I am installing Ditra Heat on ~30 sq/ft of bathroom floor and running a dedicated circuit per instructions. The Ditra Heat cable specifies Maximum Circuit Load = 15 amps while Maximum Circuit Overload Protection = 20 Amp Breaker, which has me confused. 15 amps typically requires 14 gauge wire, but then specifies 20 amp breaker, which requires 12 gauge wire.

So which is it 14 or 12 AWG, and to a 15A or 20A breaker?

• What exact componets are you using? Evidently this is a "don't cut it shorter" system so you buy a heating cable to suit the floor area. The thermostat can presumably control UP TO 15A of heating, but 30 square feet is between 338-405W or 2.8-3.4A at 120V Commented May 5 at 14:45

It depends. If your heating cable install will actually use more than 12A (due to the amount of cable installed) you need a 20A breaker as detailed in @crip659's answer due to the 80% derate for continuous loads (heating being one.)

If the area and length of cable used mean your installation will not draw more than 12A (because you are not using enough length of heating cable to do so) you get into a point where you are reading "maximum" as "minimum required" when it may not be. i.e the language you have quoted basically says "put no more than 15A worth of cable on the install, (they sell many different lengths as they don't want it to be cut) and put no more than a 20A breaker on it." You might only be putting half the maximum about of cable into the small floor area, in which case you'd have 7.5A load, and 15A breaker would be fine. Neither is above the maximum stated, so it complies with that direction.

Per installation manual, the thermostat can control UP TO 15A of heating cable, but per the instructions in it your 30 square feet is only 2.8-3.4A of actual load for this installation. So a 15A breaker and 14AWG wire is fine here.

• thanks for deciphering @Ecnerwal as the language is what had me so confused Commented May 5 at 15:49

There is a derate on the power of a breaker. I believe the number is 125% larger breaker needed(or 80% of the breaker/circuit).

You are not suppose to use the full 15 amps on a 15 amp breaker/wire, so you need to go up. A 15 amp circuit should only have up to 12 amps on it continuously, which is less than 15 amps.

You need a 20 amp breaker/wire for up to 16 amps, which is why they require a 20 breaker and 12 gauge wire.

• ...the derate is for "continuous loads" (run more than 3 hours) which heating loads are defined as being. Commented May 5 at 14:15