Transfer switch vs interlock kit: how to connect a generator to your house

Extension cords work for a fridge and a lamp. But a furnace, a well pump, or anything hard-wired needs power delivered through the house wiring. Doing that safely requires equipment that makes it impossible for utility power and generator power to meet.

Why backfeeding is dangerous

Some people connect a generator to a dryer outlet with a double-male “suicide cord.” This is called backfeeding. When the generator energizes house wiring while the main breaker is on, power flows out through the service drop and the utility transformer, which steps it up to high voltage on the lines outside. Line workers who think the line is dead can be electrocuted. NFPA and OSHA both warn never to plug a generator into a wall outlet. The exposed male prongs of such a cord are also live and can shock you.

The National Electrical Code requires transfer equipment so the generator and utility cannot be connected at the same time. Local codes and permits apply.

Option 1: Manual transfer switch

A manual transfer switch is a small subpanel mounted next to your main panel. An electrician moves selected circuits (typically 6 to 10, such as the fridge, furnace, well pump, and a few lights and outlets) into the switch. An inlet box on the outside wall accepts a generator cord.

During an outage you plug the generator into the inlet, start it, and flip each circuit’s switch from “line” to “generator.” Each switch can only be on one source at a time.

Pros: simple, prevents overloading by limiting which circuits can be powered, often includes wattmeters. Cons: only the pre-wired circuits get power; installation cost is usually higher than an interlock.

Option 2: Breaker interlock kit

An interlock is a sliding metal plate installed on the front of your main panel. It physically prevents the main breaker and a dedicated generator backfeed breaker from being on at the same time. An electrician adds the generator breaker and an outdoor inlet.

During an outage you switch the main breaker off, slide the plate, switch the generator breaker on, and then turn on only the circuits you want, managing the load yourself.

Pros: any circuit in the panel can be used; usually less expensive. Cons: you must manage the load by turning breakers off and on; it must be a kit listed for your specific panel.

Option 3: Automatic transfer switch (standby generators)

Home standby generators use an automatic transfer switch that senses an outage, starts the generator, and switches the load within seconds, then switches back when utility power returns. Some include load-shedding modules that briefly turn off an air conditioner or water heater to avoid overloading.

Which one should you choose?

• If you want a few essential circuits and simple operation: manual transfer switch.
• If you want flexibility and are comfortable managing breakers: interlock.
• If you want hands-off operation and whole-house power: standby generator with an automatic transfer switch.

In all cases, the inlet and cord must match the generator’s outlet (commonly an L14-30 for 30-amp 120/240 V generators) and the generator must supply 240 V to power 240 V circuits.

Hire a licensed electrician

This is not a DIY job for most homeowners. The work involves the service panel, permits are usually required, and the installation must match the panel’s listing. A properly installed switch is what makes your generator plan safe. Then size the circuits you selected with the generator size calculator.

How each option is used during an outage

Manual transfer switch (circuit-selector type)

1. Place the generator outdoors at a safe distance and connect the generator cord to the inlet box.
2. Start the generator and let it warm up.
3. On the transfer switch, flip each selected circuit from "LINE" to "GEN," one at a time, starting with the largest motor loads.
4. When power returns, switch circuits back to "LINE," then shut down the generator.

Interlock kit

1. Connect the generator to the inlet box and start it.
2. Turn off the main breaker. The interlock plate then lets you turn on the generator backfeed breaker.
3. Turn on the generator breaker, then turn on individual branch breakers you want to power, one at a time, staying within the generator's capacity.
4. To return to utility power, reverse the steps: generator breaker off, then main on.

With an interlock, your entire panel is available, so load management is up to you. With a circuit-selector switch, you're limited to the circuits wired to it, which naturally prevents overloading.

The inlet box and cord

Both setups usually use an exterior power inlet box, a weatherproof male inlet mounted on the outside wall, with wiring to the transfer switch or the interlock breaker. A matching generator cord connects the generator's locking outlet, often an L14-30 (30-amp, 120/240 V) or a 50-amp outlet, to the inlet. Make sure the inlet, cord, and generator outlet all match in configuration and amperage, and that the breaker protecting the circuit matches the generator and cord.

Placing the inlet box away from windows and doors helps keep the generator a safe distance from the home. Ready.gov advises keeping generators at least 20 feet from windows, doors, and attached garages.

Permits, inspections and code

Installing a transfer switch or interlock is electrical work that usually requires a permit and inspection. Interlock kits must be listed for your specific panel model to be code-compliant in most jurisdictions. Some inspectors also require the backfeed breaker to be secured with a hold-down clip. Check with your local building department, and have a licensed electrician do the work. The cost of a permit is small compared with the risk of a fire or a fatal backfeed.

What about "generator-ready" panels and smart panels?

Some newer electrical panels and smart panels have built-in generator inputs or integrated transfer functions, and some meter-base transfer devices are approved by certain utilities. These can simplify installation. Ask your electrician and utility which options are approved in your area.

Cost comparison

Prices vary widely by region and panel, but in general:

• Interlock kits are the least expensive in parts. Installation is typically a few hours of electrician time plus the inlet box.
• Manual transfer switches cost more in parts and take more labor, because circuits must be rerouted.
• Automatic transfer switches are part of standby generator systems and cost the most.

Common mistakes to avoid

• Backfeeding with a "suicide cord" (a double-male plug). This is extremely dangerous and illegal in many places. OSHA warns that backfeeding can endanger utility workers and neighbors served by the same transformer.
• Forgetting the main breaker with a homemade setup that has no interlock.
• Undersized cords or inlets for the generator's output.
• Powering too many circuits at once through an interlock, overloading the generator.
• Skipping the permit, which can complicate insurance claims after a fire.

A properly installed switch or interlock is the single biggest safety upgrade for anyone who plans to power hard-wired household loads from a portable generator.