1 Scope
NOTE This specification covers automatic transfer switch (ATS) equipment rated 600V and below for transferring connected loads between a normal source (typically the utility service) and an alternate source (typically an on-site engine-driven generator). (1.1)
1.2The transfer switch shall monitor source voltage and frequency, signal the generator to start when the normal source fails or is out of tolerance, transfer the load to the alternate source after the alternate source is stable, and return the load to the normal source after the normal source is restored and re-stable.
1.3The transfer switch's selection and configuration shall be governed by NFPA 110, NFPA 70 Articles 700/701/702/708, and the project's continuity-of-operations requirements.
1.6This standard covers low-voltage ATS equipment only and does not cover paralleling switchgear that synchronizes multiple generators onto a common bus, static transfer switches that use solid-state semiconductors for sub-cycle transfer in UPS applications, or manual transfer switches operated by hand without automatic control.
1.7Where the project requires paralleling switchgear, static transfer switches, or manual transfer switches, they shall be specified separately.
2 Referenced Standards
NOTE Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement governs unless the Engineer of Record directs otherwise in writing. (2.1)
| Standard |
Title |
| UL 1008 |
Standard for Transfer Switch Equipment |
| UL 1008S |
Standard for Solid-State Transfer Switches (where applicable) |
| NFPA 110 |
Standard for Emergency and Standby Power Systems |
| NFPA 111 |
Standard on Stored Electrical Energy Emergency and Standby Power Systems |
| NFPA 70 |
National Electrical Code (Articles 700, 701, 702, 708, 230, 250) |
| NFPA 70E |
Standard for Electrical Safety in the Workplace |
| NFPA 99 |
Health Care Facilities Code (where applicable) |
| IEEE 446 |
Recommended Practice for Emergency and Standby Power Systems (Orange Book) |
| IEEE 241 |
Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book) |
| IEEE C62.41 |
Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits |
| NEMA ICS 10-1 |
AC Transfer Switch Equipment |
| NEMA 250 |
Enclosures for Electrical Equipment (1000 Volts Maximum) |
| IBC |
International Building Code (seismic and importance factors) |
| ASCE 7 |
Minimum Design Loads and Associated Criteria for Buildings and Other Structures |
| ICC ES AC156 |
Acceptance Criteria for Seismic Certification by Shake-Table Testing |
| ANSI/NETA ATS |
Standard for Acceptance Testing Specifications for Electrical Power Equipment |
2.2Equipment and installation shall comply with the latest adopted edition of each standard listed above.
2.3Where 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.
3 Submittals
3.1 Action Submittals
3.1.1Contractor shall submit the following for review prior to fabrication.
3.1.2Installation shall not proceed until the corresponding submittals have been reviewed and returned.
- Product data for the transfer switch, controller, and accessories, with the UL 1008 listing mark indicated
- Shop drawings showing overall dimensions, conduit entry locations, mounting details, and required working clearances
- Single-line diagram showing the transfer switch in context, including normal source, alternate source, load, ratings, and pole configuration (3-pole or 4-pole)
- Control and wiring schematics, including engine-start contacts, signal interfaces, and any communications interfaces
- Withstand and Closing Rating (WCR) tables coordinated with the upstream overcurrent device(s) actually used on the project
- Seismic certification documentation per IBC/ASCE 7 where required
- Manufacturer's recommended time-delay settings and the project-specific values proposed by the Contractor for Engineer review
- Factory production-test report covering each unit furnished
☐ Product data with UL 1008 listing
☐ Shop drawings (dimensions, conduit entries, clearances)
☐ Single-line diagram showing ATS in context
☐ Control and wiring schematics
☐ WCR tables coordinated to project OCPDs
☐ Seismic certification (where required)
☐ Proposed time-delay settings
☐ Factory production-test report
3.1.3The Contractor shall submit the action submittals listed above for review prior to fabrication.
3.1.4Installation shall not proceed until the corresponding action submittals have been reviewed and returned.
3.2 Closeout Submittals
3.2.1Contractor shall provide the following at substantial completion before the transfer switch is accepted.
- Operation and maintenance manuals, including the controller programming and setting record as commissioned
- As-built single-line and control schematics reflecting any field changes
- Field acceptance test reports per NETA ATS and per NFPA 110 §8 where applicable
- Manufacturer startup and commissioning report
- Warranty documentation
- Spare parts inventory with reorder information
☐ Operation and maintenance manuals with setting record
☐ As-built single-line and control schematics
☐ Field acceptance test reports (NETA ATS / NFPA 110 §8)
☐ Manufacturer startup and commissioning report
☐ Warranty documentation
☐ Spare parts inventory with reorder information
3.2.2The Contractor shall provide the closeout submittals listed above at substantial completion before the transfer switch is accepted.
