Can I use 14/3 wire 20 amp double pole breaker for a 240 volt baseboard heater of any watts? What's the electrical code?

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    Also, you do not need XX/3 cable for electric heat. Electric baseboard heat is a straight 240V circuit, so typically we will use XX/2 cable and re-identify the white with a permanent marker. – Speedy Petey Jan 14 '15 at 21:53

Only if the heater draws less than 12 amps would 14 AWG be adequate. And that would require a 15 amp breaker, not 20, to properly protect 14 gauge wire.

Space heating devices require wiring to be 80% derated which is assumed by the electrical code to be "on" 100% of the time.

A heater rated up to 24 amps requires 10 AWG and a 30 amp breaker.

A heater rated up to 16 amps needs at least 12 AWG and a 20 amp breaker

A heater up to 12 amps needs at least 14 AWG and a 15 amp breaker.

The heater will have volts, amps, and watts on its "ratings plate".

  • Actually, any constant load requires the wire to be 80% derated, right? Not just space heaters. The wire shouldn't be subjected to more than 80% of its rated capacity on a continuous basis. – Craig Jan 19 '15 at 4:52
  • @Craig: Yes indeed. "Continuous" is defined as any load persisting for two or more hours at a time so it could potentially apply to compressors, fans, etc. too. The NEC calls out spacing heating as automatically being rated as "continuous" loads, despite the fact that some applications would never actually approach continuous. – wallyk Jan 19 '15 at 6:43
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    A continuous load is one that is expected to continue for three hours or more. NOT just a load that can or could be on for three hours. – Speedy Petey Feb 7 '15 at 17:41

If you have a 20A breaker on the circuit, you must use #12 wire or heavier, regardless of the actual load you place on the circuit.

So if you have #14 wire in the walls already, you're going to have to pull new wire, or use a 15A breaker.

  • It is far easier to change the breaker than the wire. So it makes more sense to say the 14 AWG wire must have a 15 amp or smaller breaker. – wallyk Jan 19 '15 at 6:44
  • @wallyk Fair enough, although the OP didn't specify whether wire was pulled already, just that they want to put a 20A breaker on #14 wire for a 240V baseboard heater (maybe they just have a spool of the stuff sitting around and want to use it if they can). So I stated that you must use #12 wire on a 20A breaker. I imagine the baseboard heater requires a 20A breaker. So they've got to pull #12 wire regardless. :-) – Craig Jan 19 '15 at 7:19
  • You could well be right. I assumed the wire was already inside the wall because of the "forcing" flavor of phrasing. – wallyk Jan 19 '15 at 7:46

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