Automatic Transfer Switches and Generator Switchgear: What Estimators Need to Price

The ATS System: More Than Just the Transfer Switch

An automatic transfer switch is the piece of equipment that makes the decision to switch electrical loads from the utility supply to generator power when the utility fails. It is a critical life safety component on healthcare, data centre, and commercial projects — and it is also a component that estimators frequently price in isolation, missing the full system scope around it.

The complete ATS and generator switchgear system includes:

• The ATS unit itself: Rated for the transferred load current, with utility and generator input connections and a load output bus
• Generator interconnect switchgear: The circuit breaker or fused disconnect at the generator output terminals
• Control wiring: Between the ATS and the generator control panel (remote start signal, generator ready signal, transfer permission signal)
• Emergency distribution panelboard or switchgear: The downstream panel fed from the ATS output that distributes power to the emergency loads
• Bypass/isolation switch: Allows the ATS to be taken out of service for maintenance without dropping the connected loads — required by most AHJs on life safety systems
• Remote annunciation panel: Code-required on some occupancy types, providing visual indication of ATS status at a remote monitoring location
• Load bank: For commissioning — a resistive load bank is required to load-test the generator and ATS system at full rated load before acceptance

ATS Types: What You Are Actually Pricing

Not all automatic transfer switches are the same equipment. There are three main types, and they have meaningfully different cost profiles:

Open Transition ATS

The standard and most common ATS type. On transfer, the load is briefly disconnected from the utility before being connected to the generator. There is a momentary power interruption (typically 0.1 to 1.5 seconds) during transfer. Open transition ATSs are the least expensive option and are suitable for most commercial standby applications where a brief interruption is acceptable.

Closed Transition ATS

Transfer is made in a make-before-break sequence — the load is briefly connected to both sources simultaneously (in-phase parallel) before the utility is disconnected. No power interruption occurs during transfer. Closed transition ATSs are required for sensitive loads (data centres, surgical theatres, precision manufacturing) and are significantly more expensive — typically 40–80% more than an equivalent open transition unit. They require the generator control system to support synchronisation.

Soft-Load (Programmed Transition) ATS

A variant of closed transition that ramps the generator load on and off gradually, reducing transient voltage and frequency disturbances during transfer. Used on large generators or sensitive industrial applications. The most expensive ATS type, requiring sophisticated generator controls and a higher-specification transfer switch mechanism.

Sizing the ATS: The Calculation Estimators Need to Do

ATS sizing is based on the connected load current that will be transferred to generator power. The ATS must be continuously rated for the full load current, not just the demand current — because the ATS must transfer the load at any point in the load cycle, including at peak demand.

The sizing calculation:

• Identify all loads to be served on emergency power (life safety loads, legally required standby loads, or optional standby loads)
• Sum the connected load current for all emergency loads — use the NEC branch circuit load calculations per Article 220 for residential and commercial occupancies
• Add a minimum 25% safety margin to the calculated connected load
• Select the next standard ATS rating above the calculated load — standard ratings are 100A, 150A, 200A, 260A, 400A, 600A, 800A, 1000A, 1200A, 1600A, 2000A, 3000A, 4000A
• Verify the AIC (interrupting capacity) of the selected ATS is adequate for the available fault current from the generator — this is often overlooked because generator fault current is lower than utility fault current, but it must still be checked

A common sizing mistake: including the full utility service rating in the ATS sizing calculation. The ATS only needs to be rated for the emergency loads — not the total building load. On a 2000A service with 400A of emergency loads, you need a 400A ATS, not a 2000A ATS.

