The short answer: both a transfer switch and a generator interlock kit let you power your home's circuits from a generator safely, because both make it physically impossible for generator power and utility power to be connected at the same time. An interlock kit is usually the simpler and less expensive option and lets you pick any circuit in your panel. A transfer switch is a separate device that is easier for everyone in the house to use, and it is the only choice if you want backup that turns on automatically. Either one must be installed by a licensed electrician with a permit.
Why you need one at all
Extension cords can run a fridge and a lamp, but not anything hard-wired, like a furnace blower, a well pump or ceiling lights. For those, the generator has to feed your electrical panel, and something must first disconnect your home from the utility lines. Otherwise the generator's power can flow backward out of your house and onto the grid. That is called backfeeding, and the American Red Cross warns that it "puts utility workers, your neighbors and your household at risk of electrocution." It can also start a fire or destroy the generator when utility power returns.
The Red Cross recommends having an electrician install transfer equipment that meets the National Electrical Code (NEC, published by the NFPA as NFPA 70) and local codes. A transfer switch and an interlock kit are the two common ways to do that.
Never backfeed through a dryer outlet or any wall outlet
Online you may see a cord with a plug on both ends used to connect a generator to a dryer or range outlet. Do not do this. It is dangerous and illegal. The exposed prongs are live, nothing stops power flowing out to the utility lines if someone forgets the main breaker, and that outlet was never designed to feed a whole panel. The Red Cross is direct: never try to power the house wiring by plugging the generator into a wall outlet.
What a generator interlock kit is
A generator interlock kit is a metal plate or slide that mounts on the front cover of your existing breaker panel. It works with two breakers:
- Your main breaker, which connects the panel to the utility.
- A dedicated generator breaker (often called the backfeed breaker), which connects the panel to the generator through a cord and an outdoor power inlet box.
The interlock is shaped so that only one of those two breakers can be on at a time. To switch to generator power, you turn the main breaker off, slide the interlock over, and turn the generator breaker on. Because the main is off, nothing can flow back to the grid.
Pros of an interlock kit:
- Every circuit in your panel is available, so you choose what runs during each outage.
- Usually less expensive in parts and labor than a separate transfer switch, because it uses the panel you already have.
Cons:
- You have to manage the load yourself. Every circuit can draw power, so you must switch off the ones you do not want before turning on big loads, or you can overload the generator.
- The kit must be made and listed for your exact panel model. Not every panel has one, and older panels may not.
- It is always manual. Someone has to go to the panel, flip breakers in the right order and start the generator.
What a transfer switch is
A transfer switch is a separate piece of equipment, installed next to or in place of part of your panel, that moves selected circuits (or the whole house) from utility power to generator power. It has its own switch or switches that cannot connect both sources at once. There are two main kinds.
Manual transfer switch
A manual transfer switch is the usual partner for a portable generator. The most common type for homes is a small subpanel with a short row of switches, one for each circuit your electrician wires to it. You pick those circuits ahead of time, typically the essentials: refrigerator, furnace, sump or well pump, a few lights and outlets. In an outage you plug the generator into the outdoor inlet, start it, and flip each switch from "line" to "generator."
- Easier to use: anyone in the house can follow it, and there is less chance of overloading the generator because only the chosen circuits are connected.
- Works with any panel, even one with no matching interlock kit.
- Less flexible: only the circuits wired to the switch can run. Adding one later means another visit from the electrician.
Automatic transfer switch
An automatic transfer switch (ATS) watches the utility power for you. When it senses an outage, it signals the generator to start, waits for it to come up to speed, and switches the house over on its own, usually within seconds. When utility power returns, it switches back and lets the generator cool down and shut off.
An ATS is built for permanently installed standby generators and some whole-home battery systems. Most portable generators cannot start on a signal, so they are rarely paired with one. If you want backup that works when nobody is home, automatic is the way to go, and it is also the most expensive option. Our guide on how much a whole-house generator costs breaks down where that money goes.
