The automatic transfer switch—or ATS—is what makes a standby generator system a fully automatic appliance.
They are to a standby generator what a thermostat is to a furnace. Just as a thermostat turns the furnace on and off as needed to regulate temperature, the ATS controls the generator to supply power when needed and manage power to ensure that essential appliances are able to run as needed.
Not all automatic switches are the same and selecting one that works best for your needs is just as important as correctly sizing your generator.
Transfer Switch Overview
A transfer switch controls the power source. When one power source is selected, the other power source is isolated from the system. This is important because it prevents one source from feeding the other. In a residential or commercial system, it prevents a generator from endangering utility workers by unexpectedly energizing the utility lines. It also protects the generator from damage when power is restored by the utility.
As the name implies, automatic transfer switches work automatically. When the power goes out, they can switch between utility power and generator power as required. A manual transfer switch requires someone to physically move the switch from one position to the other.
Automatic Transfer Switches come in two main types. One contains only the transfer switch and the included options. It manages the power source for a panel of circuit breakers which may serve an entire building or just a few essential circuits. The other contains both the transfer switch and the circuit breakers the switch serves.
Most circuits in a home or business are 120-volt circuits limited to 15 or 20 amperes. A few appliances require 240 volts and as much as 50 amperes.
Managing the 240-volt circuits prevents overloading a generator. Because a 20-kilowatt standby generator supplies 80 to 85 amperes at 240 volts, adding one or two high-voltage appliances to the demand can create an overload condition.
An ATS with managed power can prevent the overload by allowing the appliances to run only when power is available. If one central air unit is already running, and another wants to start, the managed power option forces the second unit to wait until the first stops.
Other loads that include electric water heaters, well pumps, electric ranges, or electric dryers are also managed, often in tiered system that prioritizes their importance.
Service Entrance Rating
According to the National Electrical Code, a service entrance must incorporate a main disconnect. The service-panel’s main circuit breaker performs this function in many homes. A separate main disconnect in its own panel is incorporated in temporary buildings or in some jurisdictions.
The main disconnect has requirements that exceed the capability of common circuit breakers. In order to serve as service entrance equipment, a transfer switch must incorporate these requirements into its design. The Service Entrance (SE) rating allows a transfer switch to replace the existing equipment already performing this function, which simplifies installation and operation.
Automatic transfer switches must safely carry their maximum rated current whether the current is coming from a generator or from the electric utility.
Even if the generator installed can only supply 80 amperes of current during an outage, an ATS installed as a service entrance in a home with 200 ampere utility service must safely handle 200 amps of current, even if the home never uses that much.
Conversely, if an ATS will only serve a limited number of circuits and operates as a sub-panel, it only requires a rating that will handle the maximum current possible. A 50-ampere circuit breaker in the main panel can feed a 50-ampere ATS during normal operation as long as the generator’s main breaker is also 50 amperes or lower—neither the utility or the generator can supply the switch with more than 50 amperes.
Most manufacturers supply automatic transfer switches designed to work with their standby generators. Each incorporates design features specific to the generators they manufacture. Choosing a transfer switch that was manufactured for use with a specific generator brand is usually the best way to go.
Package deals bundle a generator and ATS together. If you plan to purchase a 20-kilowatt standby generator for a home with 200-ampere service, selecting a package with a 200-ampere SE-rated transfer switch and 20 kilowatt standby generator is probably your best option. The features provided by the generator and by the transfer switch will govern your decision.