NEC Requirements: The Code Items That Add Scope

NEC Article 700 (Emergency Systems) has specific requirements that directly affect ATS scope and cost:

• 10-second transfer time: Life safety emergency systems must restore power within 10 seconds of utility failure. This defines the generator start-to-ready time and the ATS transfer sequence timing.
• Automatic transfer: Article 700 systems must use an ATS — manual transfer switches are not permitted on life safety loads.
• Ground fault detection: NEC 700.26 requires ground fault indication (not necessarily tripping) on emergency system switchgear. This adds a monitoring component that must be priced.
• Bypass-isolation provision: NEC 700.5(B) requires that emergency system equipment be capable of being tested under load. This effectively requires a bypass-isolation switch on the ATS for maintenance testing without dropping the connected loads.
• Testing requirements: NEC 700.3 requires annual testing of emergency systems under load conditions with the normal utility supply disconnected. This is a commissioning and ongoing maintenance requirement, not just a one-time test.
• SPD requirement (NEC 2026): Emergency system panelboards and switchgear must include surge protective devices under NEC 2026 — see our article on NEC 2026 changes for full details.

The Bypass-Isolation Switch: Priced or Missed?

The bypass-isolation switch is one of the most consistently missed scope items in ATS estimates. It is a separate switching device — typically a 4-position switch assembly — that allows the connected loads to be transferred from the ATS to a direct utility or generator feed (bypassing the ATS) for maintenance. Without it, any maintenance on the ATS requires dropping the connected loads — unacceptable for life safety systems.

Bypass-isolation switches are available in two forms:

• Integral bypass-isolation: Built into the ATS enclosure as a factory-installed option. More compact, factory-tested as a unit, and typically 20–35% more expensive than the ATS alone. The recommended approach for new installations.
• Separate bypass panel: A standalone bypass panel adjacent to the ATS, providing the bypass path independently. Used when the ATS has already been procured without bypass, or where the physical layout requires separate equipment.

When pricing ATS systems on NEC Article 700 projects, treat the bypass-isolation switch as a standard inclusion, not an optional add-on. It is a code requirement, and omitting it from the bid scope creates a compliance issue at inspection.

Commissioning: The Load Bank Question

Commissioning an ATS and generator system properly requires load testing — operating the generator at rated load (typically 100% for at least 2 hours per NEC 700.3) to verify thermal performance, voltage and frequency regulation, and ATS transfer operation at full load conditions. This requires a load bank.

Load bank supply is consistently missed in ATS commissioning scope. The options:

• Hire a resistive load bank sized to the generator rating (typically 80–100% of generator kW rating) for the commissioning test period. Load bank hire rates vary by size but are a significant line item on large generator installations.
• Use the building load as the test load — feasible only if the building is occupied and operating at sufficient load during the test period. This approach is unreliable for acceptance testing because actual load may be below rated generator capacity.
• Client-supplied load bank — the commissioning scope states that the client will provide a load bank for testing. This transfers the cost to the client but requires early agreement and coordination.

If the commissioning scope includes the load bank, price it. If it is excluded, state the exclusion explicitly. A commissioning test that cannot reach full load conditions because no load bank was provided will not satisfy the AHJ's inspection requirements.

Generator Interconnect and Protection

On larger generator installations (above 100kW), the generator interconnect switchgear — the main disconnect and overcurrent protection at the generator terminals — is a separate pricing item from the ATS. The generator interconnect requirements:

• A fused disconnect or circuit breaker at the generator output, sized to the generator's maximum output current
• Generator protection relays — overcurrent, overvoltage, undervoltage, over/underfrequency, and reverse power (for paralleling schemes) — if specified
• Metering: volts, amps, frequency, kW output — typically included in the generator control panel but should be confirmed in the specification
• Anti-islanding protection: required if the generator can export power to the utility grid — mandatory for grid-connected standby systems with export capability

Conclusion

ATS and generator switchgear estimation requires pricing the complete system, not just the ATS unit. The bypass-isolation switch, control wiring, generator interconnect protection, commissioning load testing, and code-required monitoring collectively add substantial cost to what appears on first reading to be a simple "supply and install ATS" scope. Estimators who build these items into their standard ATS scope template will consistently price these projects more accurately — and win them without the surprise costs that come from incomplete bidding.