Buyer Guide: Backup Generator Automatic Transfer Switch

Written By: MJ Logan

A Residential Automatic Transfer Switch (ATS) makes a permanent connection between a home and a standby generator in a way that protects the home, the generator, neighbors, and utility line workers. The ATS and the generator work together as a system to automatically supply electricity during a power outage.

A Standby Generator Supplies Power Automatically During an Outage

Manufacturers often package a home generator with automatic transfer switch. Purchasing a generator and ATS together as a combined package is less expensive than ordering them as separate components. There are many configurations possible and making a selection is not always straightforward. Budget restraints and possible future needs may add to the uncertainty.

Three main types of Generator Automatic Transfer Switches are most common in the United States:

  • A whole house transfer switch supplies the entire main breaker panel with electrical power during an outage.
  • A generator sub-panel switch supplies a sub-panel with a limited number of circuits during an outage. The sub-panel and switch are often combined into a single unit.
  • A service-rated load-center automatic transfer switch takes the place of the main panel and incorporates the main disconnect.

Standby Backup Generator Terms and Concepts To Understand

Electrical power requirements and budget determine the size home generator with automatic transfer switch required. Determine your minimum and maximum power needs, then choose a home backup generator and residential automatic transfer switch to meet your needs and your budget.

Power Outages and Blackouts go hand in hand with power surges cause by lightning and voltage spikes. Surges enter your home and cause damage to appliances and electronics. The damage is often gradual until the appliance or device fails. Consider a whole house surge protector to eliminate these damaging spikes and surges.

Standby Generator Buyer’s Guide: What Size Generator Do I Need?

Indoor or Outdoor Rated

Generac 200-AMP Service Rated Automatic Transfer SwitchA residential automatic transfer switch has a rating for indoor use only, or indoor and outdoor use.

An ATS with a NEMA 1 rating is for indoor use only. It is only suitable for use and installation in dry areas, though it will protect against an accidental indirect splash. The case is not watertight and will not keep the elements out. The primary function protects against accidental contact with electrical parts.

A NEMA 3R weatherproof transfer switch allows for either indoor or outdoor installation. The weatherproof transfer switch is water resistant and suitable for use in a location where it is exposed to rain or snow and is undamaged by ice. Some indoor installations such as a laundry room require a 3R rating when exposure to condensation or splashing water is possible.

Any type of residential automatic transfer switch may have either a NEMA 1 or NEMA 3R rating.

Sizing Guide for Home Standby Generators

Whole House Transfer Switches

Briggs and Stratton Whole House Automatic Transfer SwitchA whole house transfer switch installs between the utility meter and the main service panel. When an outage occurs, the ATS disconnects the incoming utility service, then connects the standby generator. The switch makes an intelligent choice about which electric supply to use. The utility is the preferred choice. When utility power is unavailable or unacceptably low, the whole-house transfer switch automatically chooses standby generator power after the generator starts and is ready to accept the load—typically less than 20 seconds after the outage occurs.

As a rule, the transfer switch current rating in amps must meet or exceed the main breaker rating in amps. A 200-Amp ATS can work with any panel rated at 200 amps or less, which includes 100-Amp and 150 Amps panels. However, the reverse is not true. A 100-amp transfer switch cannot work with 200-amp or 150-amp panel.

The main service panel the ATS supplies determines the switch’s rating. A main panel with a 200-amp main breaker requires at least a 200-amp whole house transfer switch. A 100-amp main breaker requires at least a 100-amp ATS.

Benefits of a Backup Generator Automatic Transfer Switch

Service Entrance Rated

Briggs and Stratton 100-Amp ATS with Symphony II Power ManagementA whole house transfer switch with a Service Entrance (SE) rating means that the ATS may serve as the primary disconnect for the home’s utility service. The ATS includes a circuit breaker that turns off the utility service to the home.

All homes are required to have a primary disconnect. Sometimes this is a fused switch located near the utility meter. In this configuration, the main panel might not have a main breaker and turning off the power requires turning off the disconnect switch. Other homes have a main panel that includes a main circuit breaker as the primary disconnect.

A backup generator automatic transfer switch installed for use as a whole house transfer switch requires a service entrance rating or a separate, primary disconnect located on the utility side of the switch.

Automatic Transfer Switches for Standby Generator Systems

Power Management

Kohler RXT Automatic Transfer Switch Shown with Service Entrance, Load Center, and Standard ModelsA Generator Automatic Transfer Switch with Power Management allows a standby generator with less lower capacity to supply a home with a number of high-voltage loads such as air conditioners, water heaters, dryers, well pumps, electric ranges and other 240-volt appliances.

Some appliances use large amounts of power and could overload a standby generator if too many are operating at the same time because a generator supplies a limited amount of power. For example, a 20-kilowatt standby generator can supply a maximum of 20,000 watts continuously. If another appliance added to the load exceeds 20,000 watts, the generator’s main breaker may trip and turn off power to the entire home.

Understanding Generator Watts, Volts, and Amps

Large appliances such as central air conditioners use large amounts of power. A 3-ton AC unit uses approximately 3500 watts which varies dependent on efficiency. In a home with two AC units to total power requirement is 7000 watts, plus the starting watts required to get them started. With both running at the same time, the generator may not have the power to supply everything else.

Power management options allow the transfer switch to manage the power. If one AC unit is running, the second may have to wait for the first one to finish before it starts. If the home is using very little power when both units start, power management may allow both to start and run at the same time. The benefits of using power management include a lower initial cost for the home generator with automatic transfer switch and reduced fuel costs any time the generator operates.

