What Size Wire for an Electric Furnace?

Size it from the furnace's nameplate: a listed furnace prints a minimum circuit ampacity and maximum breaker for each field circuit, and NEC 110.3(B) makes them binding. For a unit marked only in volts and watts, NEC 424.4(B) sizes the branch-circuit conductors at not less than 125 percent of the heating load and any associated motor. A 10 kW / 240 V element bank draws 41.7 A, 52.1 A at 125%, which lands on a 60 A circuit with #6 AWG copper (60 °C or 75 °C), before the blower's amps are added. At 15 kW (62.5 A, 78.1 A at 125%) the equipment is over 48 A. Over 48 A of equipment load, NEC 424.22(B) has the unit's heating elements subdivided at 48 A or less each, protected at 60 A or less by supplementary devices in, on or supplied with the unit (424.22(C)). Whether the unit then takes one field circuit or several is its listing: wire what the plate prints (110.3(B)).

Because NEC 424.22(B) requires resistance heating equipment rated more than 48 A to have its elements subdivided, each subdivided load not over 48 A and protected at not more than 60 A. A 20 kW / 240 V furnace draws 83.3 A (104.2 A at 125%), so an internal sequencer energizes the subdivided elements in stages, and the listing brings them out either as separate field-connected circuits or through a single-point connection; the nameplate says which (where it is two circuits, for the heating elements alone, a 20 kW furnace split in two needs two 60A circuits, since each 41.7A half is 52.1A at 125%; the subcircuit that also runs the blower adds that motor's load under NEC 424.4(B), so its nameplate figure can be larger). Each field circuit gets its own breaker and conductor sized to that circuit's nameplate ampacity.

Never larger than the maximum overcurrent protection printed on the nameplate for that circuit (NEC 110.3(B)). For a unit marked only in volts and watts, the arithmetic for the elements alone is a 60 A breaker at 10 kW / 240 V and 80 A at 15 kW as a single figure, before the blower; at 15 kW the unit is over 48 A, so its elements are subdivided (424.22(B)) and the nameplate gives the figures for each field connection. On a heating circuit NEC 240.4(D) holds #10 copper to 30 A: Table 240.4(G), which relaxes that for air conditioners, has no Article 424 row.

It must be. NEC 424.4(B) sizes the branch-circuit conductors at not less than 125 percent of the heating load and any associated motor, so the blower motor's amps go inside the 125% on the circuit that runs it. On a listed furnace, the nameplate's minimum circuit ampacity for that circuit is the figure to size to (110.3(B)); for a motor over 1/8 hp the plate also states its rating in volts, amperes and frequency (424.28(A)).

Yes. NEC 424.19 requires a means that disconnects the heater, the motor controller and any supplementary overcurrent devices from all ungrounded conductors at once, rated at least 125 percent of the motor and heater load and capable of being locked open. Where the unit has factory element fuses or breakers (424.19(A)) that means is within sight of them; with a blower over 1/8 hp it may also serve the motor controller where it is in sight from both and complies with Part IX of Article 430, or it may be out of sight from the motor controller where the motor and heater have a single unit switch that complies with 422.34 (424.19(A)(2)). A branch breaker that is merely lockable is the disconnect only for equipment with no supplementary devices and no motor over 1/8 hp (424.19(B)(1)). A unit fed by more than one circuit has its disconnects grouped and identified.