4 Quality Assurance
4.1 Manufacturer Qualifications
4.1.1The transfer switch shall be the product of a manufacturer regularly engaged in the production of UL 1008 listed transfer switch equipment, with a minimum of five years of documented production experience in the rating class furnished.
4.1.2The manufacturer shall maintain a North American service organization capable of dispatching a factory-trained technician on-site within 24 hours of notification.
4.2 Listing and Labeling
4.2.1The transfer switch shall be listed and labeled to UL 1008 as a complete assembly — power switching device, controller, and enclosure — by a Nationally Recognized Testing Laboratory.
4.2.2UL 1008 listing of components individually is not acceptable; the assembly as furnished shall bear the listing mark.
4.2.3Transfer switches serving emergency systems per NEC Article 700 shall be UL 1008 listed.
NOTE UL 1008 listing of emergency-system transfer switches is a code requirement, not a project preference. (4.2.4)
4.2.5Where the project includes health care facilities subject to NFPA 99 and NEC Article 517, the transfer switch shall additionally be suitable for the essential electrical system branch it serves (life safety, critical, or equipment branch) and shall be applied accordingly.
4.3 Source Limitations
4.3.1The transfer switch, its controller, and all factory-installed accessories shall be furnished by a single manufacturer responsible for the complete assembly.
4.3.2Combining a power switching device from one manufacturer with a third-party controller is not acceptable for global applications under this standard.
4.4 Testing Personnel Qualifications
4.4.1Field acceptance testing shall be performed by a firm regularly engaged in testing low-voltage power equipment, employing technicians certified by NETA or equivalent.
4.4.2Testing personnel shall have a minimum of three years of experience testing transfer switch equipment and shall be familiar with the controller furnished.
5 Environmental and Service Conditions
5.1The transfer switch shall be suitable for continuous operation under the ambient conditions of the installed location.
5.2Where conditions exceed the manufacturer's standard ratings, the Contractor shall notify the manufacturer and apply derating or alternative construction as required.
5.3 Ambient and Altitude
0 to 40°C (standard)
-20 to 40°C (cold-climate indoor)
-30 to 50°C (extended outdoor)
Below 6,600 ft (2,000 m) — no derating
6,600 to 9,900 ft (2,000 to 3,000 m) — derating required
Above 9,900 ft (3,000 m) — consult manufacturer
5.4 Outdoor and Unconditioned Installations
○ Not required (climate-controlled indoor space)
○ Provided, thermostat-controlled
5.4.1Transfer switches installed outdoors, in unconditioned spaces, or in mechanical and generator rooms shall account for the wider temperature swings, condensation, and dust exposure of those locations.
5.4.2Outdoor and unconditioned installations should include a thermostatically controlled enclosure heater to prevent condensation on the power conductors, controller, and contact surfaces.
5.5 Seismic Requirements
Not required
IBC/ASCE 7 — Importance Factor 1.0
IBC/ASCE 7 — Importance Factor 1.5 (essential facility / emergency system)
OSHPD pre-approval (California healthcare)
5.5.1Where required by the applicable building code, the transfer switch shall be seismically certified by shake-table testing per ICC ES AC156 or by analysis per ASCE 7, including the controller and any accessories as installed.
5.5.2Transfer switches serving emergency systems, life-safety branches, or essential facilities frequently carry an importance factor of 1.5 and shall be certified accordingly.
6 Ratings
6.1 Amperage Rating
100 A
150 A
200 A
260 A
400 A
600 A
800 A
1000 A
1200 A
1600 A
2000 A
2600 A
3000 A
4000 A
6.1.1The transfer switch shall be rated for the continuous load current it serves.
6.1.2UL 1008 transfer switches are typically rated for total system load and shall not be loaded above 100% of the nameplate rating unless specifically listed as continuous-duty for 100% application.
6.1.3The amperage rating shall be selected to match the upstream feeder ampacity and the downstream load with appropriate margin for future growth.
