An automatic transfer switch must suit the load it serves, the electrical system around it and the way the facility transfers between sources. Choosing an ampere rating is only one part of that decision. Voltage, phase arrangement, fault-current capability, neutral treatment and maintenance access can make two switches with the same current rating unsuitable for the same installation.
For a data center, the practical starting point is the approved single-line diagram and a calculation of the load passing through each switch. A generator's total output or a building's main-breaker rating cannot, by itself, establish the correct rating of every downstream ATS.
This guide explains the inputs procurement teams should collect before requesting equipment. The project electrical engineer must establish the final selection under the locally adopted codes and the manufacturer's instructions.
Key Takeaways
- Size the ATS for its assigned load and installation requirements, then verify protection and source compatibility.
- When calculating three-phase current from kVA, do not divide by power factor again.
- Continuous current and fault-current capability are separate ratings. Both must be suitable.
- Closed transition requires two acceptable sources; it does not eliminate the initial interruption after a utility failure.
- Compare quotations for the complete configuration. An ampere rating alone does not establish a price band.
Start with the load boundary#
Draw a boundary around the equipment supplied through the ATS. Is it the entire service, one generator-backed distribution board, a UPS input or a cooling branch? Identify what remains connected during normal operation and what can be transferred, shed or restored in stages during an outage.
A generator may supply several transfer switches. Each switch has its own load and protective arrangement; it does not automatically need to match the generator's full output. Conversely, a switch supplying a large normal-source load may need to carry more current than a smaller emergency generator can provide. That arrangement requires an appropriate emergency-load strategy.
Request a load schedule that identifies:
- Running demand and the basis for any permitted demand or diversity factors.
- Continuous loads and the applicable sizing treatment.
- Motor starting, restarting and transfer behavior.
- UPS input demand, including losses and battery recharge where applicable.
- Loads added in later phases and the point at which an upgrade becomes necessary.
- Load-shedding controls, priorities and failure behavior.
Measured demand can help validate the schedule, but a historical peak is not a substitute for the required design calculation. It may omit future equipment or an operating condition that has not yet occurred.
Calculate current using the correct units#
For a balanced three-phase system, using line-to-line voltage:
Current in amperes = kVA × 1,000 ÷ (√3 × voltage)
If the input is real power in kW instead:
Current in amperes = kW × 1,000 ÷ (√3 × voltage × power factor)
Power factor is already reflected in the relationship between kW and kVA. Applying it again to a kVA input overstates the calculated current.
| Illustrative electrical load | Calculation | Calculated line current |
|---|---|---|
| 50 kVA at 208 V, three-phase | 50,000 ÷ (√3 × 208) | Approximately 139 A |
| 40 kW at 208 V, three-phase, 0.8 power factor | 40,000 ÷ (√3 × 208 × 0.8) | Approximately 139 A |
| 500 kW at 480 V, three-phase, 0.9 power factor | 500,000 ÷ (√3 × 480 × 0.9) | Approximately 668 A |
These are arithmetic examples, not final switch selections. They assume balanced loads and do not establish the required frame, conductor size, protective device or fault rating. Use actual equipment input data where available rather than assigning a default power factor.
For a single-phase load, current equals watts divided by voltage and power factor. A mixed three-phase installation also needs phase-by-phase assessment; a balanced formula can conceal an overloaded phase.
Separate continuous current from protection requirements#
ASCO's explanation of transfer-switch functions distinguishes carrying current from interrupting overcurrent. A transfer switch connects a load to a source; protective devices clear faults. The manufacturer describes ATS equipment intended to carry its rated service current continuously.
That does not make all parts of an assembly interchangeable. Breaker ratings, conductor sizing, terminals, ambient conditions and the complete equipment listing still matter. Do not apply an automatic 80% loading limit to every ATS, or assume that a 100%-rated switch makes every associated breaker and conductor suitable for the same loading.
Likewise, a generic 10–25% allowance is not a complete engineering method. Growth allowance should be documented separately from applicable code calculations and transient-performance requirements. Buying a larger frame may be justified, but it should have an identified purpose.
For equipment at the service entrance, verify the assembly's suitability for service-equipment use and the required disconnect, protection and grounding arrangement. ASCO's service-entrance application paper addresses that equipment category. A service-entrance designation is not established simply by matching an amperage label.
Check withstand and closing ratings#
The available fault current at the installation point must be checked against the ATS's applicable withstand and closing rating, often called WCR. ASCO's UL 1008 guidance explains the rating framework.
Request the actual rating documentation for the proposed assembly. The engineer should confirm the applicable voltage, protective-device conditions and clearing time for each relevant source arrangement. A headline kA value without its conditions is insufficient.