Transfer switch vs. interlock kit: side by side
| Interlock kit | Manual transfer switch | Automatic transfer switch | |
|---|---|---|---|
| Best paired with | Portable generator | Portable generator | Standby generator or whole-home battery system |
| Circuits available | Any circuit in the panel | Only the circuits wired to it (or the whole panel on larger models) | Selected circuits or the whole house |
| How you switch over | By hand, at the panel | By hand, at the switch | Automatically |
| Load management | Up to you | Built in by limiting circuits | Built in, often with load-shedding modules |
| Relative cost | Lowest | Moderate | Highest |
| Works with any panel? | No, needs a kit listed for your panel | Yes | Yes |
How to choose: four questions to ask
- What kind of generator do you have or plan to buy? A portable or inverter or dual-fuel generator pairs with an interlock kit or a manual transfer switch. A permanently installed standby generator pairs with an automatic transfer switch.
- Who will be home during an outage? If you are comfortable going to the panel and following a few steps, an interlock works well. If a family member who is less comfortable with breakers might need to do it, a manual transfer switch with clearly marked circuits is easier. If nobody should have to do anything, choose automatic.
- How much do you want to power? Start with the wattage from our guide on what size generator you need. A short, fixed list fits a manual switch; if it changes with the season, an interlock's flexibility helps.
- Does your panel have a listed interlock kit? Your electrician can check the panel's make and model. If no listed kit exists, a transfer switch is the answer.
Either way, the generator usually connects through a 240-volt cord and an outdoor power inlet box, which your electrician sizes to the generator's output.
What the installation involves
Connecting a generator to your home's wiring must be done by a licensed electrician, with a permit and an inspection where your local code requires them. A typical job includes:
- Choosing a listed interlock kit or transfer switch for your panel and installing it.
- Mounting an outdoor power inlet box and wiring it in.
- Checking how the generator's neutral and ground are handled, which depends on the generator and the transfer equipment.
- Labeling everything and walking you through the switch-over.
Safety basics once it is installed
- Run the generator outdoors only. Ready.gov says generators should always be used outdoors and at least 20 feet away from windows. Never run one in a garage, basement or porch, even with the doors open. Plan your inlet box location with this in mind.
- Install carbon monoxide alarms with battery backup on every level of your home, as Ready.gov recommends.
- Follow the order. Utility off before generator on; generator off before utility back on. An interlock or switch enforces this, but the habit matters.
- Practice before storm season, not for the first time in the dark.
Prefer to skip fuel and fumes altogether? A home battery backup system or a solar generator can also connect to your home through transfer equipment, and smaller portable power stations can run essentials with no wiring at all. Our standby vs portable vs solar generator comparison lays out the trade-offs.
FAQ
Is a generator interlock kit legal?
In many areas, yes, when the kit is listed for your specific panel and installed by a licensed electrician to the NEC and local code. Rules vary by city and utility, so have your electrician confirm it is accepted locally and pull the permit.
Is a transfer switch safer than an interlock kit?
Both prevent backfeeding when they are listed and installed correctly. The main safety difference is ease of use: a manual transfer switch limits you to pre-chosen circuits, which makes overloading the generator less likely. With an interlock, you manage the load yourself.
Can I install an interlock kit or transfer switch myself?
We strongly recommend against it. Both mean working inside your main panel, which stays dangerous even with the main breaker off, and many areas require a licensed electrician and a permit.
Do I need an automatic transfer switch for a portable generator?
Usually not. Automatic switches are built to work with generators that can start on their own, like permanently installed standby units. A portable generator is normally paired with an interlock kit or a manual transfer switch.
Can I just plug my generator into a dryer outlet?
No. Backfeeding through a dryer outlet or any wall outlet is dangerous and illegal and can electrocute utility workers. Use a transfer switch or interlock kit installed by a licensed electrician.
This guide is general information, not electrical advice. Connecting any generator or battery system to your home's wiring must be done by a licensed electrician following the National Electrical Code and your local code and permit rules.
Have a question about backing up your home? Contact us and we will help you work out the right setup, step by step. You can also read our checklist for how to prepare for a power outage.