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Automatic Transfer Switches for Standby Generator Systems

Generac Home Standby Generator model 7032 supplying power during an outage.

A Generac Home Generator Keeps the Power On During an Outage

Automatic Transfer Switches (ATS) make permanent and safe connections between homes or businesses and backup power generators. During normal operation the switch is in the utility position and supplies power from the electric utility. When a power outage interrupts power, the generator starts and the switch moves automatically to the generator position to supply power from the generator.

A transfer switch may supply few critical circuits or an entire electrical system. It is important to select the right automatic transfer switch based on power requirements and generator capacity.

One important safety feature of any transfer switch forces it to disconnect the electric utility from any circuit supplied by power from the generator. This prevents the generator from feeding utility lines which may endanger workers or neighbors. It also keeps utility power and generator power separate once the electric utility restores service.

An Automatic Transfer Switch works with a standby generator to supply power without human intervention. Standby generator manufacturers produce various automatic transfer switches that work with their generators to provide specific features and power management capabilities. Together, the ATS and the backup generator work automatically without human intervention.

Always select a compatible ATS to work with your standby generator or choose a generator package that includes an automatic transfer switch.

Switch Only

Kohler ome Standby Generator System with 200A SE Rated ATS

Kohler Home Standby Generator System with 200A SE Rated ATS

These automatic transfer switches 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 disconnect the utility and switch the power source to the generator.

They can supply power from the utility meter to the entire main service panel, or from a circuit breaker inside the main panel to a sub-panel. If they serve the main panel, then all circuits within the panel receive electricity from the generator during an outage. In the case of a sub-panel, only the circuits served by the sub-panel receive power.

In either case, only the circuits served by the automatic transfer switch receive power from the generator during an outage.

Switch + Load Center

Kohler Standby with 12 circuit automatic transfer switch

A Kohler 8kW Standby Generator with 12-Circuit Load Center ATS

An ATS that includes a load center in the same electrical box simplifies installation by reducing wiring and electrical boxes. Power enters the automatic transfer switch load center from the utility company or the main panel during normal operation, and from the generator during an outage. The power is then distributed to the circuit breakers installed in the load center that make up the circuits in the home or business.

A service-entrance-rated automatic transfer switch has a main breaker to disconnect the panel from the utility supply. A service rated automatic transfer switch with a load center can replace an entire main service panel. A non-service rated switch can serve the same purpose with an external fused disconnect.

Manufacturers often package 8-kilowatt generators and 10-kilowatt generators with ATS Load Centers that hold limited number of circuit breakers to keep essential circuits supplied with power during an outage. 100-amp ATS load centers and 50-amp ATS load centers with 8, 10, 12, or 16 circuit breaker positions are common for this purpose.

Service Entrance Rated

Guardian Home Standby Generator Installed Outside a Home

A Standby Generator and Service Entrance Rated ATS Powers All the Appliances.

A service entrance is where electrical power enters the building from the electric utility. This includes the wires between the electric utility meter and the 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 backup generator bundled with a Service Rated ATS can supply an entire house or small business with power during an outage. An ATS with power management can allow smaller and more fuel efficient standby generators

Power Management

Briggs and Stratton 20 kW Standby with 200-amp ATS and power management.

Briggs and Stratton 20-Kilowatt Standby with 200-Amp SE Rated ATS and Symphony II Power Management

A main service panel has a main circuit breaker rating greater than the normal power demand. A home or business with a 200-amp main breaker will rarely, if ever, use that amount of current. Purchasing a generator with a 200-amperes capacity is unnecessarily expensive and uneconomical to operate. A better choice uses a smaller generator that supplies the amount of power normally required. With the addition of an automatic transfer switch with power management, the same generator can also supply heavy users of electricity such as air conditioners, hot water heaters, deep-well pumps and other 240-volt appliances.

A standby generator bundled with an ATS that includes power management controls the operation of 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 240-volt appliance to operate at any given time.

Power Management options are available for smaller and larger capacity standby generators, whether they supply an entire home or business with power or only essential circuits.

Updated October 13, 2017

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