Transfer switches allow safe and convenient connections between homes and businesses to backup-power generators. With the switch in the utility position, they supply power from the electric utility. When the switch is moved to the generator position, power is supplied by the generator. Transfer switches may supply only a few critical circuits or an entire electrical system, depending on their rating and that of the generator.
One very important function of any transfer switch is to disconnect the electric utility from any circuit supplied from the generator. This prevents the generator from feeding utility lines and endangering workers. It also keeps utility power and generator power separate once power is restored by the electric utility.
An Automatic Transfer Switch (ATS) is able to operate in conjunction with a standby generator to supply power without human intervention. Standby generator manufacturers make automatic transfer switches that work with their generators to provide specific features and power management capabilities.
Before making any purchase, ensure that the ATS you select is compatible with the generator you purchase, or choose a generator package that includes an automatic transfer switch.
These automatic transfer switches only control the flow of power from the utility and the standby generator. During normal operation, they are in the utility position and power is supplied from the electric utility. When an outage occurs, they switch the power source to the generator and disconnect the utility.
Power from the utility is taken directly from the utility meter, or from a circuit breaker inside the main service panel. The switch connects to a separate load center which contains circuit breakers that distribute power to individual branch circuits.
Switch + Load Center
This is a load center that includes an automatic transfer switch and reduces the number of electrical boxes, which simplifies installation. Power enters the ATS from the utility company or main panel, and from the generator. The switch selects between generator and utility power for distribution to circuit breakers located on the power buss-bars inside the load center.
Some hybrid versions will supply a limited number of circuit breakers from generator power, and all the circuit breakers from utility power. A service-entrance-rated switch plus load center can replace an entire main service panel.
Service Entrance Rated
A service entrance is where electrical power enters the building from the electric utility. This includes the wires between the electric utility meter and main service panel or main disconnect. A Service Entrance (SE) rated ATS is one that was specifically manufactured to handle the incoming electrical connections from the electric utility and provides a way to shut off electrical power from the utility.
Some jurisdictions require a separate disconnect or shutoff switch for use with SE rated transfer switches and other service entrance equipment.
A main service panel has a main circuit breaker rating greater than the normal power demand. This means that a home or business with a 200-amp main breaker will rarely, if ever, use that amount of current. A generator that could supply 200 amperes is unnecessarily expensive to purchase and install, and uneconomical to operate. A better choice is to install a smaller generator that can supply the normal use requirements. With the addition of a managed power option, the same generator can also supply appliances that are heavy users of electricity, such as air conditioners and deep-well pumps.
Standby generators are often packaged with an ATS that includes power management to control appliances that draw large amounts of power. Some use a scheme called load shedding that will turn off certain circuits to ensure other circuits have enough power to operate. Another method imprints an appliance’s power requirements, then keeps the appliance from starting if there is not enough generator power available. Still others will only allow one high demand appliance from operating at any given time.
Managed power options are available for both smaller and larger capacity standby generators, whether they supply an entire building with power, or only essential circuits.