Automatic Transfer Switch Buying Guide

Installing a home standby generator is a complex project that delivers seamless backup power. Crucial to this system is the automatic transfer switch (ATS). The ATS is the "brain" that senses a power outage and triggers your generator to start, powering your home as if nothing happened.

An ATS and a home standby generator must be used in conjunction, as the ATS is essential for operation; a manual transfer switch used for portable generators will not work.

How Does an Automatic Transfer Switch Work?

An automatic transfer switch connects your standby generator to your home's main electrical panel, automatically managing the flow of electricity.

The ATS constantly monitors utility power. When it detects an outage, it automatically switches your home from utility power to generator power. Once utility power is restored, the ATS switches your home back, then commands the generator to shut down.

The switch is also vital for safety, ensuring that power can never be sent back up the utility line—protecting utility workers and preventing electrical damage.

Types of Automatic Transfer Switches

Automatic transfer switches range from 50 to 400 amps and vary based on how much of your home you need to power.

Load Center Switches

These switches, also called essential circuit switches, are ideal if you only want to power your home's basic, critical appliances. They typically energize 8 to 16 critical circuits from your main breaker panel.

Load center switches work well for older homes with 100-amp service or lower. An electrician must wire the individual essential circuits from the main panel to the ATS, which can result in higher labor costs.

Service Disconnect Switches

This is the simplest option for true whole-house backup, powering your entire breaker panel. It is the right choice when the 16-circuit limit is not enough.

The switch is installed directly between your electric meter and your main breaker panel. Although the switch costs more upfront, installation is less labor-intensive because there is no wiring of single circuits.

If you have 400-amp service with two separate 200-amp breaker panels, you will need two 200-amp service disconnect switches to be code-compliant.

Sizing an Automatic Transfer Switch

Proper sizing ensures your generator can handle the required electrical load.

Sizing Service Disconnect Switches

The amperage of the switch must match the amperage of the main breaker in your electrical panel (typically located at the top of the box). If you have a 200-amp main breaker, you need a 200-amp ATS.

Sizing Load Center Switches

These switches do not have to be sized for your entire panel since they only power specific circuits. The required amperage will vary based on how much you want to back up.

Power Management (Load Shedding)

Most ATS units today include power management capability. This technology manages and prioritizes multiple electric loads (like multiple central air conditioners), temporarily turning off or "shedding" non-critical appliances when the generator detects high demand. This prevents the generator from overloading.

By understanding the difference between automatic transfer switch types and matching the amperage to your home, you can ensure your backup power system runs smoothly whenever you need it most.