We are getting a generac generator installed in a few months. Does anyone know if it uses any of the circuit breaker spaces that we have left? We are getting close to using all the circuit breaker space and trying to figure out if we need to think about a second box.

  • Have you considered what loads you want backed up by the generator? Can you post photos of your existing breaker panel(s), for that matter? Nov 14, 2021 at 21:20

2 Answers 2


You need to provide some sort of mechanical interlock so that the generator cannot possibly back-feed the utility. Transformers are 2-way, and if you back-feed your friendly neighborhood transformer, your 120/240V supply will create 9600 volts on the supply side of the transformer, and kill the poor lineman who's been up for 2 days straight trying to get your power back on.

This mechanical interlock is the deciding factor in breaker use. But there are several ways to do it.

  • A "GenerLink" meter-mounted transfer switch. This takes no breaker spaces, but is expensive and not all power companies will let you do it.

  • A huge, 200A DPDT or 3PDT switch between meter and service panel. This is all new hardware and does not impact your existing breaker spaces... in fact some variations, like a Ranch Panel with a generator interlock, will actually give you more spaces.

  • The least expensive option, a generator interlock, consumes 2 spaces in your panel and needs to be in a particular location. The sliding-plate interlock is typically $70-100 (sometimes as low as $30) and must be designed for your specific panel.

  • A generator subpanel is where you add a sub-panel strictly for generator loads. You choose the panel, so you choose the additional breaker spaces you gain. You either use a generator interlock (like above) in this panel, or an automatic transfer switch. This forces you to choose your circuits in advance, but does not limit the number of circuits you bring over. You can get any size of panel you want, and breaker spaces are cheap.

For instance a 24-space Siemens main-lug panel ($70) plus an ECSBPK01 interlock ($27) plus 3 breakers ($30) makes for a rather affordable way to switch as many as 20 circuits and get 20 additional spaces! This costs you 2 spaces in the existing panel but you'll get many back when you move critical circuits to this panel.

  • A manual 6-8-10 circuit transfer switch as any generator dealer will try real hard to sell you, because they seem to work on any panel. But they are made by third-rate vendors, VERY expensive (sales commissions aren't free), take as much space as the above subpanel but add 0 spaces, and have a bulky "octopus cable" that is difficult to install. They also can't work with GFCI or AFCI breakers. And they force you to choose your 5-10 generator circuits in advance; most others let you choose "on the fly".

Since you are out of spaces anyway and thinking of an upgrade, I would have suggested a subpanel even if you weren't getting a generator, because it's far cheaper than ripping out a perfectly good main panel simply because it's out of spaces. But since you do want a generator, it only makes more sense to use a LARGE subpanel and feed it either from a manual interlock (cheap) or standalone automatic transfer switch (simple).


It depends on the model and the design. There's one Generac model that does not occupy any breaker slots. Others require two slots, usually the two top right ones, and others require tapping in to the service feed before it gets to the breaker panel. It depends mainly on whether you bought an auto or manual transfer one, on whether you bought one big enough to power your whole house including the central A/C, or a smaller one, and on other things.

I suggest you read or watch some tutorials on different kinds of generators, read the manual of the one you bought, discuss options with the installer, and then come back here and ask further questions.

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