Generator interlock kit: how it works

What the interlock does

Your main breaker connects the house to the utility. A portable generator connects through its own breaker on the same panel. If both were on, the generator would push power back out through the meter onto the street lines. Crews working on a dead line could be shocked, and the generator would be overloaded in seconds. The interlock is a sliding metal plate that physically blocks you from turning both on. One slides up, the other is locked off.

Utilities across the country give the same advice: never plug a generator into a household outlet, and have a qualified electrician install a transfer switch or an interlock kit. The interlock is the lower-cost of the two.

What you get for the money

As of October 2026, a published cost guide puts an interlock install at about $400 to $850. Adding an outdoor inlet box and a cord takes the total to as much as $1,800. The next page, generator inlet box, shows what sets that part of the price. The numbers match the typical SoCal range on the generator interlock cost page.

What moves the price up:

• A panel that is full. A generator breaker needs two spaces, and some panels have none free.
• A long run from the panel to the inlet. Stucco, block walls and distance add labor.
• A panel that is old or damaged, so the electrician cannot use a listed kit.
• A trip to the city for inspection if the permit is not combined with other work.

Is there a kit for my panel

Every interlock is listed for a few panel models, and the maker publishes which ones. Many current panels have a kit. Some older brands do not. If you have a Zinsco or Federal Pacific panel, read Zinsco and Federal Pacific panels first. A panel in that condition is usually a replacement job before it is a generator job.

Take a photo of the panel label on the inside of the door and the main breaker. Send it with your quote request. It saves a site visit.

Using it during an outage

Your electrician should leave you a written order of steps taped to the panel. It looks like this:

1. Place the generator outdoors and away from windows, doors and vents. Carbon monoxide kills people every year in garages and on porches.
2. Plug the generator cord into the inlet box.
3. Turn off the big loads: oven, dryer, water heater, air conditioner.
4. Turn off the main breaker and slide the interlock plate over.
5. Start the generator, then turn on the generator breaker.
6. Add loads one at a time.

Reverse the order when power returns. Turn off the generator breaker, slide the plate, turn the main on, then shut the generator down.

Limits of an interlock

You run the house by hand. There is no automatic start, and if the generator runs low on fuel at 3 a.m., the house goes dark. An interlock also puts every panel circuit within reach, so you must know the wattage you are asking of the generator. A critical loads subpanel is a way to limit that risk. A transfer switch feeds only the circuits you choose.

Permit and inspection

An interlock changes your service equipment, so cities normally require an electrical permit and an inspection. Requirements differ by city, so confirm with the building department. Read electrical permits in Southern California for what the inspector looks at. The electrician should pull the permit in their own license name. Do not accept a quote that skips it.

Questions for the electrician

• Is the kit listed for my exact panel model?
• What generator size and inlet amperage do you recommend for my loads?
• Is the inlet and cord included, and who supplies the generator breaker?
• Does the price include the permit and inspection?

Care and testing

Test the setup once before you need it. Start the generator, go through the steps on the electrician’s label, and shut it down. Do it again at the start of fire season. A generator that has sat for a year may not start, and an outage is a bad time to learn that. Store fuel safely, away from the house, and keep a carbon monoxide alarm working inside.