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Short version: Can I use a 200A MCB panel as a 100A subpanel? Can I leave the 200A large-format breaker in the subpanel, or would I have to remove the 200A breaker and convert it to a MLO (obviously, there would be a 100A breaker in the main panel).

Longer version: I currently have 200A service running into my single, 200A, 30-breaker, main circuit breaker panel. I'm running out of breaker slots, and need to add on a sub panel. I've run the calcs for my power loads (the energy.gov calculator that crunches the NEC formulae), and I'm at about 175A. I don't think there's a reason to upgrade my residential service to 400A just yet, but who knows what new power-hungry toys humanity is going to need 20 years from now. If I had 2 200A MCBs, if/when I upgrade, I'd be most of the way there with the two panels ready for new feeders.

I'd of course float my neutrals, and install a separate grounding bar in the subpanel.

A 125A MLO panel and cover is roughly $150. A 200A MCB is roughly $250. If it meets Code, $100 for me is worth it for a bit of future-proofing.

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    As long as the the feed breaker(in main panel) is at or less than the sub panel's rating it is good. I think the breaker in sub panel must equal or be larger, so feeding a 200 amp breaker/panel from a 40 amp breaker is good(if you wanted). The sub panel's breaker basically becomes a disconnect use. Usually it is recommended to go big(200 amps) for sub panels, even if never expected to use that much.
    – crip659
    Commented Dec 1, 2023 at 21:18
  • The savings of using a 100A panel instead of a 200A panel are not worth it for the average DIYer IMHO. You can always use a smaller feed breaker and even obtain a smaller main breaker if you want. But a higher rate panel should cost under $100 more if you're already doing the work. Replacing it later with a bigger one is more heartache.
    – Matthew
    Commented Dec 1, 2023 at 21:41

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Yeah, that's fine. Nobody cares if a subpanel (in the same building) has a completely superfluous main breaker. And for a 100A feeder, it's always better to use a 200A subpanel with plenty of spaces than a 100A subpanel with barely enough spaces.

who knows what new power-hungry toys humanity is going to need 20 years from now.

Well right now, the wind is actually blowing in the opposite direction, as appliances only get more efficient, and energy management is starting to exist.

Technology Connections has a 2-part series on this.

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You certainly could use the 200A as is (and in this case should). The breaker feeding the panel has to be equal or smaller than the rating of the sub-panel, and breaker in the subpanel can remain even if oversized. The wire has to be rated large enough for the breaker feeding the sub-panel.

If it's in the same building you don't even need a main breaker in the sub-panel. If it's in a detached building the breaker satisfies the disconnect requiement, even if oversized since the requirement is just for a disconnect as overcurrent protection is provided in the main panel.

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  • I considered an MLO for the subpanel - in fact that's what I was going to go with first. From my quick Google searching, anything reasonably priced maxes out at 125A for a MLO - at least for QO, which was the brand I want to go with. Thanks for the comment!
    – the_meter413
    Commented Dec 3, 2023 at 1:13

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