How an Automatic Transfer Switch Works
An automatic transfer switch (ATS) is the critical component that connects your standby generator to your home electrical system. It serves three essential functions: monitoring utility power status, managing the transition between utility and generator power sources, and preventing dangerous backfeed that could electrocute utility workers. Without a transfer switch, a generator cannot legally or safely connect to your home wiring. Here is exactly how the ATS works at each stage of a power outage.
Continuous Utility Monitoring
The ATS monitors your incoming utility voltage 24 hours a day through sensors connected to your electrical service entrance. It checks voltage level and frequency continuously, looking for drops below the safe operating threshold (typically below 80% of nominal voltage). This monitoring happens in real time with no delay, which is why the system can respond to an outage within seconds.
Outage Detection and Generator Signal
When the ATS detects that utility voltage has dropped below the programmed threshold or disappeared entirely, it initiates the startup sequence. The ATS sends an electronic start signal to the generator controller through a low-voltage signal cable that runs between the two units. Most transfer switches include a brief delay (typically 2 to 5 seconds) to filter out momentary voltage dips and prevent unnecessary generator starts for split-second interruptions that resolve on their own.
The Transfer Process
Once the generator reaches stable operating speed and voltage, the ATS performs the actual transfer in a specific sequence designed for safety. First, the ATS opens the utility side contactors, physically disconnecting your home from the grid. This step happens before the generator side connects, creating a brief moment where your home is connected to neither source. Then the ATS closes the generator side contactors, connecting your home to generator power. This break-before-make design ensures your home is never connected to both the utility and the generator simultaneously, which is a critical safety requirement that prevents backfeed.
Types of Transfer Switches
Whole-House ATS (200-Amp)
A 200-amp whole-house automatic transfer switch passes your entire electrical panel through to the generator. Every circuit in your home receives power during an outage. This is the most popular configuration for 22kW and larger generators because the generator has enough capacity to handle the full panel load. Generac, Kohler, and Cummins all offer 200-amp whole-house ATS units designed to pair with their respective generators.
Load-Managed ATS
A load-managed transfer switch connects to specific circuits rather than the entire panel. It intelligently manages which loads receive power, cycling non-critical circuits on and off to keep total demand within the generator capacity. This configuration pairs with smaller generators (10kW to 16kW) that cannot handle an entire 200-amp panel simultaneously. The load management is automatic and prioritizes critical circuits like HVAC, refrigeration, and lighting.
Service Entrance Rated ATS
Some transfer switches are service entrance rated, meaning they can serve as both the transfer switch and the main disconnect for your electrical service. This simplifies installation in homes where the electrical panel and service entrance are in the same location and can eliminate the need for a separate main disconnect breaker.
Return to Utility Power
The ATS monitors the utility feed continuously even while the generator is running. When utility voltage returns and stabilizes, the ATS begins a return sequence. Most units wait 5 to 10 minutes of stable utility power before initiating the transfer back, preventing premature switching if the utility power is intermittent or unstable. The return transfer follows the same break-before-make sequence: disconnect from generator, brief pause, connect to utility. After the transfer, the ATS signals the generator to shut down. The generator typically runs for a short cool-down period before the engine stops.
Why an ATS Is Required by Code
The National Electrical Code (NEC) requires a transfer switch for any generator that connects to a building electrical system. The primary reason is backfeed prevention. If a generator feeds power into the utility grid through the electrical panel without a transfer switch, it energizes the supposedly dead power lines outside your home. Utility workers repairing those lines expect them to be de-energized. Backfeed has caused fatalities. The ATS makes backfeed physically impossible by ensuring the utility and generator connections can never be closed at the same time.
Matching Your ATS to Your Generator
Transfer switches must be compatible with your specific generator brand and model. Generac ATS units are designed for Generac generators, Kohler ATS units for Kohler generators, and so on. The communication protocol between the ATS and generator controller is brand-specific. Purchasing a matched generator and transfer switch kit from the same manufacturer ensures compatibility, simplifies installation, and is the recommended approach.
IB Lighting carries matched generator and ATS kits from Generac and Kohler. Call (800) 674-9019 for help selecting the right configuration for your home. Free freight plus insurance included.