An ATS with power management may prioritize heavy loads. The AC and well pump might have first priority while an electric water heater and dryer have a lower priority. Lower priority loads wait for higher priority loads to finish or until additional power becomes available.

Some high-voltage loads operate via a low-voltage contactor such as a thermostat for a furnace and air conditioner. The thermostat turns the furnace and air conditioner on and off by opening and closing low-voltage relays that control the high-voltage current. Many whole house switches with power management capability include the means to operate these loads.

Other high-voltage loads such as hot water heaters and dryers simply switch on and off. They require a different strategy for managing power use. A relay module, sometimes called a Power Management Module, is controlled by the automatic transfer switch to turn power on and off to the appliance.

Did you know that power surges slowly damage electronics in large appliances and small devices like phones and tablets? Over time, the damage accumulates until the device or appliance fails. A Surge Protection Device installed in your Transfer Switch or Main Panel protects your whole house from power surges.

Energy Monitoring

New to Automatic Transfer Switches, the Generac PWRview 200 Amp Automatic Transfer Switch provides real-time energy monitoring to help homeowners cut energy costs and reduce their carbon footprint. The PWRview Energy Monitor works with the free PWRview App app and works 24/7 whether the generator is running or not. See the effect starting a large appliance has on energy use. Learn which appliances cost the most money to run. Alter energy use to take advantage of off-peak rates and save hundreds off your energy bill.

Generac PWRview Automatic Transfer Switch

Residential Generator Packages with ATS Power Management

Backup Generator Sub-Panel Automatic Transfer Switches

Generac NEMA 3R 100-Amp 16 Circuit ATSA Sub-Panel Automatic Transfer Switch installs between the main service panel and the loads it controls. Under normal operating conditions, the ATS is supplied with power from the main service panel through a circuit breaker located in the main panel. During a power outage, the generator sub-panel ATS disconnects itself from the main panel and connects to the standby generator to supply its circuits with electrical power.

The circuit breakers for the circuits served by this type of ATS are located in a generator sub-panel. The ATS supplies the circuit breakers with power from either the main panel or from the generator depending on the current state of the utility power.

Two types of generator sub-panel transfer switches are common:

  • Automatic Transfer Switch only.
  • Automatic Transfer Switch Load Center.

Home Backup Generators to Power Essential Circuits

Automatic Transfer Switch Only

Cummins Power Generation 100-Amp ATSThese residential automatic transfer switches supply power to a separate sub-panel with circuit breakers. The sub-panel may have only a few breakers or it may have many. An ATS designed as a whole house transfer switch without a service entrance rating may serve as this type of switch except the utility power comes from the main service panel instead of the electric utility meter.

This type of configuration allows the flexibility to separate the circuits that will receive power during an outage from those that will not while incorporating power management for high voltage loads like air conditioners.

The circuit breaker that supplies the ATS serves as its disconnect. The main breaker or external disconnect also turns off utility power to the automatic transfer switch.

Understanding Generators: 5 Terms You Should Know

Automatic Transfer Switch Load Center

Generac 100-Amp 16-Circuit Pre-Wired Load Center ATSInstead of installing a separate sub-panel served by a backup generator automatic transfer switch, the Load-Center ATS incorporates both into the same electrical box which saves space and simplifies wiring.

A load center ATS may have only a few circuits or many. Eight, 10, 12, and 16 circuit configurations are common. An automatic transfer switch load center sold as a Pre-Wired Transfer Switch may come with the circuit breakers installed and will include the wiring necessary to connect it to the main panel and the standby generator. Purchasing a pre-wired transfer switch ensures using the correct wire gauge for each circuit and eliminates or reduces the need for purchasing spooled wire.

The number of circuits available refers to the number of single-pole (120-volt) circuit breakers it can accommodate. Double-pole (240-volt) circuit breakers require two spaces. Double-pole breakers look like a pair of single-pole breakers with the switch handles connected. An electric dryer (240-volts) will require two spaces for a double-pole, 30-amp circuit breaker. The kitchen refrigerator circuit (120-volts) will only require one space for a single-pole 15- or 20-amp circuit breaker.

Some circuits require special breakers. AFCI and GFCI breakers usually require a separate purchase and installation. The installer must supply any special breakers made by the correct manufacturer and of the correct type. Some breakers have special wiring requirements.

Choose the Right Residential Automatic Transfer Switch

Service-Rated Load-Center ATS

GenReady 200-Amp Service Entrance Rated Load Center ATSA Service-Rated Load-Center ATS can replace the home’s main service panel. The main circuit breaker inside an automatic transfer switch load center acts as the service disconnect. The transfer switch mechanism connects to the utility meter and to the standby generator. Breakers serve the branch circuits in the same manner as the breakers in a regular main service panel.

This type of automatic transfer switch load center is a good choice for including a standby generator in the design of a new home. No additional wiring for a generator sub-panel or separate load center ATS is necessary since it is already included in the installation and there are no electrical boxes to figure out where to locate.

Another possibility incorporates the SE-Rated Automatic Transfer Switch Load Center into a service upgrade. Older homes often have limited utility service. A new main service panel is required for an upgrade to meet newer building codes. Incorporating a standby generator installation in the upgrade is easy and reduces future costs because an electrician doesn’t have to take the current wiring apart to install a new transfer switch.

Backup Generator with Automatic Transfer Switch Installation Guide


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