6.2 Voltage, Phases, and Frequency
120/240V 1-Phase 3-Wire
120/208V 3-Phase 4-Wire
277/480V 3-Phase 4-Wire
480V 3-Phase 3-Wire
347/600V 3-Phase 4-Wire
6.3 Withstand and Closing Rating (WCR) / Short-Circuit Current Rating (SCCR)
NOTE UL 1008 establishes the Withstand and Closing Rating (WCR) for a transfer switch — the maximum prospective short-circuit current the switch can withstand without damage and onto which it can close. (6.3.1)
Specific breaker (manufacturer and frame named on UL 1008 tables)
Any-breaker rating (less restrictive, lower kA typically)
Current-limiting fuse (Class L, RK1, or J as listed)
Integral overcurrent protection (service entrance rated ATS)
10200
102235426585100200
Default: 65 kA
22 kA
35 kA
42 kA
65 kA
85 kA
100 kA
200 kA
NOTE The WCR is established by test in combination with a specific upstream overcurrent protective device (specific manufacturer and frame, or any-manufacturer molded-case breaker of a given trip rating, or current-limiting fuses of a stated class and rating), and the available WCR for a given switch is therefore a function of what is installed upstream. (6.3.2)
6.3.3The WCR of the transfer switch shall equal or exceed the available fault current at the terminals as determined by the project short-circuit study (see Short Circuit Study). 6.3.4The Contractor shall not select a WCR based on the upstream breaker rating alone.
NOTE A substitution of an equivalent-amperage breaker from a different manufacturer can void the rating because the UL 1008 tables tie the WCR to a specific upstream device. (6.3.5)
6.3.6Where the upstream overcurrent device is changed, the WCR shall be re-verified against the latest UL 1008 listing data.
6.4 Service Entrance Rating
NOTE A service-entrance-rated transfer switch combines the service disconnect, the service main overcurrent device, and the transfer switch into a single listed assembly. (6.4.1)
○ Not service entrance rated
○ Service entrance rated with integral main overcurrent device
NOTE Service entrance construction simplifies the equipment layout but ties the transfer switch's WCR to the integral overcurrent device. (6.4.2)
6.4.3The Contractor shall confirm that the integral device's interrupting rating, ground-fault protection, and metering provisions satisfy NEC Article 230 for the project before selecting a service-entrance-rated transfer switch.
7 Construction
7.1The transfer switch shall consist of a mechanically interlocked, double-throw power switching mechanism, a microprocessor-based controller, and an enclosure.
7.2The power switching mechanism shall be electrically operated and mechanically held in either source position so that no continuous power is required to maintain the connection.
7.3 Power Switching Mechanism
○ Contactor-based (electrically operated, mechanically held)
○ Power-frame breaker pair (two interlocked breakers — closed-transition or large frames)
○ Molded-case switch pair (interlocked)
7.3.1The power switching mechanism shall be a single, integrated double-throw device — not two independent breakers tied together by an external interlock.
7.3.2Mechanical interlocking shall prevent simultaneous connection of both sources to the load, except in closed-transition operation, where overlapping connection is permitted for a controlled, brief interval (see #Transfer Type below). 7.3.3The switching contacts shall be silver-alloy, designed for the inrush and interrupting duty associated with the connected load, and shall be inspectable and replaceable as a field-serviceable assembly.
NOTE A contactor-based mechanism is the default for typical emergency and standby applications because it provides fast transfer (a few cycles), long mechanical life, and a compact footprint. (7.3.4)
NOTE Breaker-based mechanisms are commonly used for large frames (3000A and above), for closed-transition switching where independent open/close control of each source is required, and where the transfer switch must also provide upstream or downstream overcurrent protection in a service entrance configuration. (7.3.5)
7.4 Manual Operation
7.4.1The transfer switch shall include a means for manual operation under no-load conditions to verify mechanism integrity and to permit safe maintenance.
7.4.2Manual operation shall be possible only when both sources are de-energized or as otherwise specifically permitted by the listing.
7.4.3The manual operation handle or tool shall be stored with the equipment.
☐ Normal position indicating (NO + NC)
☐ Emergency position indicating (NO + NC)
☐ Pre-transfer signal (programmable)
☐ Loss-of-normal signal
☐ Loss-of-emergency signal
☐ Not in automatic mode signal
7.5.1The transfer switch shall be provided with auxiliary contacts indicating position (connected to normal, connected to emergency) for use by remote monitoring, building automation, and load shed systems.
7.5.2A minimum of two normally open and two normally closed contacts on each position shall be wired to terminal blocks accessible without exposing energized parts.
8 Transfer Type
8.1The transfer type defines how the load is moved between sources and shall be selected based on the load's tolerance to a momentary interruption and on whether the alternate source will be operated in parallel with the normal source during transfer.
