Generator transfer switch or interlock kit: how to connect a generator to your New Hampshire home

We have been the electricians on Currier Road in Hopkinton since 2004, and every New Hampshire winter brings the same lesson: the generator is only half the job. The other half is how it connects to your house. Get that part right before the first nor'easter and an outage becomes an inconvenience instead of a scramble in the dark with extension cords.

Without transfer equipment, a generator tied into house wiring can push power back through the meter and onto the utility lines. That endangers the line crews working to restore power, and it can damage the generator and your wiring when utility power comes back. Plugging a generator into a dryer outlet with a double ended cord does exactly this. It is unsafe and it does not meet code.

The National Electrical Code, which New Hampshire adopts through its State Building Code, requires transfer equipment that keeps the generator and the utility from feeding your house wiring at the same time. All three options below meet that requirement. They differ in how the switch happens and who manages the load.

An automatic transfer switch works with a whole home standby generator. When it senses utility power is gone, it starts the generator and moves the house over within seconds, then moves it back when utility power returns. Nobody has to be home.

It can carry the whole house or a managed group of essential circuits. It is the practical choice for a house on a well, a basement that depends on a sump pump, or a property that sits empty for days at a time.

A manual transfer switch is a small panel mounted next to your main panel. It holds a set of circuits chosen at installation. The generator plugs into an outdoor inlet, and during an outage you start the generator, plug it in, and flip each circuit over to generator power.

Manual switches are common with portable generators. The tradeoff is that only the circuits wired into the switch can run on generator power, so the list has to be decided up front.

An interlock kit is a mechanical plate installed on the main panel. It physically prevents the main breaker and a dedicated generator breaker from being on at the same time. The generator connects through an outdoor inlet box wired to that breaker.

An interlock lets you run any circuit in the panel, as long as the total stays within what the generator can carry. You manage the load yourself by turning breakers on and off. The kit has to be made for your exact panel, and some older panels have no compatible kit. Fuse boxes cannot take one.

Irish Electric is based at 659 Currier Road in Hopkinton and works within about 45 minutes of the shop, including Hopkinton, Concord, Bow, Henniker, Warner, Weare, Dunbarton and Hillsborough. We sell, install and service Generac and Kohler generators, including portable units, whole home standby generators and automatic transfer switches. See generator installation and service for details.

The best time to set up a generator connection is before the first winter storm. If you already own a portable generator, the connection is often the only piece missing.