An automatic transfer switch (ATS) is a device that transfers a power supply from its primary source automatically to a backup generator when it senses a failure or outage occurring in the primary source until utility power is restored.

The ATS is connected to both primary and backup power sources, and serves as an intermediary between the power sources and your building or equipment, acting as an electrical relay. An ATS can also act as a redundant, rack-mounted power supply for equipment that is connected to a power source with only one cord.

How does a transfer switch work?

The main breaker in the transfer switch switches the power source from your utility power to a standby generator. The building cannot be connected to both the generator and utility power at the same time. The main breaker transfers power from one source to the other. This prevents the generator power from back-feeding through the utility lines and utility power from back-feeding into the generator.
The transfer switch also contains a number of circuits. You can assign loads to different circuits to power different appliances or rooms. Depending on the size of your generator, it may not be large enough to power everything you have on every circuit at the same time. In these situations, using a transfer switch will make it easy to switch between different loads. For example, to use your washing machine during a power outage, you could simply turn off the circuit powering your AC or other appliance and then turn on the circuit powering your washing machine. When you are done using the washing machine, you would do the reverse. In the event of a power outage, a transfer switch makes it easy to manage your power usage and helps you to maximize your generator’s effectiveness.

Why do I need a transfer switch?

A transfer switch is required by the NEC for any connection of power to a home. Truly, using an ATS is the only safe way to directly connect a generator to your home. An automatic transfer switch isolates your home from the power lines. This prevents back-feed, which occurs when the power goes back down the utility lines. Backfeed can not only damage the generator but has the potential to cause fires. Worse, back-feed could electrocute workers working on the power lines trying to restore utility power causing injury or even death.

A transfer switch is not only the safest way to connect a generator to your home, but it’s also the easiest. Running extension cords to and from appliances can be an awkward and time-consuming hassle especially during an outage. A transfer switch lets you use your home’s wiring system to power any appliance in your home with your generator quickly and easily. Also, some items like furnaces or well pumps can’t be connected to a generator with an extension cord, so a transfer switch is required to power these items during a utility power outage.