I am converting a room into a sauna that will frequently be very hot (100+C). It has some wiring -- a few outlets and a light, from its previous use. The insulation for electrical cables is rated up to 95C, and it isn't wise to approach this temperature.

I will put 3.5" polyiso on all the walls, but if I leave the outlets I they will obviously poke through the polyiso to the exterior. Will this be sufficient to keep the cables' insulation from melting, keeping in mind that at least a small part of the cable will be nearly outside the insulation?

Edit: Looking at the boxes, it appears that if they face into the hot room the wiring is at the back of the box, putting it behind all the insulation. If I face them away from the hot room into the adjoining room, the back of the box will be closer to the heat. Thoughts?

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    100+ C?? That is the boiling point of water. 50C scalds skin, so 100C doesn't seem like a good sauna temp. Are you quite sure you don't mean 100+F? Oct 10, 2018 at 18:45
  • definitely 100C. a good hot sauna gets from 90-100ish. Oct 10, 2018 at 19:44
  • Beware of contact between polystyrene and PVC cables - the polystyrene can leach the plasticiser out of the cable, causing it to become brittle and fail. Apr 14, 2019 at 9:54
  • i think the insulation on the cables is silicone? Apr 14, 2019 at 22:24

2 Answers 2


A low humidity sauna might be as high as 100°C measured above the upper bench. It's really no consolation that much of the wire will be behind insulation and won't get to the full temperature inside the sauna; if the insulation is compromised in the boxes, that's hazardous. The connections in the boxes may be rated for lower temperatures than the 90°C insulation.

Keep in mind that the wiring generates some heat of it's own. The ceiling light in particular would concern me. Heat rises, so it will be hotter than the average temperature in the room. The light will generate some heat too, on top of the ambient temperature of the sauna.

I don't think receptacles in the sauna is a good idea, receptacles are generally rated for 60°C operating ambient temperature. You might find some industrial specialty receptacle made to withstand high temperatures but I bet it would cost a fortune.

I think you could safely blank off the and insulate over them, leaving an insulated access panel cover to get to them. (You can't bury a junction box, it has to be accessible.)

  • thanks for this. saunas definitely do get to 100C guarantee it. there's one in this city that is 120. the heat will melt the power outlets themselves, even with high-heat wiring? Nov 17, 2018 at 2:07
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    @WalrustheCat - I don't know if they'd literally melt, but you would be way over the operating temperature range and expect them to fail in some way or other. Nov 17, 2018 at 10:35

I believe you are referring to 100 deg F as 100deg c would cook you. IF you are talking about the heat in the hot rock heater you may need high temp fixture wire out to a junction box then standard 90 deg c wire will be fine that's 194 deg F. So other than the actual heater you should be fine, you will need a wire rated for a wet location like thwn (most thhn is dual rated as thhn / thwn ).

Added from comments. Normal wire is only rated to 90 deg c so it would have to be routed outside the sauna. The 135F on the units I installed was measured at the bottom seat I am not sure how hot they got on the top seat, I remember there were a bunch of warning stickers and the first one the owners wanted me to change the thermostat but this would be a violation of a listed device, after they used it they said it was much hotter than the one at the athletic club and they recommend them to friends and I installed those also, all the internal wiring was in place I just had to assemble the box and seats then feed an external j box with 50 amps. My guess is the 135 deg at the bottom seat was a safety level and the stat was only ~2' off the floor maybe 18" the box was 7' tall so as I said it probably hotter up on the 2nd seat but I don't know how hot just what the controller said because they wanted me to change it.

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    Saunas can regularly be 160F or more, with localized higher temps around the heater exceeding boiling (100C). I don't think the wiring is in jeopardy due to heat, though--just moisture.
    – isherwood
    Oct 10, 2018 at 19:14
  • On the commercial saunas I have installed the max temp was 135 the hot rocks were probably hotter but once the room reached 135 the controls turned off the heat to the rocks.
    – Ed Beal
    Oct 10, 2018 at 19:29
  • Apparently, saunas in the US and Canada are legally allowed to get up to 90 C. That seems pretty silly to me, but I guess some people like it. sauna-talk.com/sauna-temperature.html
    – mrog
    Oct 10, 2018 at 19:37
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    I'm definitely talking about 100C, 100F would be a terrible sauna. does this change your answer? Oct 10, 2018 at 19:43
  • thanks @EdBeal for the edits. The wire would be buried 1.5" inside polyiso, which means it wouldn't be getting the whole 100C. Except of course where it connects to the outlets themselves, where it would be closer to the surface. Does this change your answer? Oct 10, 2018 at 21:07

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