10kW Heat Strip Breaker Size: Amps, Wire and a 5–20 kW Chart

A heat strip is the biggest single circuit most heat pump systems add, and its breaker comes from two NEC rules and one label on the air handler.

Short answer: a 10 kW heat strip at 240 V draws 10,000 ÷ 240 = 41.7 A. Fixed electric space heat is a continuous load, so the circuit is sized at 125%: 41.7 × 1.25 = 52.1 A, which rounds up to a 60 A breaker, typically on 6 AWG copper. But the air handler's nameplate minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP) include the blower motor, and they govern whenever they differ from this arithmetic.

How is a heat strip breaker sized?

Four steps, shown for 10 kW at 240 V:

1. Amps = watts ÷ volts. 10,000 ÷ 240 = 41.7 A.
2. Apply 125%. The NEC treats fixed electric space-heating equipment and its motors as a continuous load (NEC 424.3(B), numbered 424.4(B) in some editions), and continuous loads are sized at 125% (NEC 210.19(A), 210.20(A)): 41.7 × 1.25 = 52.1 A.
3. Round up to a standard breaker. 50 A is too small. The next standard size under NEC 240.6(A) is 60 A.
4. Pick a conductor whose ampacity covers the load and is protected by that breaker: 6 AWG copper at 75 °C, rated 65 A in NEC Table 310.16.

What about 208 volts?

Many apartments and condos get 208 V instead of 240 V. The tempting math is 10,000 ÷ 208 = 48.1 A, × 1.25 = 60.1 A: just over 60, which would round up to 70 A.

That math is wrong for a resistance heater. An element is a fixed resistance, so lower voltage means less current and less heat. For a kit rated 10 kW at 240 V:

• Element resistance: 240² ÷ 10,000 = 5.76 Ω
• Current at 208 V: 208 ÷ 5.76 = 36.1 A
• Output at 208 V: 208 × 36.1 = about 7.5 kW, roughly 75% of the 240 V rating, because output scales with the square of the voltage
• Circuit: 36.1 × 1.25 = 45.1 A, so a 50 A breaker

Heat-kit labels usually list both a 240 V and a 208 V rating. Use the one that matches your supply.

And if a heater really is rated 10 kW at 208 V, drawing 48.1 A? That's over 48 A, so NEC 424.22(B) requires its elements to be subdivided. It would be built as two circuits, not wired to a single 70 A breaker.

Heat strip breaker size chart

For kits rated at 240 V, heater current only (no blower):

The same 240 V-rated kits on a 208 V supply:

Which wire column applies. The 75 °C column assumes conductors in conduit (or SER cable) landing on terminals rated 75 °C at both ends (NEC 110.14(C)). NM-B cable is limited to its 60 °C ampacity (NEC 334.80), and so is any circuit of 100 A or less whose terminations aren't marked for 75 °C. That's why the 8 kW cable run steps up to 6 AWG. Long runs may need larger wire again for voltage drop.

* 6 AWG NM-B is rated 55 A. That covers the 10 kW heater's 52.1 A, and because 55 A isn't a standard breaker size, NEC 240.4(B) lets the next standard size up, 60 A, protect it. It works only while the load stays at or under 55 A: once the blower pushes the nameplate MCA past 55 A, the cable steps up to 4 AWG.

15 kW and 20 kW heat strips: two circuits, two breakers

NEC 424.22(B) caps the overcurrent protection for resistance heating elements at 60 A, and requires any heater rated over 48 A to have its elements subdivided into loads of 48 A or less. The protective devices for those subdivided loads are built into the heater or supplied by its manufacturer (NEC 424.22(C)).

A 15 kW kit (62.5 A) and a 20 kW kit (83.3 A) are both over 48 A, so they arrive split: typically two element groups, each with its own breaker or fuses in the kit and usually its own supply circuit. The split varies by model; your wiring diagram shows it. Examples:

The blower motor usually rides on one of the circuits, which is why the two MCA figures on the label often differ even when the element groups match.

Some air handlers accept a manufacturer's single-point wiring kit that feeds both groups from one larger circuit, with the kit's internal breakers still protecting each group at 60 A or less. Use one only when the manufacturer lists it for your model, and size that feed from the label's single-point MCA and MOP.

Why the nameplate overrules this chart

The chart is heater-only. The circuit also feeds the blower motor, which the NEC also counts as continuous, so the nameplate MCA runs higher. Illustration: an 8 kW kit plus a hypothetical 4 A blower gives (33.3 + 4) × 1.25 = 46.7 A, past the heater-only 45 A, so the circuit becomes 50 A.

• MCA (minimum circuit ampacity) is the smallest conductor ampacity allowed, read in the temperature column that applies.
• MOP (maximum overcurrent protection), often printed as "max fuse" or "max breaker", is the largest breaker allowed. Installers typically use the next standard size at or above the MCA, never above the MOP.

The air handler's rating plate often lists MCA and MOP for each approved heat kit. NEC 110.3(B) makes that labeling part of the code requirement.

Why so many amps?

Heat strips are electric resistance heat: one unit of heat per unit of electricity, a COP of 1. A heat pump moves heat instead of making it, which is why strips are the backup; the efficiency comparison is in heat pump vs electric resistance heat.

Sizing the strip to the actual gap at your design temperature, rather than the largest kit the air handler accepts, can drop a breaker size or a whole circuit; see heat pump backup heat explained. Whether your service can carry it is a load-calculation question (10 kW is 10,000 VA): see panel upgrade for a heat pump and the home electrical load calculator.

For breaker sizing on other circuits, see what size breaker do I need.

Why is this electrician work?

A heat strip circuit means 30 to 60 A double-pole breakers, 10 to 4 AWG conductors, torque-specified terminations, a disconnect at the equipment, and usually a permit and inspection. A poor connection on a large continuous load runs hot for hours at a time. Use these numbers to check a quote or a nameplate; a licensed electrician and your local AHJ make the final call.

To check a strip's real current against the chart, the electrician or HVAC technician clamps one conductor of the heat-strip circuit with the strips running; clamped around the whole cable, the meter reads close to zero. The reading can include the blower's current when it shares that circuit. The jaws go around live conductors at the air handler, so this is qualified-person work, not a homeowner check.

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Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 424.3(B) (424.4(B) in some editions): fixed electric space-heating equipment and motors are continuous loads
• NEC 210.19(A) and 210.20(A): branch-circuit conductors and overcurrent devices for continuous loads sized at 125%
• NEC 424.22(B): resistance elements protected at no more than 60 A; heaters over 48 A subdivided into loads of 48 A or less
• NEC 424.22(C): supplementary overcurrent devices for subdivided loads, factory-installed or supplied by the heater manufacturer
• NEC 424.19: disconnecting means for fixed electric space-heating equipment
• NEC 240.6(A): standard breaker sizes
• NEC 240.4(B): next standard breaker size up permitted when a conductor's ampacity falls between standard sizes
• NEC Table 310.16: conductor ampacity
• NEC 110.14(C) and 110.14(D): termination temperature limits, and torque for terminations
• NEC 334.80: NM cable ampacity taken from the 60 °C column
• NEC 110.3(B): install listed equipment per its nameplate and instructions

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Size the heat pump first: heat pump sizing calculator
• Wire and breaker size chart
• What size wire for 60 amps
• Single-pole vs double-pole breakers

More in our heating and cooling guides.