Selecting an automatic transfer switch requires more than matching a switch to the size of a generator. The correct ATS needs to fit the electrical load it will carry, system voltage and phase, pole and neutral configuration, available fault current, transfer requirements, installation environment and the way normal and alternate power sources are configured.

For facilities replacing an existing transfer switch or sourcing equipment for a new critical-power project, defining these requirements before looking at available equipment can significantly narrow the options.

CI Group maintains automatic transfer switch inventory across a wide range of amperages and configurations for commercial, industrial, data center and other critical-power applications.

How Do You Size an Automatic Transfer Switch?

Automatic transfer switch sizing should begin with the electrical system and load the ATS will serve rather than generator kW alone.

At a minimum, the project should identify:

Required continuous-current or ampere rating
System voltage
Single-phase or three-phase configuration
Number of poles
Neutral and grounding configuration
Available fault current
Required withstand and closing rating
Service-entrance requirements
Transfer type
Enclosure requirements
Bypass-isolation requirements, if applicable
Normal and alternate power-source configuration
Future load requirements

Each of these specifications can affect which transfer switch is appropriate for the application.

1. Start With the Required ATS Amperage

The ATS must have sufficient current capacity for the load it is expected to transfer and carry.

That means the starting point should be the electrical load and circuit requirements rather than simply selecting an ATS based on the generator’s advertised kW capacity.

Two facilities with similarly sized generators may require different transfer switches because their system voltages, connected loads, distribution arrangements or future expansion requirements are different.

Once the required amperage has been established, CI Group’s ATS inventory can be narrowed by capacity:

From there, equipment can be further evaluated by voltage, phase, number of poles, manufacturer, transfer configuration, enclosure and other project requirements.

Why Generator kW Alone Is Not Enough

Generator capacity is important, but generator kW and ATS amperage describe different parts of the power system.

The ATS carries an electrical load between available sources. Its required ampere rating therefore depends on the circuit and load being transferred.

For larger installations, transfer-switch selection should be coordinated with generators, UPS systems, switchgear and the rest of the electrical distribution system.

This is especially important in facilities where the ATS is one component of a larger critical-power architecture. CI Group’s Data Center Power Infrastructure & Critical Equipment page provides additional information about coordinating UPS systems, generators, transfer equipment and other critical-power systems.

2. Confirm System Voltage and Phase

Once the required amperage has been established, confirm the system voltage and phase.

Automatic transfer switches are manufactured for specific electrical configurations, which can include voltages such as:

120/208V
120/240V
208V
240V
277/480V
480V
600V

The ATS must be compatible with the electrical system on the normal source, alternate source and load side.

System voltage: What voltage is supplied by the normal and alternate sources?

Phase: Is the system single-phase or three-phase?

Frequency: What operating frequency is required by the electrical system?

Source configuration: Does the alternate power source provide the required voltage and phase configuration for the load?

A transfer switch with the correct amperage but the wrong voltage or electrical configuration is not an appropriate substitute.

3. Determine Whether You Need a 3-Pole or 4-Pole ATS

For many three-phase systems, ATS selection also requires determining whether the installation calls for a 3-pole or 4-pole automatic transfer switch.

This decision relates primarily to how the system neutral is handled.

3-Pole Automatic Transfer Switch

A typical three-phase 3-pole ATS switches the three phase conductors while the neutral remains connected.

Whether this configuration is appropriate depends on the grounding and neutral arrangement of the electrical system.

4-Pole Automatic Transfer Switch

A 4-pole ATS switches the neutral along with the three phase conductors.

A switched-neutral configuration may be required in certain systems depending on the generator grounding arrangement, whether the alternate source is treated as a separately derived system, ground-fault protection and other electrical-design considerations.

Factors that can affect the 3-pole versus 4-pole decision include:

Utility grounding configuration
Generator grounding
Neutral current
Ground-fault protection
Separately derived system requirements
Electrical-code requirements
Project engineering specifications

The project’s electrical engineer, electrical contractor or other qualified professional should confirm the required pole and neutral configuration before equipment is purchased.