For example, a documented 30 kA capability under a particular protective arrangement does not establish suitability at a location with 40 kA available under that arrangement. Selecting a switch with more continuous amperes does not automatically resolve the short-circuit issue.
The quotation should identify any required upstream fuse or breaker and the settings or limitations on which the selection depends. Check those requirements against the project's protection and coordination studies before accepting a substitute.
Evaluate motors, UPS inputs and the neutral arrangement#
A motor's running current does not describe its behavior during starting or reconnection. ASCO's motor-load guidance identifies transfer-related inrush as an application concern. Request load-specific starting and transfer data, and coordinate the switching sequence with the generator's transient capability.
For UPS-backed systems, use the UPS's input characteristics. Output IT load alone omits input losses and may omit recharge demand. SecondWatt's UPS sizing guide explains the distinction between kW, kVA and redundancy.
Do not specify a 200% neutral simply because the facility contains UPS systems or variable-frequency drives. Determine neutral loading from the actual circuit and harmonic assessment. The choice between a solid and switched neutral also depends on grounding and bonding, including whether the alternate source is treated as separately derived. Have the engineer establish the pole arrangement before sourcing a used switch.
Select transition mode and maintenance access separately#
| Configuration | What it does | Question to resolve |
|---|---|---|
| Open transition | Disconnects one source before connecting the other | Can the supported load tolerate the interruption, or is ride-through provided elsewhere? |
| Delayed transition | Includes an intentional disconnected interval | Does the load require time before reconnection? |
| Closed transition | Briefly connects both acceptable sources during a permitted transfer | Are synchronization, protection and utility requirements satisfied? |
| Bypass-isolation | Provides an alternate path and isolation arrangement for maintenance | Can the proposed operating procedure maintain the required service safely? |
ASCO's transition-mode paper introduces open and delayed transition. Its bypass-isolation design paper addresses a separate maintenance feature.
Closed transition is not an uninterruptible power supply. When the normal source has failed and the generator has not yet become available, overlapping two healthy sources is impossible. Specify outage response separately from planned testing and retransfer behavior. For downstream IT distribution, also review ATS versus STS selection.
Distinguish NFPA 110 Type from Level#
The ten-second timing classification is Type 10. It should not be described simply as a consequence of “Level 1.” Type addresses the permitted interruption interval; Level addresses the consequences of system failure, as distinguished in Cummins’ NFPA 110 training presentation. Cummins' explanation of NFPA 110 time to readiness describes the Type 10 timing basis.
The applicable system classification and locally adopted requirements must be established for the project. Generator starting, acceptable voltage and frequency, controls and transfer operation all contribute to the restoration sequence. An ATS operating-time specification alone does not demonstrate whole-system compliance.
What an ATS quotation should include#
An equipment-only price becomes comparable only after the scope is defined. Request the continuous current, voltage, frequency, poles, transition mode, WCR basis, service-equipment status, bypass arrangement, enclosure, controls and communications.
For used equipment, add the model and serial number, manufacturing information, photographs, drawings, maintenance records, test results and a list of missing or modified components. Confirm controller support and replacement-parts availability for the actual assembly.
Separate purchase price from removal, freight, engineering, cable adaptations, installation, testing and commissioning. This article does not establish a verified market-wide ATS price range or a standard percentage premium for closed transition or bypass-isolation.
Frequently Asked Questions#
What size ATS do I need for a 200 A service?#
A whole-service application requires a service-equipment selection appropriate to that service and its design. A switch serving only selected downstream loads has a different boundary. Confirm the load calculation, protection and assembly requirements rather than using the service label alone.
Must every ATS match the generator's amperage?#
No. A generator may serve several separately protected load branches. Each ATS must suit its assigned circuit and load, with source capacity and any load management checked at system level.
Is a 50 kVA, 208 V three-phase load about 175 A?#
No. The balanced three-phase calculation gives approximately 139 A. Dividing by 0.8 power factor again would incorrectly treat the kVA input as though it were kW.
Does closed transition keep the load powered through an outage?#
It can avoid a transfer interruption when both sources are acceptable and permitted to overlap. It does not supply energy while a failed utility source is unavailable and the generator is starting.
What should I send with an ATS sourcing request?#
Send the approved equipment schedule, single-line diagram, load calculation, fault-duty requirements and transition sequence. Include the required condition and delivery location. These inputs are more useful than an ampere rating alone.
Compare the surrounding switchgear, generators and UPS systems, then submit an equipment request with the project specification. Availability and commercial terms require confirmation for each proposed unit.