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I'm going to upgrade my existing electrical service from 100 amps to 200 amps. Along with the service upgrade, I'll be moving the location of the service drop (part of a larger home remodel). I want to keep the existing 100A panel (which mostly serves old K&T and original NM spec wiring) as a subpanel. I'll add new circuits to the new 200A panel, and over time decommission the circuits fed by the 100A panel.

I'll add a 100A breaker in the new panel. But my existing panel has a non-removable main breaker (rated at 100A). Is this allowed?

Are there other considerations?

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2 Answers 2

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Main breaker

There's no problem with having a main breaker in a subpanel. It's required if your panel is in a separate building and has more than six breakers, but there's nothing prohibiting it in any other case.

Wire size

You need to ensure the wire between the panels is sufficient size. See What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?

Neutral-ground bond

You need to ensure the only bond between ground and neutral is in the main panel. The subpanel must have separate ground and neutral bus bars. Since your panel is used a main panel now, it will currently have a bond between ground and neutral, so that'll have to be removed.

  • It's usually a metal bar or connector that screws between the two bus bars, or between the cabinet and neutral bar.
  • If you currently have neutrals and grounds mixed on both bus bars, you'll have to separate them out
  • If you have only a single bus bar (with grounds and neutrals mixed) you'll have to add a neutral bar (which is not bonded to the cabinet). Check with manufacturer first to see if they have a documented way of doing this. If not, I'd suggest you check with your AHJ (inspector) first to be sure whatever you do will be acceptable.
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  • Nice summary. You say "add a ground bar" if grounds and neutrals are mixed; wouldn't you actually need to add a neutral bar ? I gather this is also called an "insulated ground bar"; I guess most ground bars are bonded to the cabinet through their mounting, as I'm having trouble purchasing parts for separate ground and neutral bars in a Siemens subpanel I'm adding..
    – RustyShackleford
    Commented Mar 22, 2017 at 2:53
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    @RustyShackleford Thank you, yes, you should add a bar not bonded to panel, so it would make more sense to be a 'neutral bar'. I updated my answer. Some manufacturers don't have add-on bars or ways of officially adding a bus bar, so that's why I suggested talking to your AHJ before you just stick some random one in or come up with some other solution.
    – gregmac
    Commented Mar 22, 2017 at 19:50
  • Thanks for the answer. Somewhat off-topic, but for my own edification: what's the reason that the neutral and ground bars aren't bonded together in a subpanel?
    – the_meter413
    Commented Mar 24, 2017 at 19:29
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All grounds go back to the service panel to prevent ground loops from forming if the ground is connected at every subpanel.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here.
    – Daniel Griscom
    Commented Mar 13, 2020 at 21:42
  • This doesn't answer the question asked
    – ThreePhaseEel
    Commented Mar 14, 2020 at 3:53

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