8.2 Transfer Type Selection
○ Open transition (break-before-make)
○ Delayed transition (open with programmed neutral time)
○ Closed transition (overlapping, ≤100 ms parallel)
○ Soft-loading (closed transition with ramped load transfer)
8.3 Open Transition
NOTE Open-transition transfer breaks the connection to one source before making the connection to the other. (8.3.1)
NOTE During open-transition transfer the load is briefly de-energized — typically less than 0.1 seconds for a contactor-based mechanism — and equipment with internal energy storage (motors, contactors, lighting ballasts) experiences a momentary loss of voltage. (8.3.2)
NOTE Open transition is the default and the appropriate selection for the large majority of emergency and standby applications, because it does not require any coordination, parallel-operation permission, or relaying with the utility. (8.3.3)
8.4 Delayed Transition
NOTE Delayed transition is open transition with a programmable time delay in the neutral (off) position between sources, allowing residual voltage on motor loads to decay before connection to the alternate source, which prevents out-of-phase reclosing on motors and the resulting torque transients and breaker trips. (8.4.1)
8.4.2Delayed transition shall be specified where the load includes significant motor content or where motor in-rush following retransfer has historically tripped breakers or damaged equipment.
NOTE The neutral time is typically programmable from 0 to 60 seconds. (8.4.3)
8.5 Closed Transition
NOTE Closed-transition transfer briefly parallels the two sources (typically for less than 100 ms) so that the load is never de-energized during retransfer. (8.5.1)
8.5.2Closed-transition operation requires that the alternate source be synchronized to the normal source (matched in voltage, frequency, and phase angle) before the transfer is initiated, and requires the explicit permission of the serving utility because the on-site generator is, for a brief interval, paralleled with the utility.
8.5.3Closed transition shall not be assumed without confirmed utility approval.
8.5.4The Contractor shall obtain written interconnection approval and shall coordinate any required protective relaying, anti-islanding protection, and metering with the utility before procurement.
8.5.5The closed-transition window shall be set so that the parallel interval is consistent with the utility's approval — typically 100 ms or less.
8.5.6If the alternate source cannot be synchronized within the controller's permitted window, the controller shall fall back to open or delayed transition automatically.
8.6 Soft-Loading (Closed Transition with Ramped Transfer)
NOTE Soft-loading is closed-transition operation in which the controller actively ramps the real and reactive load between the two sources over several seconds while they are paralleled, minimizing voltage and frequency transients on the load, and is appropriate for sensitive process or healthcare applications. (8.6.1)
8.6.2Soft-loading requires more sophisticated generator controls (typically isochronous load sharing with utility interface) and additional utility approval beyond simple closed-transition operation.
9 Pole Configuration
9.1The transfer switch's pole configuration shall be selected based on whether the alternate source is a separately derived system per NEC 250.30 and on whether the neutral is switched.
9.2 Pole Configuration Selection
○ 3-pole (solid neutral, neutral not switched)
○ 4-pole (switched neutral, fully separated)
○ 3-pole with overlapping neutral (where listed)
9.3 3-Pole (Solid Neutral)
NOTE A 3-pole transfer switch switches the three phase conductors only; the neutral passes through unbroken and is solidly connected between the normal source neutral, the load neutral, and the alternate source neutral. (9.3.1)
NOTE With a 3-pole ATS, the alternate source (generator) is not a separately derived system — the service main bonding jumper at the utility service is the single neutral-to-ground bond, and no system bonding jumper is installed at the generator. (9.3.2)
9.3.3The 3-pole arrangement is mechanically simpler and is acceptable for many smaller and non-critical applications, but it requires the generator neutral to be bonded only at the service main and not at the generator itself.
9.4 4-Pole (Switched Neutral)
NOTE A 4-pole transfer switch switches the neutral together with the three phase conductors. (9.4.1)
9.4.2With a 4-pole ATS, the alternate source becomes a separately derived system per NEC 250.30, and a system bonding jumper shall be installed at the generator (or at the first system disconnecting means on the generator side) and shall not be installed in parallel with the service main bonding jumper.
NOTE The 4-pole arrangement isolates the load neutral from the normal source neutral when running on the alternate source, which eliminates ground-fault sensing confusion, prevents objectionable neutral currents on grounding conductors, and is the default for systems with ground-fault protection on either or both sources. (9.4.3)
9.4.4A 4-pole switch shall be specified where the system has ground-fault protection per NEC 230.95 (most services 1000A and larger at 480/277V), where multiple service mains feed the same load through transfer switches, and where the design requires the generator to be a separately derived system.
