Generator Interlock Vs. Transfer Switch – Which One Should Use? - Spruce Generator (2022) (2024)

One way of connecting a generator to a house during a power emergency is through an interlock device or a transfer switch. And most generator owners get stuck in choosing between the two. So, what’s the difference between a generator interlock and an automatic transfer switch?

A generator interlock is a small device installed on a power panel next near the main home breaker to allow you to safely power your home with a generator without back-feeding the utility line. On the other hand, an automatic transfer switch manages the utility line and the generator power during an outage for the same purpose as an interlock device.

Both devices help transfer the power source from the utility line to the backup generator and vice versa when there is a power emergency. The choice depends on the power need at hand. This post will take you through these devices, their differences, and where to use them.

What are the Similarities and Differences between Transfer Switch and Interlock Device?

An emergency backup generator, portable or standby, comes in handy as a lifesaver during a power outage. The generator will restore power to selected essentials such as lights, ventilation, computers, and other critical systems when the storm ranges.

The power has to be transferred from the mains/utility line to the backup power. The device you use is a transfer switch or an interlock device for this task, depending on your power requirements and demands. Usually, the type of power transfer device to choose determines what happens in the event of a power outage.

Each of these power transfer devices determines how your backup power from the generator flows through the building. It’s a must that the equipment be installed according to the NEC® Electrical Ordinances.

Both devices prevent the danger of back-feeding the mains line using the generator power, which can cause electrocution of any service personnel working on it or damage your electrical appliances.

Allow me to explain each of these devices deeper before telling you the best choice for your needs.

Similarities and Differences between Generator Interlock and Transfer Switch

What is a Generator Interlock Kit?

A generator interlock kit is a device designed to allow you to transfer utility power off and turn on the portable generator backup power during a power outage.

An interlock kit manages the power source to the electrical panel. It ensures that the generator circuit breaker and the main circuit breaker cannot be turned on simultaneously. This makes it safe for anyone working on the electrical lines and for preventing dangerous feedback.

It helps manage the two power sources manually, ensuring only one breaker, home mains breaker, or generator backup breaker stays on at any more moment. It helps ensure you don’t back-feed your home mains, a situation that can potentially electrocute electricians working hard to restore the power or even damage appliances in your home.

A generator interlock device offers the same solution provided by a transfer switch. However, you have to operate it manually and designate some breakers if your generator cannot deliver enough power to energize the whole panel.

It’s easy to operate; very little training is needed to know how it works.

How Does a Generator Interlock Kit Work?

A generator interlock device’s primary purpose is to make sure you only switch on one breaker between the main home breaker and the generator backup breaker.

It comes with two sliding hard plastic or steel plates that you slide to make sure one of the two breakers is OFF while the other is ON.

You have to operate it manually when there’s a power outage before starting your generator. That means you’ll have to turn OFF the mains breaker and slide the plate to allow you to turn ON the emergency generator breaker.

When the power is restored, you have to head to the power panel, turn OFF the generator breaker, slide the plate to block it, and get the allowance to turn the home breaker ON.

The sliding plate cannot allow you to turn both breakers ON. You can only have only one ON when the need arises. It’s one way to make sure you don’t back-feed the mains and risk getting charged for manslaughter for electrocuting a power service personnel.

What is a Transfer Switch?

A transfer switch is a power-source switching device designed to ensure power sources switch safely during an emergency. It’s available in two options; a manual transfer switch or an Automatic Transfer Switch, also known as ATS.

The manual transfer switch has a system that allows you to power a separate service panel with its kit. Doing so ensures you don’t back-feed the utility line while also powering selective units in the house.

On the other hand, an automatic transfer switch switches between the main power and backup generator automatically. It automatically switches the generator line on when the mains power goes out and switches it off when the power gets restored. You don’t have to do anything.

How a Transfer Switch Works

Manual Transfer Switch: With the manual transfer switch, which has a separate set of breakers, you have to manually switch between the power sources, even if you have an automatically starting standby generator.

Automatic Transfer Switch: When there is a power outage, the ATS switches the backup line breaker ON and sends a signal to start the backup standby generator.

It monitors the voltage in the mains line; when it drops, the transfer switch turns off the main breaker and switch on the generator breaker automatically

The best part is that it’s a technology that ensures you don’t have to worry about a power outage. You don’t even have to get out of your home to start the generator unless you’re using a portable generator.

In that case, when there’s an outage, the ATS will disengage the mains breaker and engage the generator breaker waiting for you to connect the portable generator to energize the panel.

You have the option of running to the breaker panel and turning off the breakers that you want to power with the generator if it cannot hand the whole house.

But when using a standby generator, you don’t have that option since it has a mechanism that starts the engine when it receives a signal from the ATS that there is a power outage.

A transfer switch, though expensive, offers a safer and convenient way of switching between power sources during an emergency safely and prevents back-feeding. But again, it has to be installed by a qualified electrician and follow NEC® ordinances.

ATS or Interlock Device – Which one is best for your application?

Before deciding whether to use a generator interlock or a transfer switch, you must understand your power needs and your backup power source.

Consider the type of generator you have, whether a portable generator or a standby unit, application area, residential or commercial, and your budget.

When to Use an Interlock

An interlock device is a simple-to-use and inexpensive device that you can use when working with a portable generator requiring you to start it manually.

It’s also an excellent choice if you must manually select the breakers to run during an emergency if your generator cannot power the whole house.

It’s used mainly by small household owners working hooking up a portable emergency generator to their home through the power panel.

You can only use it in small commercial applications that rely on portable generators as they backup power source.

It’s a legal alternative to transfer switch, but for it to be installed legally, its installation must be done but a professional electrician and follow the set NEC® ordinances.

When to Use a Transfer Switch

You can use an automatic transfer switch in all situations that need power source switching, especially in cases that need quicker automatic power switching.

You will mostly find transfer switches in residential homes or commercial buildings that can only be powered by automatic starting standby generators.

It does comply with National Electrical Code® and Building Codes, and that’s why it’s applicable primarily in commercial buildings.

Under NEC® ordinances, the transfer switch is the most applicable for permanently installed generators. However, some portable generators are transfer-switch-ready, meaning you can connect to your home through the ATS.

It’s, however, an expensive option that might not be necessary for a small home considering an interlock device is a legal alternative.

Is it a must you use a generator interlock or a transfer switch?

YES! According to the National Electrical Code® ordinances, you must install a power transfer switch device when installing a generator through the power panel. Powering your home with a backup generator without it can land you in jail or a fine for not following these rules.

What’s more, if an electrocution incident happens because you back-fed the utility line, the charges might even increase. Another thing, if the mains power was restored with the generator back-feeding it, there is a high chance your appliances will be fried, and your generator won’t be spared either.

So, it’s a must you use either of these devices for safe switching of a power source when the need arises.

Related Questions

How much will it cost to install an interlock kit?

Installing an interlock kit for a generator can cost anywhere from $100 to $350, depending on the job’s complexities, the cost of electrical labor, and the hardware you use. An interlock kit can cost anywhere from $25 to $150, with labor costs ranging from $50 to $150.

How Much Does It Cost To Install a Transfer Switch?

It depends on where you reside, the type of your installation (single-phase or three-phase), and the sort of transfer switch you’re utilizing (manual or automatic.) When the electrician charges $50-100 per hour, the bill will often vary from $200 to $400. In most cases, a professional will need 3-4 hours to complete the installation. A device designed for commercial usage will be more expensive.