4. Check Available Fault Current and ATS Withstand Rating

The ampere rating tells you how much current the ATS is designed to carry during normal operation.

It does not, by itself, determine whether the ATS is suitable for the available short-circuit current at the point where it will be installed.

Automatic transfer switches also have a withstand and closing rating, commonly referred to as WCR.

The equipment needs to be coordinated with:

Available fault current
Upstream breakers or fuses
Overcurrent-protection configuration
Required short-circuit rating
Project electrical specifications

This means two transfer switches with the same amperage, voltage and pole configuration may not necessarily be interchangeable.

Fault-current requirements should therefore be established before selecting equipment, particularly for larger commercial, industrial and data center applications.

5. Determine Whether Service-Entrance-Rated Equipment Is Required

Where the ATS sits within the electrical system can also affect equipment selection.

If the transfer switch will be installed at the facility’s service entrance, the application may require service-entrance-rated automatic transfer switch equipment.

A service-entrance-rated ATS differs from equipment installed downstream from the building’s service disconnect and may incorporate additional disconnecting and overcurrent-protection requirements.

Before sourcing equipment, confirm:

ATS location within the electrical system
Whether an upstream service disconnect already exists
Whether service-entrance-rated equipment is specified
Required overcurrent protection
Grounding and bonding requirements
Applicable project and code requirements

For a more focused explanation, see What Is a Service-Entrance-Rated Automatic Transfer Switch?

6. Identify the Required Transfer Type

Not every ATS transfers a load between sources in exactly the same way.

Depending on the facility and load requirements, project specifications may call for:

Open-transition transfer
Delayed-transition transfer
Closed-transition transfer
Bypass-isolation configuration
Automatic or non-automatic operation

The appropriate configuration depends on how sensitive the load is to an interruption and how the normal and alternate power systems are designed to operate.

Bypass-Isolation Automatic Transfer Switches

A bypass-isolation ATS incorporates additional switching capability that can allow the primary transfer mechanism to be isolated for maintenance or service while another path remains available.

This can be particularly useful where maintaining power availability during transfer-switch maintenance is an important part of the facility design.

For more information, see What Is a Bypass-Isolation Automatic Transfer Switch and When Is It Used?

7. Consider the Installation Environment

The physical environment where the ATS will be installed also affects equipment selection.

Before sourcing a transfer switch, determine whether it will be installed:

Indoors
Outdoors
In a conditioned electrical room
In an exposed or potentially wet location
In an industrial environment
At a service entrance

The required enclosure should match the intended installation environment.

Physical dimensions matter as well. Larger ATS equipment, bypass-isolation configurations and service-entrance-rated units can require substantially more installation space than smaller standard transfer switches.

When replacing existing equipment, verify that the replacement can be accommodated within the available space and electrical layout.

8. Check Conductor and Connection Requirements

Even when the primary electrical ratings appear correct, connection requirements can affect whether an ATS fits the project.

Before purchasing equipment, review:

Conductor size
Number of conductors per phase
Copper or aluminum conductor requirements
Lug configuration
Cable-entry space
Neutral connections
Grounding connections
Existing feeder arrangement

This is especially important during a replacement project where new equipment needs to integrate with existing conductors and distribution infrastructure.

9. Consider Manufacturer and Control Requirements

Manufacturer and controller requirements can also influence ATS selection, particularly when equipment is being integrated into an existing facility.

CI Group carries ATS equipment from multiple manufacturers, and different product families can vary in controls, available configurations, transfer operation and application features.

For projects evaluating ASCO equipment, see ASCO 7000 Series vs. ASCO 300 Series Automatic Transfer Switches.

That comparison can help narrow the equipment family before evaluating the specific amperage, voltage, pole configuration and transfer requirements of the project.

