I purchased Thermosoft mats and planning to install it soon. I was wondering what cable to use from the thermostat to the power supply panel?

I'm planning to use 240V, 20Amp circuit breaker. From my research a cable 12/3 should be sufficient. Is that correct? Do I also need a conduit? It will be exposed on the ceiling but inside the house, not in the unheated garage or something similar. This is in Ohio.

  • 12 gauge wire is the right size for 20A, but are you sure of the electrical requirements of the mats? Electrical usage is going to be directly related to the size/number of mats, so make sure you know what you need.
    – JPhi1618
    Commented Nov 30, 2021 at 17:44
  • according to the manual on a sole thermostat, 240V mats cover up to 327 ft². I have around 200 ft². I got the ones for 240V.
    – Grasper
    Commented Nov 30, 2021 at 17:55
  • 1
    /annoyed Why are you making us drag the sizing information out of you? You've been around SE long enough to know our quality standards for questions. Commented Nov 30, 2021 at 19:24

1 Answer 1


Yes, because you are using the 240V version

According to the web site, they sell mats that are 3 feet wide x various lengths ranging from 4 to 30 feet. It appears to my eye that these mats use 11 watts per square foot. Crunching the numbers on a bunch of mat sizes proves this out. So...

200 square feet x 11 watts = 2200 watts, or 9.17 amps raw power.

Since this is a heater, it requires a 125% derate, which will put us about 12.23 amps that we need to 'provision'.

The minimum legal circuit size is 15A, and that is more than 12.23 so it will suffice.

Some of these numbers are a bit marginal. If you can compute based on the actual amps of your actual mats, instead of making us guess, the numbers might be a bit lower and allow you more flexibility.


If this is a dedicated circuit, you may use #14 wire since it is <=15 amps. (however I myself don't actually own any #14 wire, as #12 is allowed everywhere #14 is, so why tie up capital in a second set of wire spools?)

Do you need /3 (3-wire +ground) cable? No, you do not. The heaters don't need neutral. You can use /2 cable - *But if you do, you must paint, tape, or shrink-tube the white wire with black or colored tape, to indicate that it is a "hot wire".

If you want to buy a 5-pack of colored electrical tape and use "red", that is allowed, though Code does not require red.

Can you make this a Multi-Wire Branch Circuit and also serve 120V loads?

Yes. You can use /3 cable, bring along the neutral also, and also power various 120V loads.

If they are hardwired loads, it's pretty straightforward -- you must think of each 120V "half of the circuit" independently (applying 240V loads to both legs)... and make sure the hardwired loads on that side don't exceed the breaker trip rating with any 125% derates included.

For instance, on leg A:

  • 9.17A of heater x 125% derate = 12.23 amps
  • 7A of microwave oven x 100% (no derate on microwaves) = 7A
  • Total 19.23 amps. That works on a 20A circuit.

Can I have receptacles also on the circuit? Maybe. Again on a per-leg basis, you must total up the actual, not derated amps of each hardwired load per NEC 210.23(A)(2). If the hardwired loads are less than 50% of circuit ampacity, you can have receptacles. For instance:

  • Heater at 9.17 amps (not applying derate for this calc)
  • LED light at 0.20 amps (ditto)
  • Total 9.37 amps is > half of 15A, so receptacles not allowed on 15A circuit.
  • 9.37 < half of 20A, so receptacles allowed on 20A circuit.

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