A generator interlock is one of the most practical electrical upgrades a homeowner can make — and one of the most misunderstood. It is not a full transfer switch, but for homeowners who already own a portable generator, it is a safe, code-compliant way to power their home during an outage without the risks that come from improvised connections. Mid Penn Electric installs generator interlocks throughout Lebanon, Lancaster, and Dauphin counties.

How a Generator Interlock Works

A generator interlock kit is a mechanical device installed on your electrical panel. It physically prevents the main utility breaker and the generator input breaker from being on at the same time. This eliminates the possibility of back-feeding power into the utility grid — a serious hazard for utility lineworkers restoring power after an outage. With the interlock in place, you can safely connect your portable generator to a dedicated inlet, select which circuits to power, and run the home normally from generator power until utility service is restored.

We recently installed generator interlocks as part of larger projects — including the Elizabethtown whole-house rewire, where a family of eight wanted reliable backup power in their newly expanded home.

Frequently Asked Questions

What is the difference between a generator interlock and a transfer switch?

A transfer switch is a dedicated panel — either manual or automatic — that isolates selected circuits and switches them between utility and generator power. A generator interlock is a simpler mechanical device installed on your existing panel that achieves the same safety result at a lower cost. An interlock allows you to use your existing panel’s breakers to select which circuits to power. For most homeowners with a portable generator, an interlock is the right solution. A whole-house automatic transfer switch makes more sense if you have a standby generator.

Is a generator interlock legal and safe?

Yes, when properly installed. Generator interlocks are listed by UL and other testing agencies and are permitted under the NEC when installed to manufacturer specifications on a compatible panel. The key safety requirement is that the interlock physically prevents simultaneous connection to both utility power and generator power — eliminating back-feed risk. Improper connections using extension cords or direct wiring are dangerous and illegal. A licensed electrician should handle the installation.

What do I need in addition to a generator interlock?

In addition to the interlock kit, you need a generator inlet box installed on the exterior of your home and a properly rated power cord to connect the generator to the inlet. We install the inlet and interlock together as a complete system. You also need a generator with sufficient wattage to power the circuits you want to run — we can help you think through what that looks like based on your home and generator.