10. Include Future Load Growth in the Discussion

A transfer switch may remain in service through multiple changes to a facility.

Before sizing the ATS strictly around today’s requirements, consider known plans for:

Facility expansion
Additional critical loads
Data center expansion
AI or GPU infrastructure
Larger UPS systems
Increased cooling or mechanical loads
Generator expansion
Electrical-distribution changes

This does not mean automatically selecting oversized equipment. It means known future requirements should be included in the engineering and equipment-selection process rather than discovering later that the ATS has become a limiting component.

11. Consider Inspection, Maintenance and Replacement Requirements

ATS selection is not only about initial installation.

Transfer switches contain mechanical, electrical and control components that need to remain operational for the equipment to perform when normal power is lost.

For existing equipment, the project may need to determine whether the current ATS should be maintained, repaired or replaced rather than simply selecting new equipment based on its existing nameplate.

For a broader discussion, see Automatic Transfer Switch Maintenance: Inspection, Testing & Replacement Considerations.

For the more specific question of inspection frequency, see How Often Should an Automatic Transfer Switch Be Inspected and Tested?

CI Group also provides Automatic Transfer Switch Service & Maintenance for existing critical-power equipment.

Automatic Transfer Switch Sizing Checklist

Before requesting ATS pricing or availability, gather as much of the following information as possible.

Electrical Requirements

Required ampere rating
System voltage
Single-phase or three-phase
Frequency
3-pole or 4-pole requirement
Neutral configuration
Available fault current
Required withstand and closing rating

Source Information

Normal power source
Alternate power source
Generator voltage and configuration
Existing upstream protection

Application Requirements

Service-entrance rated or non-service rated
Required transfer type
Bypass-isolation requirement
Indoor or outdoor installation
Required enclosure
Critical-load requirements

Project Information

New installation or replacement
Existing ATS manufacturer and model, if applicable
Preferred manufacturer
Required delivery schedule
Equipment condition requirements
Future expansion plans
Project location
Electrical one-line or equipment specification, if available

Having this information available before reviewing inventory makes it much easier to narrow the search to equipment that fits the application.

Browse Automatic Transfer Switches by Amperage

If the required ATS amperage has already been established, review CI Group’s available automatic transfer switch inventory by capacity:

Available inventory varies by manufacturer, amperage, voltage, pole configuration, enclosure, transfer type, condition and other specifications.

Related Automatic Transfer Switch Resources

ASCO 7000 Series vs. ASCO 300 Series Automatic Transfer Switches

Compare two commonly used ASCO transfer-switch families and the factors that may affect equipment selection.

Automatic Transfer Switch Maintenance: Inspection, Testing & Replacement Considerations

Review inspection, testing, maintenance and replacement factors for existing ATS equipment.

What Is a Bypass-Isolation Automatic Transfer Switch and When Is It Used?

Learn how bypass-isolation equipment differs from a standard ATS and when this configuration may be considered.

What Is a Service-Entrance-Rated Automatic Transfer Switch?

Understand how the location of the ATS within the electrical system affects service-entrance requirements.

How Often Should an Automatic Transfer Switch Be Inspected and Tested?

Review the factors that influence automatic transfer switch inspection and testing schedules.

Need Help Identifying ATS Equipment for a Project?

Automatic transfer switch sizing involves more than choosing an amperage range.

Before equipment is selected, the project should confirm the required load, voltage, phase, pole configuration, grounding arrangement, fault-current requirements, service-entrance requirements, transfer type and installation conditions.

CI Group can help identify available automatic transfer switch equipment based on established project specifications and coordinate ATS sourcing with related generators, UPS systems and critical-power equipment.

Browse Automatic Transfer Switch Inventory

For an existing transfer switch that needs inspection, testing, repair or maintenance, visit Automatic Transfer Switch Service & Maintenance.

Automatic Transfer Switch Sizing Guide: Amperage, Voltage, Poles & Application Requirements