So I purchased a 200A Siemens panel to replace the old 100A sub panel in my house. The main panel is in the garage and is also 200A Siemens and has a branch circuit to the house of 100A.

I bought a 100A main breaker for the new 200A sub panel since the sub panel will still effectively be 100A, but I noticed on the box of the breaker it says "Do not install on 150A-225A Load Centers." Can I really not install this in the 200A panel? If so, that's kind of a bummer... I guess I'd have to return the 200A panel and downsize to a 100A or 125A panel instead. The breaker fits just fine.

2 Answers 2


Breakers protect wires. The wire to the sub-panel can only be protected by the breaker in the main panel, and that must be correct for the wire's type and size.

Like Ed Beal says, you don't need a master breaker in a sub-panel. The breaker in the sub-panel wouldn't protect the wire to the sub-panel. It protects the sub-panel itself, which is listed for 200A. It can also be nice if you ever get sick of paying the tenant's electric bill and have the electric company feed a separate meter to that panel - voila, the master breaker is there already!

(I'm saying "master breaker" to avoid saying "main breaker", which might be confused with "breaker in main panel".)

If you are hoping for the sub-panel breaker to trip before the main-panel breaker, that doesn't work. Breaker trip curves are complex and unpredictable.

If the 100A breaker is not listed or labeled to work in the 200A panel, then you cannot use it, end of subject. That's the law.

This is a situation where buying from a proper electrical supply house (and not a big-box home improvement store) will be very helpful. You tell them what you want, they will sell you the right thing in the first place, and stand behind the combo. On price, for the behind-the-counter stuff, I find them more than competitive with big-box. Their customers are, after all, electricians who deal in volume and drive past 3 big-box stores to buy there. (Electrical supplies do tend to wildly overprice the impulse-buy grab-candy at the front of the store, so don't go off those prices.)

  • I ended up returning the 200A panel and 100A main breaker to get a smaller 16/24 125A panel. I just decided the 200A was wayyyyy too big for my small house... the 125A is the perfect size and a small upgrade over the original 100A panel. I'm also going to forgo the sub panel "master" breaker for now since it's not needed. Maybe some day I will get another 100A "master" just so there is an easy shut-off in the house without having to run to the garage.
    – tbox
    May 17, 2016 at 21:05
  • As long as you'll have enough spaces, that's the key thing. May 17, 2016 at 21:51
  • Yep, I will. I'm changing all of my 20A outlets to 20/20s. I should have about 5 free spaces left which would be plenty if for some reason I (or someone else) make an addition to the house.
    – tbox
    May 17, 2016 at 22:20

With a 100A breaker in your main panel you do not even need a main breaker. I would leave the 200A in the sub as a disconnect. Take back the new 100A and get your $ refunded. If you ever decide to upgrade the service you will need the larger breaker.

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