Generator Interlock Kit: What to Know

The generator interlock kit -- also known simply as an interlock kit -- is an inexpensive and easy to use alternative to a manual transfer switch. But if you don't know what interlock kits or manual transfer switches are, don't worry. We'll explain it all in this blog post.

To start, imagine you're sitting in your house enjoying a nice dinner or watching television. Suddenly, your power goes out. What's your solution? You have a solution, right?

Hopefully you've thought through a plan that includes a generator of some kind. (If you haven't, use our website for a personalized assessment of your home power needs.)

If you use a permanent standby generator, the backup power will kick in as soon as traditional power from the grid is lost. That's not the case with a portable generator, however.

You'll need to connect your portable generator directly to appliances or to your home's electrical circuit. Be sure you place the generator at least five feet from any windows, doorways or soffit vents to avoid carbon monoxide poisoning.

There are several hazards to be careful of when connecting your portable generator. An interlock kit is designed to protect against these hazards.

But before we talk about the interlock kit, let's address its alternatives and some challenges associated with them.

Before the generator interlock kit, there was...

Extension cords

One option when utilizing your portable generator is to connect it directly to appliances via extension cords. Besides being a potential fire issue, extension cords zigzagging around your home can cause you or a family member to trip and fall. On top of all that, there are certain machines -- such as your furnace and water heater -- that can't be connected by cord. With this plan, you won't have those items to use until traditional electricity is restored.


An even more dangerous option -- which in some places is illegal -- is to connect your portable generator directly to the electrical service panel. This is where your circuit breakers are stored.

When your portable generator powers your home, it is done through a process called backfeeding. This is because the generator is backfeeding power to your home. This can cause appliances to fry and can damage your generator, but there's also a bigger danger.

If your main breaker is on when your portable generator is backfeeding your system, you're endangering the lives of utility workers. That's because keeping the main breaker on allows backfed electricity from your generator to pump thousands of electrical volts into your neighborhood power lines. Workers trying to restore power could suffer a fatal shock.

This could happen because you forgot to turn the main breaker off. Or it could happen if someone else in the house switches it back on before cutting off electricity from the generator.

A transfer switch

A transfer switch is a safe option that predates the interlock kit. It's a separate set of circuit breaker switches that allows you to isolate certain circuits from the traditional power grid. By flipping a single switch, you can power circuits by your generator's electricity instead.

That's easy, and it's safe, but it's also expensive. To purchase a transfer switch and to have it installed can cost as much as $1,200.

Fortunately there is less expensive option that is just as safe.

The origin of the generator interlock kit

A solution that ensures the main breaker and the generator breaker can't be on at the same time is important. The transfer switch, for example, is so important it saves lives.

But does this solution need to cost $1,200?

Douglas Albertson, managing partner of Generator InterLock Technologies in Richmond, Virginia didn't think so. He saw the need for the generator interlock kit back in 2003. In September of that year, Hurricane Isabel tore through Virginia and North Carolina, leaving 6 million people without power -- many for weeks.

There was suddenly an influx of people using generators for the first time. That high demand also created a long wait time for professional electricians to install safety precautions like a transfer switch.

The result was that many people hooked up portable generators to their home in unsafe ways. As we mentioned earlier, some had extension cords draped all over their homes. Others utilized something nicknamed the "Generator Suicide Cord."

Albertson felt there needed to be a simpler and safer way for the public to connect their portable generator with their existing electrical system. He wanted this to be a viable option for anyone. That included those on a budget, and consumers without the space for a large standby generator or a bulky transfer switch.

In 2005, he officially introduced his interlock kit to the world.

What is a generator interlock kit?

An interlock kit is a less costly alternative to transfer switches. And, rather than installing a separate breaker in the form of the transfer switch, an interlock kit is mounted on your existing electrical panel.

The kit is simple. It acts as a manual version of the transfer switch by ensuring the main and generator breakers can't be on at the same time. The interlock kit isolates those two systems. In one position, electricity from the generator can't surge into the utility lines. In the other position, electricity from the utility line can't damage your generator. Only one can be activated at any given moment.

