What Size Wire for a Baseboard Heater?

Match the wire to the circuit breaker, and size the circuit for 125% of the heater wattage because NEC 424.3(B) treats fixed electric space heating as a continuous load. Most 240V baseboard runs on a 20A circuit with #12 copper, which legally carries up to 3,840 watts of heat (16A x 240V). Lighter loads up to 2,880 watts fit a 15A circuit with #14 copper. 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, but 240V is far more common and more efficient for whole-room heat because it draws half the current of a 120V heater of the same wattage, 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 424.3(B) classifies fixed electric space-heating equipment as a continuous load, running three hours or more, without you having to prove it. Continuous loads are limited to 80% of the circuit rating, or equivalently the circuit is sized at 125% of the load (NEC 210.19(A)(1) and 210.20(A)). That is why a 3,000W / 240V heater (12.5A) needs a 20A circuit rather than a 15A: 12.5 x 1.25 = 15.6A, which is over the 15A rating.

Not necessarily its own circuit, but a heating circuit should serve only heaters (and their thermostat), not receptacles or lighting, and the total wattage must stay within the 80% continuous limit. A line-voltage thermostat is wired in series ahead of the heaters it controls. Each heater also needs a disconnecting means (NEC 424.19), which the branch-circuit breaker can satisfy if it is within sight or lockable.