Does radiant floor heat need a GFCI?

Yes. NEC 424.44(G) requires GFCI protection for personnel for heating cables in electrically heated floors of bathrooms, kitchens, and hydromassage bathtub locations.

Can I use a GFCI breaker instead of a GFCI thermostat?

Yes, either satisfies the rule. Do not use both in series, because the combined leakage from a heating mat causes nuisance tripping.

What size circuit does bathroom floor heat need?

NEC 424.3(B) requires conductors and overcurrent protection at not less than 125 percent of the heating load, since fixed electric space heating is treated as a continuous load.

Does electric floor heat in a bathroom need to be GFCI protected?

Yes, and this one is not optional or AHJ dependent. Section 424.44(G) of the NEC requires ground fault circuit interrupter protection for personnel for cables installed in electrically heated floors of bathrooms, kitchens, and hydromassage bathtub locations. The bathroom is named directly in the rule.

Where the protection lives

Most floor heating systems ship with a thermostat that has built in GFCI protection at the 5 milliamp personnel level, and that satisfies the requirement. If the thermostat you are installing is a plain line voltage stat with no ground fault device, put a GFCI breaker on the circuit instead. Do not stack a GFCI breaker under a GFCI thermostat. Two devices in series nuisance trip constantly because of the small leakage current every heating mat produces along its length.

Watch the difference between a 5 milliamp personnel GFCI and a 30 milliamp equipment ground fault protection device. Section 424.44(G) calls for personnel protection, so a 30 milliamp GFPE device is not a substitute in a bathroom.

Circuit sizing and the other 424 rules

Fixed electric space heating is a continuous load. Section 424.3(B) requires the branch circuit conductors and overcurrent device to be sized at not less than 125 percent of the total heating load. A 12 amp mat therefore wants a 15 amp circuit at 12 x 1.25 = 15 amps, and anything larger goes to 20 amps on 12 AWG.

Section 424.44(A) requires the cables to be applied only to dry locations unless identified for wet use, 424.44(D) sets spacing so the cable is not less than 2 inches apart except at returns, and 424.44(E) requires the cable to be separated at least 8 inches from the edges of outlets and openings used for mounting luminaires. Section 424.44(F) requires cables to be secured and the manufacturer instructions govern under 110.3(B), so follow the mat spacing on the sheet in the box.

Do not forget the rest of the room

Section 210.8(A)(1) requires GFCI protection for all 125 volt through 250 volt receptacles in a dwelling bathroom, and 210.11(C)(3) requires at least one 20 amp branch circuit for bathroom receptacles with no other outlets on it. The floor heat cannot ride on that bathroom receptacle circuit if the circuit serves receptacles in more than one bathroom.

Field takeaway

Use the manufacturer GFCI thermostat, size the circuit at 125 percent of the mat load, and keep the heat off the dedicated bathroom receptacle circuit.