Generator Transfer Switch vs Interlock Kit: How to Connect a Generator to Your House

What a transfer switch does, and why backfeeding is dangerous

A transfer switch lets a generator feed your home’s circuits while keeping it disconnected from the utility lines. CPSC warns never to power house wiring by plugging a generator into a wall outlet, a practice called backfeeding; it puts utility workers and neighbors served by the same transformer at risk of electrocution and bypasses some household circuit protection.^[CPSC] OSHA says connecting a generator to a building’s wiring without a properly installed transfer switch can energize lines for great distances, and that a qualified electrician should install it.^[OSHA]

The electrical code says the same thing. NEC 702.5 requires transfer equipment for standby systems connected to premises wiring, and that equipment must be listed and installed so the generator and the utility can never be connected at the same time.^{[NEC 702.5]}

Manual transfer switch: how it works

A manual transfer switch is a small panel, wired to the circuits you choose, that you flip to generator power after starting a portable generator. An electrician mounts a power inlet box outside, runs a cable to the switch and wires it to your selected circuits.^{[This Old House]} Generac’s HomeLink manual switches are made for portable generators, prevent back-feeding, and can be upgraded later for use with a Generac home standby generator (up to 11 kW).^{[Generac HomeLink]}

• Amps: HomeGuide says most smaller portable generators are 30 or 50 amp, and the switch amperage should match the generator.^[HomeGuide] Match the inlet and cord to your generator’s outlet; your electrician will pick parts with the same plug type and amp rating.
• Circuits: HomeGuide says a 6-circuit switch covers essentials in most homes, while 10 circuits or more suits larger generators.^[HomeGuide]
• Why choose it: because the circuits are pre-selected, it limits overload risk compared with an interlock.^{[This Old House]}

Generator interlock kit: is it legal and code-compliant?

Yes, when the kit is listed for your panel. IAEI says panelboard interlock kits are considered transfer equipment under NEC 702.5 and can be used with portable generators; they are evaluated as part of the panel under UL 67 and must be identified on the panel’s markings for that specific kit.^[IAEI] Consumer Reports says an electrician can tell you whether an interlock works with your panel and meets local codes.^[CR]

With an interlock, the inlet feeds a breaker in your main panel, and a metal plate makes it impossible to turn on the main breaker and the generator breaker at once.^[HomeGuide] You can power any circuit up to the generator’s capacity, but you have to manage the load yourself by turning circuits on one at a time.^{[This Old House]}

Automatic transfer switch for standby generators

An automatic transfer switch comes with a home standby generator and needs no action from you. Generac says it detects an outage, signals the generator to start, turns on within seconds, and turns off when grid power returns.^[Generac] It is part of the standby install covered in our home generator installation guide; for brand pricing see Generac generator cost, and for options compare best home generators.

Transfer switch installation cost in 2026

Most homeowners pay about $400 to $1,300 for a manual transfer switch, $400 to $850 for an interlock kit, or $600 to $2,500 for an automatic transfer switch, installed.^[HomeGuide] HomeGuide puts labor at $200 to $500 for a straightforward job (2 to 4 hours), a power inlet box at $20 to $100 and a heavy-duty cord at $30 to $350.^[HomeGuide] Consumer Reports budgets $500 to $1,500 for a transfer switch with installation and says an interlock is significantly cheaper.^[CR] Costs rise if the panel needs work first; HomeGuide lists a new panel at $850 to $2,500.^[HomeGuide]

How to connect a portable generator to your house safely

Have a licensed electrician install the inlet and the transfer switch or interlock, then follow the same switching order every outage. This is how the setup is used, not a guide to wiring it yourself.

1. Hire a licensed electrician. Have a licensed electrician install a power inlet box outside and either a manual transfer switch or a listed interlock kit at your panel, with the permit your area requires.
2. Place the generator outside, far from the house. Run the generator outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents.
3. Connect the cord to the inlet. With the generator off, plug a generator cord rated for the generator outlet and inlet into both.
4. Switch the house off the utility. Turn off the main breaker (an interlock blocks the generator breaker until you do) or set the manual transfer switch circuits to off.
5. Start the generator and switch to generator power. Start the generator, then turn on the generator breaker or move the transfer switch to generator.
6. Add circuits one at a time. Turn essential circuits on one at a time, such as the fridge, furnace and well pump, and stay within the generator rating. Reverse the steps when utility power returns.

Distance and exhaust rules are from CPSC^[CPSC]; the switching order follows This Old House and HomeGuide.^{[This Old House][HomeGuide]} Never run a generator in a garage, even with the door open. Install a battery-backed CO alarm.^[CPSC]

Do I need a permit for a transfer switch?

Usually, yes. HomeGuide says most areas require a permit and inspection for changes to a home’s electrical system, at $50 to $350,^[HomeGuide] and Generac says most jurisdictions require an electrical permit and inspection for a transfer switch.^[Generac] The electrician normally pulls it.

What about battery power stations?

If you plug appliances straight into a power station or generator with outdoor-rated extension cords, you don’t need a transfer switch.^[CPSC] Feeding home circuits from any backup source still needs transfer equipment installed by an electrician. Compare plug-in options in best portable power stations, and check your state’s outage minutes at outages by state.

Keep reading

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