Garage Heater Wire Size Calculator
Enter your heater and the distance from the panel. Get the current, the breaker, the wire gauge and the voltage drop, then check your subpanel can take it.
Your heater
- Current draw
- 31.3 A
- Sized at 125%
- 39.1 A
- Voltage drop at 50 ft
- 1.0%
- Output
- 25,590 BTU/hr
Planning estimate only. Local code, temperature, conduit fill and the heater's nameplate decide the final answer. Have a licensed electrician confirm it.
Will my garage subpanel handle it?
You are 3 A over the 80% planning line. Run the heater and the big tools at different times, or talk to an electrician about a larger feeder.
Add up what really runs together: an EV charger, a compressor, a welder, a dust collector, lights. This is a quick check, not an NEC load calculation.
Wire and breaker chart for 240 V heaters
| Heater | Current | Breaker | Copper NM-B |
|---|---|---|---|
| 2,000 W | 8.3 A | 15 A | 14 AWG |
| 4,000 W | 16.7 A | 25 A | 10 AWG |
| 5,000 W | 20.8 A | 30 A | 10 AWG |
| 7,500 W | 31.3 A | 40 A | 8 AWG |
| 8,500 W | 35.4 A | 45 A | 6 AWG |
| 10,000 W | 41.7 A | 60 A | 6 AWG |
How the calculator picks a size
It divides watts by volts for the current, multiplies by 1.25 because fixed heat is a continuous load, and takes the next standard breaker. It then picks the smallest wire that carries that 125% current and that the breaker can protect, using 60°C ampacity for NM-B and 75°C for THHN. It keeps the usual limits of 15 A on 14 AWG, 20 A on 12 AWG and 30 A on 10 AWG. Where a wire's rating falls between breaker sizes, the code allows the next standard breaker up, which is why 6 AWG NM-B rated 55 A can sit on a 60 A breaker. If voltage drop passes 3%, it steps up a size.
Not sure how many watts you need? Start with the garage heater calculator, or read our electric garage heater guide for 120 V versus 240 V.
Questions people ask
What size wire do I need for a 5,000 watt heater?
At 240 V a 5,000 W heater draws 20.8 A. Sized at 125%, that points to a 30 A breaker and 10 AWG copper on a short run. Long runs may need the next size up.
What size breaker for a 7,500 watt heater?
A 7,500 W heater at 240 V draws 31.25 A. At 125% that is about 39 A, so the next standard breaker is 40 A, with 8 AWG copper NM-B on a short run.
What size wire for a 10,000 watt heater?
At 240 V a 10,000 W heater draws 41.7 A, which points to a 60 A breaker and 6 AWG copper NM-B on a short run.
Why is the heater sized at 125%?
Fixed electric space heating counts as a continuous load, so the National Electrical Code sizes branch-circuit conductors at no less than 125% of the load.
Can I run a 240V heater on a 120V circuit?
No. A heater needs the voltage on its nameplate. A 4,000 W heater also would not be practical at 120 V, because it would pull over 33 A.
Can a garage heater share a subpanel with an EV charger?
Only if the panel has room for both at once. Both can run for hours, so add their amps together and keep the total at or under 80% of the panel rating. If it does not fit, an electrician can look at a larger feeder or a load management device.
Why does my garage heater keep tripping the breaker?
Usually the circuit is too small or something else shares it. A 4,000 W heater at 240 V needs a 25 A circuit. If a correctly sized breaker keeps tripping, stop using the heater and call an electrician.
Sources
Sources last checked October 31, 2026.
- NEC 424.4(B) branch-circuit sizing, quoted by Fast Trax (accessed October 2026)
- Mike Holt on NEC 424.3(B) (accessed October 2026)
- IRC E3702.4 branch circuit ratings, UpCodes (accessed October 2026)