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I am looking forward to purchasing a sauna heater and the one I like is 9KW. Under 240V the amperage will be 37.5A. The heater is about 10 feet away from the electric panel.

I already have the wire roughed in and covered by the drywall. The wire I put is 3C 8AWG.

Is it code compliant to put this heater with 8AWG wire and 40A breaker? I know that it would be much better to use 6AWG wire and 50A breaker but it is too late to change that.

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    We need to know more about the cable type. Can you give us the text writen along the side of the cable? It repeats every 12 inches. Jan 5, 2022 at 22:35

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A 37.5A heater requires a 125% "derate" - so your (legal, and not burning your house down) choices are change, or never turn it on.

Alternatively, since the heater appears not to be purchased yet, learn to like one that draws 32A, maximum, if you have pre-purchased and pre-installed 8AWG cable and a 40A breaker.

Had you installed 8 AWG copper wires in conduit, as opposed to cable in the wall, those would potentially be good for 50A, if the receptacle (or terminals if it's not a plug-in heater) and breaker are both rated for 75°C (which is common) - using cable you are limited to 60°C and 40A.

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    Isn't it possible that a sauna (being an enclosed area, and thermostat-controlled) might not be considered a "continuous" load and thus not necessarily subject to the 80% continuous limit? The same way ranges and ovens can draw >80% of their OCPD rating.
    – nobody
    Jan 6, 2022 at 1:49

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