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Related to this question (Can a sub-panel have a larger main breaker than the feeder breaker in the main panel?) on having a larger "main" breaker in a subpanel than in the panel feeding it, my question is similar, but has to do with a subpanel "main" breaker larger than the busbar ampacity/panel rating.

In this case, I have a subpanel (no ground/neutral bond) with a 200A panel rating and a 200A breaker in the panel feeding it. Can I install a 225A "main" breaker in the subpanel to act as a shutoff switch for that subpanel or is that prohibited by code since its rating exceeds the rating for the downstream busbar?

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The NEC requirement for protection of the panel is straightforward:

408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supplyside of the panelboard.

So the panel must be protected by an OCPD at or below it's rating, but that OCPD can be the main or upstream. As long as the upstream OCPD satisfies this rule, the main breaker does not have to; a main breaker larger than the panel rating doesn't satisfy the requirement and doesn't protect the panel bus but it's not a violation to have it there serving as a switch / disconnect.

Keep in mind that the NEC also requires that the panel and the main breaker be listed, and used in accordance with it's listing and the manufacturer's instructions. The combination of a main breaker and panel with a lower rating may violate the manufacturer's instructions. That of course would vary case by case.

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