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I need to upgrade a 40amp to a 50amp. This 40amp supplies power to a sub panel that already has a 50amp breaker on it. How do I know if I need to change the wiring for the 40amp or if I can just change the breaker head itself and be done? It sounds like I need a 6 gauge wire but I’m not sure if the main panel is using 8 gauge.

Why would an electrician use a 40amp from the main panel to a 50 amp on the sub panel? Does that mean they possibly already have a 6 gauge behind the walls?

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    The 40 amp is protecting the wires. The 50 amp is almost just a disconnect right now for the subpanel. Will need to check the wire gauge, should have writing near the breakers to read. Giving the reason to upgrade/change the 40 amp breaker in your question will be useful also. Unless it is tripping often, then changing it might not be needed.
    – crip659
    Mar 6 at 0:07
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    @crip659 Write that up as an answer and I'll +1
    – manassehkatz-Moving 2 Codidact
    Mar 6 at 0:14
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    @manassehkatz-Moving2Codidact Would like to know the reason first. If reason is just because, then will make it an answer.
    – crip659
    Mar 6 at 0:33

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Breakers from 15A to 60A are priced the same.

Therefore, the installer certainly used the largest breaker that is possible given the wiring they installed. In house wiring this is most likely 8 AWG Type NM or UF.

So you can get us some info on the information on the cable jacket, on the hopes the guy made a mistake... but honestly, don't get your hopes up.

Now, why are you up-cycling old wire to another purpose? I ask because the usual reason these days is EV charging, and that is practically magic - it can be adjusted to work on any power available. So we can get it done, for sure, on what you got now.

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The only way to know for sure is to check the gauge of the wire by inspecting it. Since the wire is visible at both ends (i.e. at main and sub panel), this is easy to do. The gauge is probably written on the insulation. Otherwise, compare the diameter of the copper. Do not assume anything about the wire sizes based upon the breakers.

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