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I am installing a ~100-amp sub-panel in my "attached-detached" garage (an "attached-detached" garage is a detached garage that shares a continuous roof with the home). The sub-panel will be fed by my main 150-amp service panel.

I currently am only planning on having three 20-amp circuits in my garage but would like to buy a sub-panel that can accommodate more in case I expand in the future.

Does my sub-panel require a main breaker panel or a main lug panel? I have read various reports on NEC code (225.31) requiring a means of disconnect for sub-panels that are on a separate building from the main service panel but I'm confused with the exception for sub-panels with only 6 breakers (225.33)? Since my sub-panel will only have 3 breakers but may have more than 6 slots for breakers what kind of panel do I need?

  • Breaker panels are no place to pinch pennies. We have an endless parade of "I'm out of breaker slots, what do I do?" And never yet had a "What do I do with all the extra spaces?" question. Generally the incremental cost of a better panel is a few latte's or frappucino's, or a couple of pizzas. – Harper - Reinstate Monica Jan 10 at 17:27
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Correct. An Outbuilding needs a main shutoff switch even if it has one breaker. It does not need a main breaker, but it absolutely needs a shutoff switch.

It is unclear whether a "connected by roof" building is an outbuilding or not. Code plainly says it's not an outbuilding and does not need a main switch. Your local inspector is the final word on the subject.

Spend some time in the Eaton price book, and you find out that if you optimize for "cheap" or "compact", the best way to get a main shutoff switch is to get a panel with a big switch that by wild happenstance is a circuit breaker too. We don't care about that, we just need a shutoff switch. We only care that its "circuit breaker" trip value does not unnecessarily limit us. If your supply is 100A, then any main breaker 100A or larger will suffice.

This thing is not a breaker for us, and coordinating the breaker trip is hopeless - expecting this local breaker to trip first for our convenience violates Murphy's Law.

The one special characteristic this main "switch" might have is GFCI -- using an oversize hot tub panel is one way to provide necessary GFCI protection to every garage circuit at once, at the cheapest cost. The problem is, hot tub panels are woefully small, though this is helped by our ability to use double-stuff breakers if AFCI is not needed.

And "small panel" is death to a project like this. A person who brings 100A of service to their garage means to run some 240V loads. Those go through breaker spaces like congressmen through taxpayer dollars. Most loads in a garage need to be GFCI with some AFCI, so these will be full-space breakers.

The upshot is, don't even think of a panel less than 24-space...

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Let's start with why there should only be one main (service) disconnect. In this case its primarily there for safety. Meaning if in the event of a disaster you can go to your panel and kill all power to your building with one stroke. Your question falls to another section of the NEC.

The NEC allows more that one main (service) disconnect under NEC Article 230.71 if it meets ALL of the requirements listed in the article as well as the requirements of Articles 230.72 and 230.79. Article 230.72 is about grouping and access of disconnects.

Article 230.729 states that the total addition of the amperage of breakers installed or in SOME CASES spaces for disconnects cannot exceed the total amperage of the service conductors. I say in some cases because it all depends on the interpretation by you local AHJ. For example if you install a panel with only 6 spaces and install just 3 20A breakers. A AHJ could say that the remaining spaces could exceed the 6 disconnect rule if you use 1/2 sized breakers or you COULD install 2 70A 2 pole breakers and adding 70+70+20=160A which mean you have exceeded you 100A service feeder and the MLO Panel is in violation of local code.

It all depends if the local AHJ allows only the new installation of 3 breakers or views future expansion of the panel. Since AHJ are generally conservative in their thinking I would say they lean to the future expansion.

In conclusion I would suggest you go with a 100A Main and absorb the initial cost rather than spend time and money trying to make it right later on down the road.

Hope this helps and good luck

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