NOTE When in doubt, 4-pole is the safer default. (9.4.5)
9.4.6The Contractor shall coordinate the resulting system bonding jumper and grounding electrode connection at the generator with Grounding And Bonding and Generators. 9.5 Overlapping Neutral
NOTE Some listed transfer switches provide an overlapping neutral — a neutral pole that makes the alternate connection before breaking the original connection — to avoid a momentarily open neutral during transfer. (9.5.1)
NOTE An open neutral on a 4-pole transfer of an unbalanced load can cause line-to-neutral voltage excursions on the load. (9.5.2)
9.5.3Overlapping neutral construction prevents open-neutral voltage excursions and shall be considered for 4-pole switches feeding heavily unbalanced single-phase loads.
10 Bypass-Isolation
10.1Bypass-isolation transfer switches add a manually operated bypass mechanism that allows the load to be carried directly from either source while the automatic transfer mechanism is electrically isolated and physically removed from the live bus for inspection, testing, or replacement.
10.2 Bypass-Isolation Selection
○ Not required (standard automatic transfer switch)
○ Bypass-isolation construction (no-load interruption during maintenance)
10.3 Bypass-Isolation Application
10.3.1Bypass-isolation construction shall be specified where continuity of operation cannot tolerate the planned outage that would be required to service a non-bypass switch.
NOTE Typical applications include hospital essential electrical systems, data centers, critical industrial processes, telecommunications central offices, and other facilities operating under NFPA 110 Level 1 with strict outage limits. (10.3.2)
10.3.3Bypass-isolation construction adds substantial size, weight, and cost; it should not be selected by default but where it is justified by the load criticality.
10.3.4The bypass operation shall be capable of being performed by a single qualified person without specialized tools and shall not require de-energizing the load.
11 Controller
11.1The transfer switch controller shall be microprocessor-based, with a non-volatile memory retaining all settings and event logs across power outages.
11.2The controller shall provide voltage and frequency sensing on both sources, programmable time delays, manual and automatic transfer modes, an exerciser clock, an event log, and a local human-machine interface.
11.3 Controller Display
Alphanumeric LCD with status LEDs
Graphical LCD with menu navigation
Color touchscreen
11.4 Source Sensing
☐ Three-phase under-voltage on normal
☐ Three-phase over-voltage on normal
☐ Phase loss / single-phase on normal
☐ Phase rotation / phase reversal on normal
☐ Under-frequency on alternate
☐ Over-frequency on alternate
☐ Phase angle / synchronization (closed transition only)
11.4.1The controller shall sense voltage on all phases of both the normal and alternate sources and shall sense frequency on the alternate source.
11.4.2Pickup and dropout setpoints for voltage and frequency shall be independently programmable.
11.4.3The controller shall initiate engine start on under-voltage, over-voltage, under-frequency, over-frequency, or phase loss on the normal source, with each condition independently enabled or disabled.
11.4.4Voltage sensing shall include both pickup and dropout setpoints (with hysteresis) to prevent nuisance transfers from short transients.
11.4.5Frequency sensing shall be applied to the alternate source to confirm the generator has reached operating frequency before transfer.
11.5 In-Phase Monitor
○ Not provided
○ Provided, programmable phase window
11.5.1For open-transition or delayed-transition transfer of motor loads, the controller may provide an in-phase monitor that delays retransfer until the two sources are within a programmed phase angle window.
NOTE The in-phase monitor reduces motor torque transients on retransfer without requiring synchronization or closed-transition operation, and is appropriate for motor-dominated loads (elevators, large pumps, compressors). (11.5.2)
11.6 Exerciser Clock
Weekly, no load (controller starts generator, does not transfer load)
Weekly, with load transfer
Monthly, with load transfer (NFPA 110 Level 1 minimum)
Custom schedule per O&M plan
11.6.1The controller shall include a programmable exerciser that starts the alternate source on a scheduled basis to confirm readiness and to exercise the generator and its fuel system.
11.6.2NFPA 110 Level 1 systems require a monthly exercise of at least 30 minutes under load or per the manufacturer's recommended test cycle.
11.6.3NFPA 110 Level 2 systems and NEC Article 700/701 systems have similar but not identical exercise requirements that shall govern.
11.7 Load Shed and Priority
○ Not required (generator sized for full load)
○ Programmable load-shed outputs
○ Source priority coordination across multiple ATS
11.7.1Where the alternate source has insufficient capacity to carry the entire load, the controller shall provide load-shed outputs to drop non-critical loads in a programmable sequence as the generator approaches capacity, and load-add outputs to restore them as capacity allows.
