Interlocks & Transfer Switches –
What’s the Difference?

by Gail Brumfield

If you’ve got a portable generator, and you want to safely connect it to your home’s electrical system, you and your licensed electrician have a choice to make: Whether to install a transfer switch or an interlock. There’s just one problem: You don’t know the difference. Don’t worry – we’ll fix that!

What’s a Transfer Switch?

Simply put, it’s a device that allows you to safely run power to your home’s electrical system. Transfer switches force you to choose between using utility power and generator power; they prohibit you from connecting your household electrical system to both at the same time – a dangerous circumstance that can severely harm devices, household systems, and utility workers.

In addition to controlling the connection between home, generator, and utilities, transfer switches also dictate which circuits are powered. Because most portable generators can’t power entire homes, transfer switches restrict power to the circuits (and their connected devices) that you’ll need most during an outage. You and your electrician will agree upon these circuits (and devices) prior to installation.

Transfer switches look like smaller versions of your home’s electrical panel. Because fewer circuits will be powered, you’ll see fewer breakers. You’ll also see a switch that allows you select whether you want to connect to the utility, the generator, or neither one. Once you flip the switch to the generator setting, the pre-arranged circuits will receive power as needed.

Depending on where your electrical panel (and transfer switch) are located, you may be able to plug your generator directly into an outlet on the switch; this will be the case if your panel and generator are located outside of your home. If your electrical panel is located inside your home, a power inlet box will also need to be installed on the outside of your home; your switch will connect to the generator via the power inlet box (which is basically an outlet).

What’s an Interlock?

Interlocks perform the same role as transfer switches, but they use a simpler method to do it. Interlocks are sliding metal plates that are installed onto the electrical panel; where these plates are positioned dictates whether or not the panel is connected to the generator or to utility power.

Unlike transfer switches, interlocks don’t require your electrician to perform any rewiring; overall, a bit less installation time is required for interlock kits. Again, depending on where your electrical panel is located, a power inlet box may also need to be installed.

Interlocks don’t dictate which circuits are powered; this is something that you’ll need to do manually. As mentioned above, portable generators are limited in which household systems they can power. You and your electrician will want to confirm what your generator can – and can’t – safely power. This will determine which circuits you’ll engage in the event of an outage; engage too many, and try to power too much, and you’ll risk overloading your generator.

There you have it: Relative to interlocks, transfer switches offer slightly easier operation, but more expense. What matters most to you?