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I need to set up a sub-panel with four 15 amp breakers. What size breaker would I require in the main board to feed this?

110V in Canada.

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    It really has to do with your expected simultaneous current need, not the number or rating of the breakers.
    – isherwood
    Nov 5 '17 at 15:59
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    ...and the size of the wire between panels is also a factor.
    – Tyson
    Nov 5 '17 at 16:22
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    What loads are you putting on the subpanel? Are you using existing wiring for the feeder, or running a new feeder? Nov 5 '17 at 16:22
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    What size of wire exists between the main panel and the subpanel? If more than one size, what is the smallest (highest AWG)? Nov 5 '17 at 16:22
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    @JimStewart You can have as many gauges as you want. The ampacity of the circuit will be decided by the smallest gauge. And yes, there are reasons to do that. When I say "more than one cable" I mean in a string, not paralleled. Can't parallel except in very rare circumstances involving BIG wire, and special disconnects which handle the paralleling correctly. I have one. Nov 5 '17 at 23:44
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If you are running a proper sub-panel on the North American model (i.e. 240V + neutral and ground) a 30A double-pole breaker and 10Ga wire will permit 4 15A 120V circuits to run at full capacity.

If you are using existing wire, that sets the upper limit of what you can feed it with, whether or not that is adequate for your needs. Else, you will be replacing the wire to meet your needs.

If the panel might be expanded later (i.e. 4 circuits now, but with space for additional circuits) and you use larger wire, you can feed it up to the ampacity of the feed wire and/or sub-panel rating.

If you are running a single-phase (120V) feed you will probably not be happy, and you would need large wire to feed it anywhere near full capacity - which is why the normal/"proper" method is a 240V feed.

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