What Size Wire for a Baseboard Heater?

Match the wire to the circuit breaker, and size the circuit for 125% of the heater wattage: NEC 424.4(B) requires the branch-circuit conductors to carry not less than 125 percent of the fixed heating load. A 20A circuit with #12 copper carries up to 3,840 watts of heat at 240V (16A x 240V), and a 15A circuit with #14 copper up to 2,880 watts. Convert watts to amps first (watts / volts), multiply by 1.25, then pick the breaker and the matching wire.

As many as fit under 80% of the breaker, which is the same as sizing at 125%. A 20A/240V circuit carries 3,840 watts total, so that is roughly two 1,500W heaters or three 1,000W heaters wired together. A 15A/240V circuit carries 2,880 watts. Add up the wattage of every heater on the circuit; the sum, not the count, is what matters. A line-voltage thermostat controlling the group counts as part of the circuit.

Both exist. A 240V heater draws half the current of a 120V heater of the same wattage (resistance heat turns all of its input into heat at either voltage), so a single circuit carries twice the heat. A 120V baseboard is usually a small supplemental unit. On 120V, a 20A/#12 circuit tops out at 1,920 watts and a 15A/#14 at 1,440 watts, half the 240V figures.

Because NEC 424.4(B) says so for fixed electric space heating: the branch-circuit conductor ampacity shall not be less than 125 percent of the load of the heating equipment and any associated motor. That puts a 3,000W / 240V heater (12.5A) on at least 15.6A of conductor (12.5 x 1.25), so #12 rather than #14. The breaker follows the same 125%: a heater that runs three hours or more is a continuous load, and NEC 210.20(A) sizes its overcurrent device at not less than 125 percent of it, so 15.6A needs a 20A breaker rather than a 15A. Keeping the total heat at or under 80% of the breaker is the same rule read backwards.

Not necessarily its own circuit. A branch circuit that supplies two or more fixed heater outlets is rated not over 30A (NEC 424.4(A)), and a receptacle built into a baseboard heater must not be connected to the heater circuit (NEC 424.10). A 15 or 20A circuit that also supplies lighting or plug-in loads may carry fastened-in-place heat of no more than 50 percent of its rating (NEC 210.23(B)(2)); the watt budgets on this page are for a circuit that serves only heaters. A line-voltage thermostat is wired in series ahead of the heaters it controls. Each heater also needs a disconnecting means (NEC 424.19): for a heater with no motor over 1/8 hp and no supplementary overcurrent devices, the branch-circuit breaker satisfies it where it is within sight of the heater or capable of being locked open (424.19(B)(1)).