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.