductor shall be installed at the location selected and at no other point, and the neutral shall not be bonded to the equipment grounding system anywhere downstream of that point.
16.6.5 Where the transformer secondary constitutes the service, the grounded conductor and the grounding electrode connections shall comply with NEC 250.24 and with Electrical Service EntranceElectrical Service EntranceResolves to the current adopted revision.sync/electrical-service-entrance.
16.6.6 Where the X0 bushing is furnished with a removable strap and the system bonding jumper is located downstream, the strap shall be removed before energization and its removal recorded on the as-built documents.

16.7 Labeling and Signage

16.7.1 Field-installed labels shall be provided as indicated in the datasheet and shall comply with Equipment LabelingElectrical Equipment LabelingResolves to the current adopted revision.sync/equipment-labeling.
Transformer Labelingcheckbox
☑ Equipment designation matching the one-line diagram
☑ kVA, primary and secondary voltage, and winding connection
☑ Upstream source and disconnecting means identification
☑ Available fault current at the secondary terminals with the calculation date
☑ Arc-flash warning label per NFPA 70E
☐ High-voltage danger signage per NEMA 260 and ANSI Z535.4 on a pad-mounted unit
☑ Tap position as set at energization
☐ Insulating liquid identification and less-flammable listing on a liquid-immersed unit
16.7.2 The available fault current at the secondary terminals shall be calculated from the utility's stated primary fault contribution and the transformer's design impedance, and shall be marked at the secondary terminals where NEC 110.24 requires it.
16.7.3 A pad-mounted unit accessible to the public shall carry the safety labels NEMA 260 specifies on the exterior of each compartment door.

16.8 Energization

16.8.1 Before energization, the Contractor shall remove every shipping brace, blocking, desiccant, and packaging material from the compartments and the enclosure, and shall record their removal on the field test report.
16.8.2 Before energization, the Contractor shall set the tap changer to the position as indicated on the one-line diagram, or where none is indicated to the rated-voltage tap, and shall record the position on the as-built documents.
16.8.3 Before energization, the Contractor shall verify the torque of every accessible connection against the manufacturer's values with a calibrated torque tool.
16.8.4 Before energization, the Contractor shall confirm that the field acceptance tests have been completed and accepted, that the liquid level of a liquid-immersed unit is at the 25°C mark, and that every compartment door is closed and locked.
16.8.5 The transformer shall be energized from the primary side with no load connected on the secondary, held for not less than one hour while the sound level, the temperature indicators, and the compartments are observed, and then loaded.

17 Delivery, Storage, and Handling

17.1 A liquid-immersed unit shall be shipped filled with its insulating liquid, with the gas space at a positive pressure, and the pressure shall be checked and recorded at receipt; a unit that arrives at zero or negative pressure shall be reported to the manufacturer before it is set, and its liquid shall be tested before energization.
17.2 A liquid-immersed unit shall be lifted only by the tank lifting lugs with a spreader that keeps the slings clear of the bushings, the cabinet, and every accessory, and shall not be lifted by the cabinet or by any accessory mounting.
17.3 A dry-type unit shall be shipped with its windings protected against moisture and shall be stored indoors in a clean, dry, heated or ventilated space, on its shipping base, until it is set.
17.4 A dry-type unit shall not be stored outdoors, and a dry-type unit that has been exposed to rain, condensation, or a damp space shall be dried in accordance with the manufacturer's instructions and its insulation resistance verified before energization.
17.5 A liquid-immersed unit may be stored outdoors on its pad or on a level surface with the compartment doors closed and locked and the tank pressure maintained.
17.6 Where the transformer will stand more than 90 days between delivery and energization, the Contractor shall inspect it monthly, shall record the tank pressure of a liquid-immersed unit and the condition of the windings of a dry-type unit at each inspection, and shall keep the inspection log with the closeout documents.
17.7 Transformers damaged in transit, in storage, or in handling shall be repaired or replaced as the manufacturer directs and as the Engineer of Record accepts in writing, and a unit whose tank has been breached or whose windings have been wetted shall be replaced.

18 Warranty

18.1 The manufacturer shall warrant each transformer against defects in materials and workmanship for the period indicated in the datasheet, measured from the date of substantial completion.
Manufacturer's Warranty Periodrange
years
123510
18.2 The warranty shall cover repair or replacement of the transformer, the insulating liquid, and the accessories furnished with it, including the labor and rigging to remove and reinstall the unit at the site.
18.3 Where the contract documents state a longer warranty period than the datasheet, the longer period shall govern.
18.4 The Contractor shall warrant the installation, including every connection, the anchorage, the grounding, and the field-installed accessories, for the project warranty period.
18.5 A repair or replacement performed under this warranty shall itself be warranted for a full new term equal to the original period from the date the repair is completed, or for the remainder of the original period, whichever ends later.

19 Spare Parts

19.1 The Contractor shall deliver the spare parts indicated in the datasheet to the Owner before the transformer is accepted, and shall obtain a signed receipt for them.
Spare Parts to Be Furnishedcheckbox
☐ One set of expulsion fuse elements for each fuse rating on a fused unit
☐ One set of current-limiting fuses for each rating on a unit so fused
☐ One loadbreak elbow connector and one insulated parking bushing for each voltage class
☐ One elbow arrester for each voltage class on a unit so equipped
☐ One gasket set for each pad-mounted unit
☐ One winding temperature sensor for each dry-type unit
☐ One cooling fan for each unit furnished with a forced-air stage
☐ Touch-up finish in the exterior color
NOTE A blown internal fuse is otherwise an outage that lasts until a replacement is sourced, and a fuse of a different rating installed to end the outage defeats the coordination study, so a fused unit without spare elements on the site is a fused unit waiting for a substitution. (19.2)
19.3 Spare parts shall be of the same manufacturer and rating as the installed items, shall be delivered in their original packaging, and shall be labeled with the equipment designation of the unit they serve.