Generator transfer switch
A transfer switch lets a generator power your home's circuits while keeping the house disconnected from the utility grid — so you're on utility power or generator power, never both.[2]
if you want a generator to run anything hard-wired — a furnace, a well pump, a sump pump on its own circuit — you need a transfer switch or an approved interlock, installed by a licensed electrician, usually with a permit.[1] Plugging a generator into a wall or dryer outlet instead ("backfeeding") can kill utility workers and is illegal in many places.[2]
Why you can't just plug a generator into the house
Backfeeding means connecting a portable generator directly to your home's wiring — at the panel or through an outlet. Without a transfer switch, generator power can flow back out onto utility lines that crews believe are dead.[1] Generac lists three consequences: injury or death for line workers, fire and appliance damage when utility power comes back, and code violations.[2] Oregon's building codes division adds that removing an improvised connection after power returns puts the homeowner at risk too.[1]
Your options
| Option | How it works | Who it suits | Watch out for |
|---|---|---|---|
| Manual transfer switch | A separate panel next to your main panel. You plug the generator into an inlet and flip selected circuits from "line" to "generator." | Portable generator owners who want a few chosen circuits (fridge, furnace, sump pump, lights). | Only the circuits wired to it can be powered; you switch by hand each outage. |
| Automatic transfer switch (ATS) | Detects the outage, starts a standby generator and transfers the load without anyone home. | Permanently installed standby generators. | Highest cost; sized to the generator and service. |
| Interlock kit | A mechanical plate on your main panel that physically prevents the main breaker and a generator breaker from being on at the same time. | Portable generators where you want flexibility to choose circuits at the panel. | Must match your exact panel model; whether it's accepted as "transfer equipment" is up to your local inspector.[3] |
What the code requires
- Transfer equipment that prevents paralleling. The National Electrical Code's article on optional standby systems (Article 702) requires transfer equipment that prevents the normal and alternate sources from being connected at the same time, and that equipment must be approved for its intended use.[3]
- Approval is local. For transfer equipment the NEC language requires approval, which means your local authority (the inspector) decides — some may require a field evaluation.[3] This is why interlock kits are accepted in some towns and questioned in others.
- Permit and licensed electrician. Oregon, for example, requires a transfer switch or other safety device, a permit, and a properly licensed electrician for this work.[1] Check your state and city — rules and the adopted NEC edition differ.
Sizing: match the switch to the generator
Start with what you need to run, then pick the generator, then the switch. The generator size calculator gives you running and starting watts for your appliance list; your electrician sizes the switch and inlet to the generator's output.
What it costs
We haven't published price ranges yet: the installed-cost figures circulating online vary several-fold and mostly come from unverified sources. The main cost drivers are:
- the type of equipment (interlock kit, manual switch, automatic switch);
- amperage and the number of circuits you want backed up;
- distance from the panel to where the generator will sit (inlet box and wiring);
- permit and inspection fees in your area.
Get at least two written quotes from licensed electricians that list equipment, labor and permit separately.
Sources
- Electrical Safety: Generator Connections — Oregon Building Codes Division, document dated 2021-04. Accessed 2026-11-03.
- Why you should never back-feed your electric panel with a generator — Generac. Accessed 2026-11-03.
- Transfer Equipment Used in Optional Standby Systems for Commercial Applications, Part I — IAEI Magazine, document dated 2009-05. Accessed 2026-11-03. Discusses the 2008 NEC; check the edition adopted in your area.