Fixed Electric Space-Heating Branch Circuit Calculator
Enter the nameplate rating of electric baseboard, a unit heater, or an electric furnace and get the branch-circuit breaker and minimum conductor. NEC 424.3(B) treats fixed electric space-heating equipment and any associated motor as a continuous load, so the circuit is sized at 125% of the total load, and 424.22(B) governs how the elements are subdivided above 48 A.
How to use this calculator
- Pick a common heater from the list, or read the nameplate and enter its heating rating. Baseboard and furnaces are usually marked in watts (for example 1500 W or 10000 W); switch the unit to amps if the label gives amperes.
- Set the nominal voltage and phase. Residential baseboard is 120 V or 240 V single-phase; commercial unit heaters are often 208 V, 277 V, or 480 V, and three-phase units enter watts with the phase set to three-phase (or enter the nameplate line amps directly).
- If the equipment has a blower or fan motor (electric furnaces and many unit heaters do), enter its full-load current in the optional associated-motor field. NEC 424.3(B) folds it into the same continuous load.
- Adjust the conductor material, insulation and termination ratings, ambient temperature, and conductor count if your install differs from the standard basis.
- Read the minimum breaker and conductor. If the resistance elements exceed 48 A, the result flags the 424.22(B) subdivision the manufacturer must provide, and you follow the nameplate's marked minimum circuit ampacity and maximum overcurrent device.
NEC reference
Sizing follows NEC 2020. Section 424.3(B) states that fixed electric space-heating equipment and any associated motors shall be considered a continuous load for the purpose of sizing branch circuits, so the conductors and overcurrent device are sized at not less than 125% of the total load (the general continuous rule of 210.19(A)(1) and 210.20(A)). Section 424.22(B) requires resistance-type heating elements rated over 48 A to be subdivided into loads not exceeding 48 A, each protected at not more than 60 A by a factory-installed supplementary overcurrent device. The breaker is a 240.6(A) standard rating, and the conductor comes from Table 310.16 with 310.15 derating, the 110.14(C) termination cap, and the 240.4(B)/(D) conductor-protection limits. These rules are unchanged across the 2017, 2020, and 2023 editions; only the section numbering shifts.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Common electric-heat circuits: copper, standard conditions
| Nameplate | Current | Min circuit (125%) | Min breaker | Copper conductor |
|---|---|---|---|---|
| 1500 W @ 240 V (baseboard) | 6.25 A | 7.81 A | 15 A | 14 AWG |
| 4000 W @ 240 V (unit heater) | 16.7 A | 20.83 A | 25 A | 10 AWG |
| 7500 W @ 240 V (unit heater) | 31.3 A | 39.06 A | 40 A | 8 AWG |
| 10 kW @ 240 V (furnace) | 41.7 A | 52.08 A | 60 A | 6 AWG |
| 15 kW @ 240 V (furnace, subdivided) | 62.5 A | 78.13 A | 80 A | 4 AWG |
Standard conditions (86°F, 90°C insulation on 75°C terminations, 2 current-carrying conductors, copper, resistance elements only). The 15 kW row exceeds 48 A, so 424.22(B) requires the manufacturer to subdivide the elements; the branch circuit above is the minimum ampacity, and the nameplate governs the internal fuse blocks. Aluminum, hot ambients, and long runs (voltage drop) change the conductor; use the calculator above with your actual conditions.
The 125% continuous-load rule for electric heat
The rule that governs an electric-heat circuit is that the load is continuous. NEC 424.3(B) makes that determination for you: fixed electric space-heating equipment and any associated motor are a continuous load, full stop, without any test of how long the heater actually runs. NEC 210.20(A) then applies the continuous-duty requirement, the 125% factor, to both the overcurrent device and the conductors.
So a 10 kW / 240 V furnace is not on a 45 A circuit. It draws 41.67 A, needs 41.67 x 1.25 = 52.08 A of protection and conductor ampacity, and lands on a 60 A breaker with 6 AWG copper. Sizing electric heat to the raw current instead of 125% is the most common mistake, and it produces a circuit that nuisance-trips or overheats the terminations under a full heating season.
The associated motor is part of the continuous load
Electric furnaces and many unit heaters run a blower or fan whenever the elements energize. NEC 424.3(B) is explicit that the equipment and any associated motors are considered together as one continuous load, so the motor full-load current is added to the resistance-element current before the 125% factor, not sized on its own Article 430 circuit. A 10 kW furnace (41.67 A) with a 3 A blower is a 44.67 A total, 55.83 A at 125%, still a 60 A circuit here. Enter the blower full-load current in the optional associated-motor field so the tool folds it in.
Elements over 48 A: the 424.22(B) subdivision
Fixed space heating does not have a single "150% maximum breaker" rule the way a water heater does under 422.11(E). Instead, NEC 424.22(B) caps the resistance heating elements themselves: any element load over 48 A must be subdivided by the manufacturer into loads not exceeding 48 A, each protected at not more than 60 A by a factory-installed supplementary overcurrent device. This is why a 15 kW or 20 kW furnace has internal fuse blocks or is fed as more than one circuit rather than a single large breaker on a single conductor.
This calculator sizes the branch-circuit minimum ampacity and its overcurrent device from the 125% continuous rule, and flags when the elements exceed 48 A so you know the equipment must carry the internal subdivision. The nameplate's marked minimum circuit ampacity and maximum overcurrent protective device govern the field installation, so match the breaker and conductor to the nameplate whenever it is marked.
Frequently asked questions
What breaker for a 10 kW electric furnace?
41.67 A at 240 V x 1.25 = 52.08 A, so a 60 A breaker on 6 AWG copper (75°C). Add the blower motor full-load current before the 125% if the furnace has one. Enter 10000 W above to see the aluminum equivalent and your exact conditions.
Is 12 AWG or 10 AWG right for a baseboard heater?
It depends on the wattage. A single 1500 W or 2000 W / 240 V baseboard (6.25 A or 8.33 A) fits 14 AWG on a 15 A breaker. Loading several heaters up toward 12 A or 16 A on one 20 A circuit moves you to 12 AWG. Enter the total connected watts on the circuit to be sure, and never assume from the breaker alone.
Does this size the ground wire?
No. The equipment grounding conductor is sized from the breaker rating under NEC Table 250.122, not the 125% circuit ampacity. Use the Equipment Grounding Conductor Size Calculator linked below with the breaker size this tool returns.
What about voltage drop on a long run?
A unit heater in a detached shop or at the far end of a long run can push the conductor larger than the ampacity minimum to keep voltage drop within the recommended 3%. This calculator sizes for ampacity and overcurrent only; run the distance through the Voltage Drop Calculator before finalizing.
Related calculators
- Water Heater Circuit Calculator: the sibling continuous-load tool for a storage water heater (NEC 422.13).
- Breaker Size Calculator: the general continuous/noncontinuous load sizing this tool is built on.
- MCA & MOCP Calculator: heat pumps and air conditioners size by nameplate MCA/MOCP under Article 440.
- Equipment Grounding Conductor Size Calculator: size the EGC from the breaker rating (Table 250.122).
- Voltage Drop Calculator: check the run length before finalizing the conductor.
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For deeper NEC training on electric heat branch circuits and continuous-load sizing, Mike Holt's NEC courses are the industry standard.
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