NEC Article 424 (Fixed Electric Space Heating)
What Size Wire for an Electric Furnace?
An electric furnace is a large continuous heat load, sized to the nameplate. Here is the current by kilowatt, when it needs multiple circuits, and the calculator.
Size to the Nameplate, Expect Multiple Circuits
An electric furnace is resistance heat, so the amps are simply watts divided by 240V, and NEC 424.3(B) makes it a continuous load sized at 125%. The catch is scale: past about 10 kW the current is larger than one practical circuit, so the manufacturer divides the elements into two or three field-connected subcircuits, each energized in stages by an internal sequencer. The nameplate prints a minimum circuit ampacity and a maximum overcurrent protection for each connection, and those figures already include the blower motor. Wire each circuit to its own listed ampacity and breaker; the table below is a typical guide, not a substitute for the data plate.
Electric Furnace Current by Size (240V)
| Furnace Size | Load Amps | At 125% | Typical Circuit(s) |
|---|---|---|---|
| 10 kW | 41.7 A | 52.1 A | 1 x 60A, #6 Cu |
| 15 kW | 62.5 A | 78.1 A | 2 - 3 circuits (see nameplate) |
| 20 kW | 83.3 A | 104.1 A | 2 - 3 circuits (see nameplate) |
| 25 kW | 104.2 A | 130.3 A | 2 - 3 circuits (see nameplate) |
The 125% column is the minimum ampacity a single circuit would need. Wire shown is at the 75°C column, which furnace terminals are rated for; on 60°C NM-B cable a 60A circuit needs #4 rather than #6. Once the load passes ~60A the furnace almost always divides it; a 20 kW unit is commonly two 40A / #8 circuits, for example. The nameplate's per-circuit minimum circuit ampacity and maximum breaker are the code values.
Size a Single Circuit
Enter one circuit's nameplate ampacity to get the wire, or the whole-furnace amps if it is a single-circuit unit. Split multi-circuit furnaces by their listed per-circuit values.
Frequently Asked Questions
What size wire do I need for an electric furnace?
Size it from the furnace's nameplate minimum circuit ampacity, and expect it to be a large load. An electric furnace is a resistance heat load: watts / 240V gives the amps, and NEC 424.3(B) makes it continuous, so multiply by 1.25. A 10 kW / 240V furnace draws about 42A, or 52A at 125%, which lands on a 60A circuit with #6 copper. At 15 kW (~78A) and above, the current exceeds what one convenient circuit carries, so most furnaces split the heat into two or three field-connected circuits, each with its own breaker and wire. The nameplate lists the minimum circuit ampacity and maximum breaker for each connection; follow it and the install manual.
Why does an electric furnace use multiple circuits?
Because the total load is too big for a single practical circuit. A 20 kW / 240V furnace draws about 83A, which at 125% is over 100A, so putting it on one breaker would need very large conductors and a big double-pole breaker. Instead, the furnace has an internal sequencer that energizes the heating elements in stages, and the manufacturer divides them into two or three subcircuits (for example, two 40A circuits). Each subcircuit gets its own breaker and conductor sized to that circuit's nameplate ampacity.
What size breaker for an electric furnace?
Use the maximum overcurrent protection printed on the nameplate for each circuit, not a value you calculate. For a single-circuit 10 kW / 240V furnace that is commonly a 60A breaker; a 15 kW single circuit is often 80A. Multi-circuit furnaces list a separate max breaker per connection. Because the elements are a continuous load, the breaker and wire are already sized at 125%, and you must not exceed the listed maximum.
Is the electric furnace blower included in the wire size?
Yes, the nameplate minimum circuit ampacity already accounts for the air-handler blower motor plus the heating elements, so you do not add it separately. The manufacturer includes the 125% factor on the largest motor and the continuous heat load when it prints the minimum circuit ampacity. That is one more reason to size from the nameplate rather than adding up parts yourself.
Does an electric furnace need a disconnect?
Yes. Fixed electric space-heating equipment needs a disconnecting means for the heater and any motor controller and supplementary overcurrent devices (NEC 424.19). In practice the branch-circuit breakers serve as the disconnect if they are within sight of the furnace or capable of being locked; otherwise a separate disconnect is installed at the unit.
Related
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The smaller electric-heat load: watts-per-circuit budget and by-wattage wire.
Tankless Water Heater Wire Size
Another large multi-circuit continuous load sized to the nameplate.
Wire Size for Appliances
Dryer, range, water heater, AC, and more: the circuit for each.
Wire Size Calculator
Size any load per NEC 310.16 with derating and voltage drop.