And the interlock kit achieves this at a price point far less than the transfer switch. One kit typically costs between $50 and $150.

How does a generator interlock kit work?

Interlock kits are custom designed for the specific service panel on which it will be mounted. You'll want to make sure you purchase one compatible with your breaker box.

Still, once you have the correct kit purchased and installed, they all work the same way.

First, you should turn off your main breaker as well as the rest of your breakers. Next, you'll go out to your yard. You'll hook up the generator power cord to your generator and your generator power inlet box. Turn your generator on.

Back at your service panel, you should slide your interlock kit up. This will reveal your generator breaker, which was previously covered by the kit. When you slide it up, it will now block your main breaker. This is how the interlock kit ensures only one of the main breaker or generator breaker can be switched on at a time.

Turn on your generator breaker. Now your home is ready to receive power from your generator, rather than the public utility.

Switch each of your individual breakers you need back to the on position. The only breaker that must remain off is the main breaker, which is still blocked by the kit. Your generator is now powering the appliances and outlets you need.

When the traditional electrical grid is restored, you can turn off the individual breakers. This includes your generator breaker. You'll then slide your interlock kit down. Doing this will reveal the main breaker and cover the switched off generator breaker.

Switch on your main breaker and then turn your individual breakers back on. Your home is now back to being powered by the public electrical grid.

Unplug your generator power cord and return your portable generator to its storage area for when you need it next.

Advantages of a generator interlock kit

There is a lot to love about using an interlock kit. Here are some of those advantages:

  • An interlock kit eliminates the maze of extension cords you'll need if you connect your generator directly to appliances. Also, moving around large appliances like a refrigerator to plug them into an extension cord can be difficult work.
  • The kit doesn't require a separate breaker panel. It's installed on the main breaker panel. This means cost savings and that no additional wall space is required.
  • When your portable generator is plugged into appliances, it only uses the amount of power required by those specific appliances. This is often far less than its capacity. Interlock kits and transfer switches allow the generator to utilize its full load capacity and power more of your home.
  • The cost of an interlock kit is much less than that of a transfer switch.
  • Installing a transfer switch or a generator sub-panel can take an electrical contractor as many as eight hours because of the rewiring involved. An interlock kit, on the other hand, can be installed in less than two hours. All it takes is mounting the correct kit to the front of the panel and wiring a generator power inlet to your generator breaker.

Disadvantages of a generator interlock kit

No solution is foolproof. Interlock kits have disadvantages, as well. Here are some of the most notable:

  • Underwriter Laboratories (UL) is a longtime, third-party certification company. Its standards are well-respected. Many types of interlock kits are tested to UL Standards by independent labs, but they don't carry the UL Listing mark. As a result, some inspectors won't allow their use.
  • As we've mentioned, the kits are installed on the cover of your electrical panel. While the cover is on, the interlock kit prevents the main and generator breakers to be in the on position at the same time. If the cover is removed, however, both breakers could be switched on. This mistake is very dangerous for utility workers.
  • An interlock kit saves space on your wall by not requiring an additional panel full of circuit breaker switches. The kit does take up space on your existing beaker panel, though. You will need to reserve two or three spaces for it.
  • Some interlock kits need to be drilled into the panel cover. Once they are installed, they cannot be removed without causing additional issues for your electrical panel.
  • When power from the electrical grid is restored, a standby generator automatically turns off. The traditional grid takes over the work of powering your home. When appliances are plugged directly into your portable generator, the other items in your house will turn on when the public utility is fixed. An interlock kit, however, requires you to shut off your main breaker. That makes it difficult to know when the utility is restored.

Determining if an interlock kit is right for you

If your backup power strategy includes a portable generator, then a generator interlock kit could be right for you.

And if you decide you're interested in an interlock kit, they should be installed by a certified professional. Our staff can help you find the experts in your area to help bring your backup power plan to life.

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