Generator transfer switch vs interlock: which is right?

Both do the same critical job

A transfer switch and an interlock kit both let you safely power your home from a portable generator, and both exist to stop one deadly problem: backfeed. Backfeeding through a dryer outlet with a double-ended cord sends power back up the utility line and can kill a lineworker restoring your neighborhood. Both of these are the code-legal way to avoid that, and both require a permit and inspection in Oregon.

The difference is how they hand power to your circuits. A transfer switch is a separate device that powers a fixed, pre-selected set of circuits. An interlock is a metal plate on your existing panel that lets you back-feed the whole panel through a dedicated breaker while mechanically making it impossible to have the generator and the utility on at the same time.

The manual transfer switch

A manual transfer switch is a small sub-panel installed next to your main panel. During installation an electrician moves the circuits you care about, furnace, fridge, well pump, a few lights, some outlets, over to it. When the power goes out you plug the generator into an inlet, then flip those circuits from utility to generator on the switch.

• Powers a fixed set of circuits, usually 6 to 12, chosen at install
• Simple and idiot-proof to operate: flip the labeled switches
• Each circuit has its own meter, so you can watch your generator load
• You are limited to the circuits it was wired for, so plan them carefully

The interlock kit

An interlock kit is a bracket that mounts on your existing panel. It ties a new generator breaker to the main breaker with a sliding plate so that only one can be on at a time. To run on generator power you turn off the main, slide the plate, and switch on the generator breaker, which energizes the whole panel from a generator inlet. It is the more flexible and usually cheaper option, with a catch: the kit has to be listed for your exact panel make and model, and not every panel has one available.

• Lets you run any breaker in the panel, one circuit or many, as long as you stay within the generator's output
• You manage the load yourself: turn off the big draws and juggle what runs at once
• Costs less than a transfer switch in most cases
• Requires a listed kit for your specific panel brand, which not every older panel has

How to choose

• Want the simplest possible operation and a set-and-forget list of circuits: a transfer switch
• Want flexibility to power different things on different days, and your panel has a listed kit: an interlock
• Have an older or oddball panel with no interlock available: a transfer switch, or pair the work with a panel upgrade
• Want the generator to start and switch over on its own during an outage: that is an automatic transfer switch, which pairs with a permanently installed standby generator

Installed right, in Oregon

Whichever you pick, this is permitted, inspected work, and the inlet, the breaker sizing, and the interlock or switch all have to be done to code. A safe generator installation also means a properly rated inlet box, the correct gauge cord, and a generator run outdoors, never in a garage, because of carbon monoxide. We size the setup to your generator's real output and the loads you need to keep running, pull the permit, and coordinate the inspection. Backfeeding with a homemade cord is illegal and lethal, so do not go that route.