Generator interlock kit

Using an interlock kit during a power outage

At the top of the panel, the steel plate sits between the main breaker and the 2-pole generator breaker. While the main breaker is on, the plate stops the generator breaker from moving, so the two cannot both be turned on by accident.

When the power fails, follow these steps:

1. Start the generator outdoors and connect its cord to the inlet box mounted on the outside wall.
2. Switch the main breaker off, since the house has to be separated from the street before generator power reaches it.
3. Slide the plate into place and switch on the generator breaker.
4. Once utility power comes back, switch off the generator breaker and turn the main back on.

Order matters because of back-feeding. Back-feeding happens when generator electricity travels out through house wiring onto street lines, and that can kill a lineworker who is fixing the fault. Sending power back that way without a transfer switch or an interlock breaks the law, and sending it back through a dryer outlet is never safe.

Is an interlock kit right for your generator?

With a portable generator, it usually makes sense. The kit lets that portable unit supply any circuit in your panel, so you pick the loads to power as you go; it also usually costs less than a manual transfer switch. By contrast, a manual transfer switch serves only the circuits wired to it.

Nothing in the interlock caps how much power you draw, so keeping the running watts below your portable's rating is up to you. A home standby generator is a different story, because an interlock will not work with one. Standby units need an automatic transfer switch, which detects the outage and changes over in about 10 seconds with nobody there to operate it.

Finding a kit that fits your panel

Every kit is designed for a single panel brand and a single panel layout, so a Square D panel calls for a Square D-style kit rather than a general-purpose design. Panels from Square D, Eaton, Siemens, GE and Leviton each need their own matching kit. A kit from the panel's own manufacturer is the safest pick for passing an inspection.

Making room for the generator breaker

Before buying, check whether your panel has room in the right spot. The generator breaker needs to occupy the slot directly beside the main breaker so the plate can move across it. When space in your panel runs short, the electrician may need to shift a circuit or two to clear that slot.

Most places require a licensed electrician and a permit. Under the NEC, a listed interlock counts as transfer equipment, and some towns want to inspect it before the first use.

Comparing interlock and transfer switch costs

That installed price range covers the kit, inlet, breaker, cord and labor needed to plug in a portable generator. The automatic switch prices only matter if you're buying a home standby generator, which needs that switch instead of an interlock.

Staying safe with a portable generator

Always run the generator outdoors, at least 20 ft away from doors, windows and vents, and make sure carbon monoxide alarms keep working inside your home. That 20 ft distance and the indoor alarms follow CDC and CPSC advice, and they help protect your family from carbon monoxide.

Choose a cord that suits both the inlet and the generator. Never use a cord with a plug on each end.