Transfer switch vs. interlock kit: which is right for your generator?
Short answer
Both are safe, approved ways to connect a generator to your home's wiring, and both prevent dangerous backfeeding. A transfer switch is a separate panel that moves a set of chosen circuits from utility power to generator power. An interlock kit is a mechanical plate on your main panel that physically prevents the main breaker and the generator breaker from being on at the same time. Interlocks are usually cheaper and let you choose any circuit; transfer switches are simpler to operate and work with any panel. Either one should be installed by a licensed electrician, typically with a permit.
Why you need one at all
Plugging a generator into a wall outlet with a double-ended cord, often called a "suicide cord," sends power backward through your panel and out onto the utility lines. That can electrocute line workers repairing the grid, start fires and destroy the generator when power returns. It is illegal in most places. A transfer switch or interlock makes it physically impossible for your home to be connected to the generator and the grid at the same time.
How each one works
Manual transfer switch
A small subpanel is mounted next to your main panel. A few important circuits, such as the fridge, furnace, well pump and some lights, are rewired to run through it. During an outage you plug the generator into an outdoor inlet box, then flip each circuit on the transfer switch from "Line" to "Generator." Many units include wattage meters so you can see the load.
Interlock kit
A metal plate is mounted on the cover of your main panel next to a breaker that is fed by the generator inlet. The plate slides so that only one of two things can be on: the main breaker (utility power) or the generator breaker. During an outage you switch off the main, slide the plate, switch on the generator breaker, and then turn on whichever circuits you want, staying within your generator's capacity.
Side-by-side
| Feature | Transfer switch | Interlock kit |
|---|---|---|
| Circuits you can power | Only the ones wired into it (commonly 6 to 10) | Any circuit in the panel, as long as total load fits |
| Typical cost | Higher: the switch plus more wiring work | Usually lower: the plate, a breaker and an inlet |
| Ease of use | Very simple: labeled switches for each circuit | Requires managing breakers yourself to avoid overload |
| Panel compatibility | Works with almost any panel | Must be listed for your exact panel model |
| Space needed | Wall space next to the panel | Two adjacent breaker slots at the top of the panel |
Which one should you choose?
- Choose an interlock if your panel has a listed kit available, you want flexibility to power different circuits at different times, and you are comfortable flipping breakers to stay within your generator's limit.
- Choose a transfer switch if you want the simplest possible switchover for other people in the household, your panel has no approved interlock, or you only ever need a fixed set of essentials.
- Consider an automatic transfer switch only with a home standby generator. It detects an outage and starts the generator on its own.
Both options need an outdoor generator inlet box and a cord rated for your generator's 240 V outlet. Size the generator first with the generator size calculator.
What to ask your electrician
- Is there an interlock kit listed for my panel, and does my local code accept it?
- Do I need a permit and an inspection? In many areas the answer is yes.
- What size inlet and cord match my generator's outlet (for example L14-30 or 14-50)?
- Which circuits should be on the generator, and what will the starting load be?
- Does my furnace or well pump need anything special to run on generator power?
This is not a DIY job. Work inside an electrical panel can be fatal, and a mistake can backfeed the grid. Have a licensed electrician do the installation, and keep the generator itself outdoors, at least 20 feet from the house.
Frequently asked questions
Is an interlock kit as safe as a transfer switch?
When it is listed for your panel and installed correctly, yes. Both make it physically impossible for the generator and the utility to be connected at the same time.
Do I need a permit to install a generator interlock or transfer switch?
In many US jurisdictions, yes. Your electrician will know the local requirements and can arrange the permit and inspection.
Can I connect a generator to my house without a transfer switch?
Not safely through your wiring. Without a transfer switch or interlock, the only safe option is to plug appliances directly into the generator with outdoor-rated extension cords.
Can a power station use a transfer switch?
Some larger power stations with a 240 V or 30 A output can feed a transfer switch or inlet, and some brands sell their own home panel kits. Check the manufacturer's instructions and have an electrician install it.
Sources and method
Runtime figures assume 85% of battery capacity reaches AC appliances and typical US appliance ratings. See exactly how we calculate. Found a mistake? Tell us and we will fix it.