Interlock Kit vs Transfer Switch

How each one works

Interlock kit. A metal plate or sliding bracket mounts on your panel's cover. It physically blocks the main breaker and a back-fed generator breaker so only one can be on at a time. To switch over, you turn off the main, slide the plate, and turn on the generator breaker. After that, you pick which circuits run by flipping breakers in the panel you already have.

Manual transfer switch. A small separate panel goes next to your main panel. An electrician moves a set of chosen circuits, such as the refrigerator, furnace fan, well pump or sump pump, into it. During an outage you start the generator and flip those circuits from utility to generator one at a time.

Automatic transfer switch. This is the standby generator setup. The switch senses the outage, starts the generator and moves the load without you touching anything.

Side by side

The automatic range is wide because it covers everything from a small essential-circuit switch to large service-entrance units. Generac alone sells automatic switches from 100 A up to 800 A.

Why the interlock wins for most portable setups

It's cheaper, and it doesn't box you in. With a transfer switch you live with the circuits you chose on install day. With an interlock you can run the well pump in the morning, shut it off, and run the microwave at lunch, as long as you stay under the generator's rating.

The catch is that the panel has to have a kit made and listed for it. Eaton's kits, for example, list exact load center catalog numbers and state they meet NEC Article 702. No listed kit, no interlock. That's where the transfer switch earns its price.

When a transfer switch is the better call

• Your panel brand or series has no listed interlock kit.
• You want a setup where nobody in the house has to think about which breakers to turn off. A switch with a short list of labeled circuits is harder to overload.
• You're buying a standby generator. Those run through an automatic transfer switch, which Generac calls the piece that kicks the generator into action.
• You want the option to go automatic later. Generac's HomeLink manual switch has an automatic upgrade for small standby units up to 11 kW.

What both setups still need

Neither device connects the generator by itself. You still need an outdoor power inlet box and a cord, and Champion notes those come in 30-amp or 50-amp versions to match the generator's L14-30 or 14-50 outlet. A whole-house manual switch can be sized the same way: Generac sells 200 A utility switches with either a 30 A or a 50 A generator input.

Both also need the right size generator. If you haven't done that math yet, start with generator sizing.

Permits apply either way. Champion warns that skipping them can mean fines or redoing the install. Check with your local building department before the electrician starts.

What to do next

1. Open your panel door and photograph the label.
2. Check whether a kit is listed for it on our interlock kits by brand page.
3. If one is, get an electrician's quote for the kit, inlet and permit. If not, ask for a manual transfer switch quote instead.
4. If you'd rather never touch a breaker in a storm, price a standby generator with an automatic switch.