Generator interlock kit vs transfer switch: what's the difference?

If you want your generator to power the furnace, well pump or kitchen circuits, you need a safe way to connect it to your home's wiring. There are two common ones: a generator interlock kit and a manual transfer switch. Both do the same core job. They make sure your house can never be connected to the generator and the utility lines at the same time.

This page explains how they work so you can talk to an electrician with confidence. It is not a how-to. This is licensed-electrician work.

Why you need one at all: backfeed

When a generator is tied into house wiring without a proper switch, its power can flow out of your house and onto the utility lines. That's called backfeeding.

OSHA warns it "can energize wiring systems for great distances," putting utility workers and others at risk of electrocution. Generac says it can injure or kill line workers who are trying to restore power, and can cause shocks and damage your home's wiring.

Never do these:

• Never plug a generator into a wall outlet or dryer outlet.
• Never use a double-ended "suicide cord" or "backfeed cord."
• Never wire a generator into your panel without a transfer switch or listed interlock.

OSHA's rule is plain: don't attach a generator to a building's electrical system "unless a qualified electrician has properly installed the generator with a transfer switch."

What is a transfer switch?

A manual transfer switch is a small panel mounted next to your main panel. An electrician moves a handful of important circuits into it, like the fridge, furnace, sump pump and some lights.

During an outage you plug the generator into an outdoor power inlet box, then flip each switch from "Line" to "Generator." Champion's FAQ explains that a transfer switch isolates generator power from utility power and lets selected circuits run.

• Only the pre-wired circuits can get generator power.
• It's hard to overload by mistake, because you picked circuits to match the generator.
• It's listed as a unit. One maker's 50 A manual switch, for example, is certified to UL 1008, the standard for transfer switches.

An automatic transfer switch switches over by itself, and some can also tell the generator to start. Those are usually part of a permanent standby setup, not a portable one.

What is a generator interlock kit?

An interlock kit is a metal plate that mounts on your existing main panel's cover. It slides so that the main breaker and a generator breaker can never be on at the same time. To use the generator, you must switch the main off first.

One interlock maker describes it as two sliding plates that block either breaker, "but never allowing both to be in the on position simultaneously." A few details matter:

• Kits are panel-specific. A kit for one panel brand won't fit another, even at the same amp rating.
• Look for a listing. Choose a kit listed by a testing lab, such as UL or MET, for your panel.
• It needs a generator breaker in the panel, and usually a breaker retainer to hold it in place, plus an outdoor inlet box.
• Every circuit stays available. That's the big plus, and the catch.

Interlock vs transfer switch at a glance

	Interlock kit	Manual transfer switch
Where it goes	On your existing main panel cover	A separate small panel beside it
Circuits you can power	Any circuit in the panel	Only the circuits wired to it
Overload risk	Higher: you manage breakers by hand	Lower: chosen at install
Install work	Usually less	More wiring
Needs a panel-specific part	Yes	No
Who installs	Licensed electrician	Licensed electrician

With an interlock, you have to be careful. Turn off big loads like the AC, electric water heater or dryer, then turn on circuits one at a time. Our generator size calculator above helps you see what your generator can carry.

Do you need a permit?

In most places, yes. This is a change to your home's electrical system. NFPA says a transfer switch should be installed by a qualified electrician "in accordance with the National Electrical Code® (NEC) and all applicable state and local electrical codes." One interlock-kit seller tells buyers to check with their town about local requirements before ordering.

Ask your electrician to pull the permit and have the work inspected.

Grounding and 240 V: things your electrician will check

• Grounding and neutral. Generac notes that when a generator feeds a panel or transfer switch, grounding rules change. Some generators have a floating neutral and others a bonded one. Your electrician will match the setup to your generator.
• 240 V circuits. A well pump or other 240 V load needs a generator with a 240 V outlet, like an L14-30 or 14-50. A 120 V-only generator can't run it. See 30 amp vs 50 amp generator cords.
• Inlet size. The inlet box, cord and generator breaker must all match your generator's outlet and amp rating.

Which one should you choose?

• Choose a transfer switch if you want it simple and hard to overload, and you know which circuits matter.
• Choose an interlock if a listed kit exists for your panel and you want the option to power any circuit. You must be willing to manage breakers by hand.
• Either way, hire a licensed electrician, get the permit, and ask them to walk you through the switch-over steps.

Your generator still has to run outdoors, at least 20 feet from the house, even when it's connected by an inlet box. Read our generator carbon monoxide safety guide.