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What kind of breaker do I have to use for a sub panel of 125 amps? May I use a 125 amp breaker (same amp rating as the sub panel) from main panel to feed the sub panel?

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  • What make and model is the subpanel? How big is the feeder wiring? – ThreePhaseEel Jun 11 '17 at 14:51
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The breaker size is decided by the wire connecting the panels. For instance if the wire is 8 AWG, the breaker will be 40A. Regardless of the wire size, the breaker cannot exceed 125A, presuming that is the rating of the panel.

As always, make sure you are buying a subpanel that is large enough. It is very common for someone to save $40 on the panel, only to later have to spend a lot more than $40 for twin breakers and other work... because they got a panel that left them too little room for expansion. Panel space is cheap.

It is totally fine to have a subpanel whose "main breaker" is larger than the breaker in the main panel serving it. For instance I have a 60A circuit serving a too-small 4-space subpanel. I replace that with a 42-space subpanel with a 225A main breaker. That is fine. The big main simply serves as a shutoff switch. (And I made out because the bundle came with 22 ”bonus breakers" which I use elsewhere.)

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