11.7.2Where multiple transfer switches share a single alternate source, the controllers shall coordinate source priority so that critical switches transfer first and non-critical switches wait for capacity.
11.7.3The Contractor shall coordinate load-shed strategy with Generators and the load calculations. 12 Time Delays
12.1The transfer switch controller shall provide independently programmable time delays for each transfer event.
12.2The defaults specified in each time-delay section reflect typical NFPA 110 Level 1 settings and shall be adjusted to suit the project; the final values shall be recorded in the commissioning report.
12.3 Time-Delay Engine Start (TDES)
NOTE TDES is the time the controller waits after detecting a loss of normal source before signaling the generator to start. (12.3.1)
NOTE A short TDES (1 to 5 seconds) prevents nuisance starts from very brief utility disturbances while still complying with the 10-second start requirement of NFPA 110 Level 1. (12.3.2)
12.3.3For Type 10 systems, total time from loss-of-normal to load-on-emergency shall not exceed 10 seconds.
12.4 Time-Delay Transfer to Emergency (TDTE)
NOTE TDTE is the time the controller waits, after the alternate source is stable, before transferring the load. (12.4.1)
NOTE A short delay (1 to 3 seconds) confirms the alternate source has stabilized; longer delays may be appropriate where the generator is shared with other transfer switches and load shedding must occur first. (12.4.2)
12.5 Time-Delay Retransfer to Normal (TDRN)
030
15101530
Default: 15 min
NOTE TDRN is the time the controller waits, after the normal source is restored and stable, before retransferring the load. (12.5.1)
NOTE A long retransfer delay (typically 10 to 30 minutes) confirms that the utility restoration is stable and avoids cycling the generator and the load on a marginal utility recovery; NFPA 110 commentary recommends 30 minutes for stable utility verification. (12.5.2)
12.6 Time-Delay Engine Cool-Down (TDEC)
NOTE TDEC is the time the generator runs unloaded after the load is retransferred to normal, to allow controlled cool-down of the engine. (12.6.1)
12.6.2The required TDEC value is set by the generator manufacturer and shall be coordinated with Generators. NOTE Skipping cool-down shortens engine life. (12.6.3)
12.7 Time-Delay Neutral (TDN, delayed-transition only)
NOTE TDN is the time the load is intentionally held in the neutral (off) position during a delayed-transition transfer, to allow motor residual voltage to decay before connection to the new source. (12.7.1)
12.7.2TDN shall be set based on the motor decay characteristics of the load; 1 to 5 seconds is typical for general-purpose motors.
13.1The transfer switch shall provide dry, isolated contacts to start the engine-generator.
13.2Two contacts shall be furnished: a normally closed contact that opens to call for start on loss of normal source (the standard engine-start contact), and an additional normally open contact for use by the generator controller's manual remote-start or test inputs as required.
13.3Contacts shall be rated for the generator starting circuit voltage (typically 12 or 24 VDC) and shall be wired to terminal blocks accessible without removing the controller.
13.4 Engine Start Wiring
○ Two-wire start (single contact closure)
○ Three-wire start with separate run signal
13.4.1Engine start wiring shall be installed between the transfer switch and the generator in dedicated raceway, separated from power conductors where required by NEC 700.10(D) for emergency systems.
14 Communications
14.1The transfer switch controller shall provide communications appropriate to the project's monitoring and building management integration requirements.
14.2 Communications Interface
☐ Dry contacts only (auxiliary contacts, no protocol)
☐ Modbus RTU (RS-485)
☐ Modbus TCP/IP (Ethernet)
☐ BACnet IP
☐ SNMP
☐ Manufacturer's proprietary protocol over Ethernet
○ Not required
○ Provided per NFPA 110 §5.6.5
14.2.1Where the project includes a building automation system or generator monitoring platform, the transfer switch shall be integrated using the protocol selected above.
14.2.3Where remote monitoring is required for NFPA 110 Level 1 systems, the remote alarm annunciator and the transfer switch communications shall be coordinated and tested as a single system.
15 Enclosure
15.1The transfer switch enclosure shall match the environmental conditions of the installed location and shall comply with NEMA 250 for the rating selected.
