The automatic transfer switch is what connects your generator to your facility’s electrical system when utility power fails. Size it wrong and the switch becomes the weakest link in your backup power plan, either undersized and at risk of overload, or oversized and incompatible with the circuit it is meant to protect. Getting the sizing right requires understanding generator output, connected load, transfer switch type, and voltage configuration before anything gets ordered.
What ATS Sizing Actually Controls
An automatic transfer switch does not store power. It switches the source of power to a connected load from the utility feed to the generator, and back again when utility service is restored. The transfer switch must be rated to carry the full ampere load it will handle under that switch condition, continuously, not just momentarily.
ATS sizing is primarily an ampere rating question, not a kilowatt question. The generator’s kW output tells you how much power the unit can produce. The ATS ampere rating tells you how much current the switch contacts can safely carry to the connected loads. Both numbers have to be compatible with the same system, and neither can be assumed from the other without doing the calculation.
Calculating the ATS Ampere Rating You Need
The starting point is the generator’s full load ampere output at its rated voltage and power factor. For three-phase industrial systems operating at 480 volts with a 0.8 power factor, the most common configuration for industrial generators, the calculation is:
Amps = (kW x 1,000) / (Voltage x 1.732 x Power Factor)
At 480V and 0.8 PF, that denominator works out to approximately 665. A 500 kW generator at those parameters produces roughly 752 amps of full load current. The ATS needs to be rated at or above that figure, which in practice means selecting the next standard rating up, typically 800 amps.
The table below shows approximate full load ampere values and corresponding minimum ATS ratings for common industrial generator sizes at 480V, three-phase, 0.8 power factor.
| Generator Output | Approximate Full Load Amps (480V, 3-phase, 0.8 PF) | Minimum ATS Rating |
|---|---|---|
| 100 kW | ~150A | 200A |
| 200 kW | ~301A | 400A |
| 300 kW | ~451A | 600A |
| 500 kW | ~752A | 800A |
| 750 kW | ~1,128A | 1,200A |
| 1,000 kW | ~1,503A | 1,600A or 2,000A |
| 1,500 kW | ~2,255A | 2,500A or 3,000A |
These figures assume 480V, three-phase operation. Systems running at 208V or 240V will produce higher ampere values at the same kilowatt output, which means the ATS rating requirement increases substantially at lower voltages. Always run the calculation at your actual system voltage, not a generic assumption.
Transfer Switch Type and Configuration
Ampere rating is only part of the specification. The transfer switch configuration, how many poles it switches and how it handles the transition, has to match the electrical system it is installed in.
Three-pole transfer switches switch all three hot conductors and are standard for solidly grounded three-phase systems. Four-pole transfer switches switch the neutral as well, which is required in systems where the generator and utility neutral are separately derived sources, or where the installation must comply with NEC requirements for separately derived systems. Many industrial standby applications require a four-pole switch; a licensed electrical engineer should confirm which configuration applies before any equipment is ordered.
On the transition method side, the two primary options are open transition and closed transition. Open transition switches drop the utility source completely before connecting the generator, creating a brief power interruption during transfer. Closed transition switches the load while momentarily connecting both sources in parallel, allowing a transfer with no interruption. Closed transition requires tighter synchronization controls and typically costs more, but it eliminates the voltage dip that open transition creates, which matters for sensitive loads like variable frequency drives, UPS systems, and process control equipment.
Load Type Affects Sizing More Than Most Buyers Expect
The ampere calculation above accounts for steady-state load. Motor loads add a layer of complexity because motors draw significantly more current at startup than they do during normal operation. Large motors can pull six to eight times their rated current for the first few seconds of a start cycle, and that inrush load hits the transfer switch as well as the generator.
In facilities with large motor loads, including HVAC compressors, pump motors, and production equipment, the ATS and generator both need to be sized to handle the largest anticipated inrush event without tripping or derating. Ignoring inrush when sizing a transfer switch is one of the most common installation errors on industrial projects, and it typically surfaces as nuisance tripping or switch contact damage within the first year of operation.
Critical load management is the other variable. If the transfer switch serves only a defined critical load panel rather than the full facility distribution, the sizing calculation changes accordingly. Sizing to the critical load rather than the full facility load is common in healthcare, data center, and process-sensitive applications where NFPA 110 compliance governs emergency power system design. NFPA 110 sets specific performance requirements for transfer time, load restoration sequence, and testing protocols for life safety and emergency standby systems.
Time Delay Settings and Transfer Sequence
Automatic transfer switches include programmable time delays that control how the system behaves during both loss of utility power and restoration. These settings affect both the generator and the electrical system connected to the switch.
The most common delay settings include:
- Time delay on engine start (TDES): Prevents the generator from starting during momentary power fluctuations. Typically set between 2 and 10 seconds depending on how sensitive the application is to brief interruptions.
- Time delay on transfer to emergency (TDTE): Holds the transfer until the generator has reached stable voltage and frequency. Typically 10 to 15 seconds after generator start signal.
- Time delay on retransfer to normal (TDTR): Delays switching back to utility after power is restored, allowing the utility supply to stabilize before the transfer occurs. Typically 5 to 30 minutes depending on the application.
- Time delay on engine cooldown (TDEC): Keeps the generator running unloaded after retransfer to allow proper cooldown before shutdown. Typically 5 minutes.
These settings are configured at commissioning and should match the generator manufacturer’s recommendations alongside the specific requirements of the connected load. Facilities with NFPA 110 compliance requirements will have specific maximum transfer time thresholds that constrain how the delays can be set.
Sizing Mistakes That Create Problems After Installation
The most common ATS sizing errors are predictable, and most of them stem from specifying the switch before the load calculation is complete.
Matching the ATS to generator nameplate kW without converting to amps at the actual system voltage is the most frequent error. The nameplate rating does not account for voltage, power factor, or phase configuration, all of which affect the ampere load the switch will carry.
Selecting a three-pole switch when a four-pole is required creates a compliance problem that often only surfaces during inspection or after a neutral switching fault. Confirming the neutral switching requirement before purchase avoids an expensive correction later.
Underestimating motor inrush, particularly on facilities with rooftop HVAC units or large pump systems, leads to contact wear and nuisance faults. If the load profile includes large motors, the electrical engineer needs to account for starting current in the ATS specification, not just running load.
Finally, specifying an open transition switch for a facility with VFDs, medical imaging equipment, or precision manufacturing process controls creates problems that show up as equipment faults or damage during transfer events. Those applications need closed transition or a bypass isolation switch to handle the load transfer without disruption.
What Turnkey Industries Can Tell You About the Generator Side of This Decision
Sizing the ATS correctly starts with knowing the generator’s confirmed output at rated voltage and power factor, and that data has to come from the generator itself, not from a nameplate estimate or a prior installation’s assumption. Every used generator sold through Turnkey Industries goes through a 22-point inspection and load bank test, and the resulting documentation includes confirmed output data at rated load conditions. That data is what an electrical engineer needs to complete an accurate ATS sizing calculation before any switch is ordered.
Buyers comparing generator options by output range can use the Shop by kW tool to filter available inventory to the right capacity for their application. The used industrial generators inventory includes inspection-backed units across a wide range of output levels from major manufacturers. For buyers working through the generator and ATS specification at the same time, contact the Turnkey Industries team for confirmed generator output documentation before the switch is ordered.
