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I'm no pro, so I'm sorry in advance for doing my own work, but I enjoy the learning and experience. I've run aluminum 4/3 out to a new sauna with a sub-panel, roughly 100', which houses 2x single pole 15A breakers, and a dual pole 50A for my 9Kw sauna heater. If my math is right, the heater draws 37.5A per pole. My question is, the sub-panel to the heater is only going to be a short run of < 10', and I have a spool of 10/3 wire leftover from a previous job, is it good enough, or must I use 8/3?

Thanks!

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Assuming you are in the US and subject to NEC, 10 gauge copper is strictly limited to 30A. So, no.

I don't think 8 gauge is big enough, either, unless it's THWN in conduit and the connections at both ends are rated for 75C. The breaker protects the wire, so you can't play games with sizing the wire for 37.5A. Given it's a heater, you need to provision for 125% of the load as a continuous load, which is 46.875 amps, which gets to why you have a 50A breaker, not a 40A breaker.

If using cable, which is limited to 60C sizing, you need 6 gauge copper or 4AWG aluminum. Which begs the question of what size breaker you have on the 4/3 aluminum you ran out there, since that's limited to 55A if it's 4/3 cable.

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  • They don't make 55A breakers, so OP would be allowed to round up to 60A. However OP could only provision (loosely, "plan to use") 55A. Nov 22, 2020 at 22:05

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