15.2 Enclosure Rating
NEMA 1 — Indoor general purpose
NEMA 3R — Outdoor rainproof
NEMA 4 — Indoor/outdoor watertight
NEMA 4X — Watertight, corrosion-resistant (stainless or non-metallic)
NEMA 12 — Industrial, dust-tight
NOTE NEMA 1 is appropriate for typical indoor electrical rooms with clean, dry, climate-controlled conditions. (15.2.1)
NOTE NEMA 3R is appropriate for outdoor installation in areas not subject to direct wash-down. (15.2.2)
15.2.3NEMA 4X shall be specified for coastal, chemical, food-processing, or wash-down environments where corrosion resistance is required.
NOTE NEMA 12 is appropriate for industrial spaces with significant airborne dust. (15.2.4)
15.3 Doors, Access, and Conduit Entry
○ Wall mounted (smaller frames, ≤400 A typical)
○ Floor standing on concrete housekeeping pad
15.3.1The enclosure shall include hinged front access doors with provisions for padlocking in the closed position.
15.3.2Doors shall provide access to the controller display, the manual operation handle, and the maintenance terminals without exposing energized power conductors.
15.3.3Provisions for conduit entry shall accommodate the source and load feeders, control wiring to the generator, and communications cabling.
16 Identification and Labeling
16.1Identification of the transfer switch shall follow Equipment Labeling conventions and shall additionally include the items selected below. ☐ Equipment designation matching one-line diagram
☐ Normal source identification (panel/switchgear of origin)
☐ Alternate source identification (generator designation)
☐ Load identification (loads served)
☐ UL 1008 listing label visible
☐ Available fault current and date determined (NEC 110.24)
☐ Arc flash warning per NFPA 70E
☐ Service entrance warning (where service entrance rated)
☐ Emergency system identification (Article 700 systems)
16.2Identification of the transfer switch shall follow Equipment Labeling conventions and shall additionally include the items selected above. 16.3The available fault current at service equipment and at the line side of the transfer switch shall be field-marked, including the date of the determination, per NEC 110.24.
16.4Where the transfer switch is in an Article 700 emergency system, the enclosure shall be marked as such per NEC 700.5(C) and 700.10.
16.5Labels shall be permanent, weather-resistant where exposed, and legible from the front of the equipment without obstruction.
17 Testing
17.1 Factory Tests
17.1.1The manufacturer shall perform the following production tests on each transfer switch before shipment, with results recorded on a certified test report:
- Dielectric withstand test per UL 1008
- Mechanical operation (minimum five complete transfer cycles, normal-to-emergency and emergency-to-normal)
- Electrical operation of the power switching mechanism and controller, on the rated control voltage
- Controller functional test, including all programmable time delays, source sensing setpoints, and engine-start contacts
- Continuity and polarity of all auxiliary contacts and communications terminals
- Visual and dimensional inspection
○ Standard production test report (default)
○ Witnessed factory acceptance test by Owner's representative
17.1.2The manufacturer shall perform the factory production tests listed above on each transfer switch before shipment, with results recorded on a certified test report.
17.2 Field Acceptance Tests
17.2.1Field acceptance testing shall include, at a minimum, the following:
- Visual and mechanical inspection, including verification of conductor torque per the manufacturer's specifications using a calibrated torque tool
- Insulation resistance testing, phase-to-phase and phase-to-ground, in both source positions
- Contact resistance measurement on all power-pole connections
- Mechanical operation verification, including manual operation and lockout in each position
- Controller setting verification, with each programmable parameter compared to the approved settings record
- Source-failure simulation, with measurement of TDES, generator-stable time, TDTE, and total time from loss-of-normal to load-on-emergency (≤10 seconds for NFPA 110 Type 10 systems)
- Source-restored simulation, with measurement of TDRN and TDEC
- Operation under load on the alternate source for not less than 30 minutes per NFPA 110 §8.4.2
- Verification of engine start contacts, alarm contacts, communications outputs, and remote annunciator points
- Closed-transition timing verification (closed-transition switches only) — peak parallel time shall be within the utility's approved limit
○ NETA acceptance testing plus manufacturer commissioning
○ NETA acceptance testing only
○ Manufacturer commissioning only
○ Not applicable (NEC Article 702 optional standby only)
○ Required per NFPA 110 §8 (emergency/legally required standby)
17.2.2The Contractor shall engage a qualified independent testing firm to perform field acceptance testing per ANSI/NETA ATS and the additional requirements of NFPA 110 §8 where applicable.
17.2.3Field acceptance testing shall occur after installation is complete and before the system is placed in service.
17.2.4Field acceptance testing shall include, at a minimum, the items listed above.
17.2.5Field acceptance testing shall be coordinated with the generator startup and testing required by Generators so that the complete normal/alternate-source/transfer-switch chain is verified as a system, not as isolated components. 17.2.6A failure or anomaly observed at the transfer switch during integrated testing shall be addressed before placing the system in service.
18 Installation
18.1 Coordination and Sequencing
18.1.1The Contractor shall coordinate transfer switch installation with the upstream service or distribution equipment, the alternate source, the loads served, and the building automation interface.
18.1.2Conduit and conductor routing for the engine-start signal, generator power feeder, and communications cabling shall be planned to avoid conflicts and to maintain separation requirements where the system is an Article 700 emergency system.
18.2 Concrete Housekeeping Pad
18.2.1Floor-standing transfer switches shall be mounted on a reinforced concrete housekeeping pad extending a minimum of 3 in. beyond the base of the equipment on all sides.
18.2.2The pad shall be a minimum of 4 in. above finished floor for indoor installations and 6 in. above finished grade for outdoor installations.
18.2.3Anchor bolt locations shall be coordinated with the manufacturer's shop drawings prior to concrete placement.
18.3 Working Clearance
18.3.1Working space at the transfer switch shall comply with NFPA 70 Article 110.26 based on the nominal voltage and the energized parts likely to require examination, adjustment, servicing, or maintenance while energized.
18.3.2Working space shall not be used for storage and shall not be obstructed.
18.4 Conductor Termination
18.4.1Source and load feeders shall be terminated on the lugs provided by the manufacturer.
18.4.2Where field-furnished lugs are used, they shall be listed for the conductor material and size, and they shall be torqued to the manufacturer's specified torque using a calibrated torque tool.
18.4.3Aluminum conductors shall be terminated with listed Al/Cu lugs and anti-oxidant compound where required by the lug manufacturer.
18.5 Grounding and Bonding
18.5.1Grounding and bonding shall be in accordance with Grounding And Bonding and NEC Article 250. 18.5.2For 4-pole (switched-neutral) transfer switches, the generator becomes a separately derived system per NEC 250.30 and shall have a system bonding jumper installed at the generator (or at the first system disconnecting means on the generator side), and a grounding electrode conductor connected at the same point.
18.5.3The system bonding jumper shall not exist at more than one point.
18.5.4For 3-pole (solid-neutral) transfer switches, the generator is not a separately derived system and shall not have a neutral-to-ground bond at the generator.
18.6 Article 700 Routing Separation
18.6.1For emergency system transfer switches per NEC Article 700, the wiring of the emergency system shall be kept entirely independent of all other wiring and equipment, in accordance with NEC 700.10(B).
18.6.2The Contractor shall confirm that the feeder from the alternate source to the transfer switch, and the feeder from the transfer switch to the emergency loads, are routed in dedicated raceways and enclosures as required by the article.
19 Delivery, Storage, and Handling
19.1Transfer switches shall be delivered in the manufacturer's original protective packaging with all listing marks intact.
19.2Equipment shall be stored indoors in a clean, dry, climate-controlled location.
19.3Where indoor storage is not available, the manufacturer shall provide weatherproof packaging and the Contractor shall protect the equipment from condensation, precipitation, dust, and physical damage.
19.4Equipment stored for more than 30 days in an unconditioned environment shall have its enclosure heater energized, where furnished, or shall be otherwise protected from condensation.
19.5Shipping braces and protective covers shall be removed only at final installation.
20 Warranty
1 year from substantial completion
2 years from substantial completion
5 years from substantial completion
☐ Annual preventive maintenance visit
☐ Controller firmware updates included
☐ 24/7 emergency response with 4-hour commitment
☐ Parts inventory commitment for the warranty period
20.1Warranty shall cover defects in materials and workmanship under normal use and service conditions.
20.2The manufacturer shall maintain a service organization capable of providing emergency replacement parts and field service within 24 hours during the warranty period.
20.3Where the transfer switch serves an emergency or legally required standby system, the response time commitment shall be coordinated with the facility's continuity-of-operations plan.
21 Spare Parts
☐ One set of replacement control fuses
☐ One set of replacement indicating lights / LEDs
☐ One set of auxiliary contact assemblies
☐ One spare main contact assembly (frames ≥800 A)
☐ Manual operation handle / tool
☐ Complete set of keys for all locks
21.1The manufacturer shall provide the spare parts selected above at substantial completion.
21.2Spare parts shall be of the same type, rating, and configuration as the installed components and shall be fully interchangeable.
21.3Spare parts shall be stored in a clearly labeled cabinet or container in the electrical room or generator room, accessible to maintenance personnel without specialized tools.