Sauna heater breaker and wire size, by kW
Updated November 4, 2026 · 240-volt, single-phase heaters · How we check
A 9 kW sauna heater draws 37.5 amps at 240 volts. That means a 40-amp breaker, or a 50-amp breaker if your inspector treats the heater as a continuous load (sized at 125%). A 6 kW heater draws 25 amps: a 30-amp or 35-amp breaker. Your heater's manual and your local code decide which.
Some makers list a different breaker than this math. A licensed electrician must confirm what your home needs.
Breaker and wire size for each heater size
| Heater | Amps at 240 V | Breaker: next size up | Copper wire for that breaker | Amps × 125% (continuous load) | Breaker at 125% | Copper wire for that breaker |
|---|---|---|---|---|---|---|
| 4.5 kW | 18.8 A | 20 A | 12 AWG | 23.4 A | 25 A | 10 AWG |
| 6 kW | 25.0 A | 30 A | 10 AWG | 31.3 A | 35 A | 8 AWG |
| 8 kW | 33.3 A | 35 A | 8 AWG | 41.7 A | 45 A | 8 AWG (6 AWG if NM cable) |
| 9 kW | 37.5 A | 40 A | 8 AWG | 46.9 A | 50 A | 8 AWG (6 AWG if NM cable) |
| 10.5 kW | 43.8 A | 45 A | 8 AWG (6 AWG if NM cable) | 54.7 A | 60 A | 6 AWG (4 AWG if NM cable) |
| 12 kW | 50.0 A | 50 A | 8 AWG (6 AWG if NM cable) | 62.5 A | 70 A | 4 AWG |
How each number is worked out: amps = watts ÷ 240. "Next size up" is the next standard breaker size (NEC 240.6) at or above the amps. The 125% columns multiply the amps by 1.25 first, the rule for loads that run 3 hours or more; whether it applies to your sauna heater is your inspector's call. Wire: the smallest copper size whose rating in NEC Table 310.16 covers the breaker (75°C column for wire in conduit, 60°C for NM house cable), with 12 AWG capped at 20 amps and 10 AWG at 30 amps (NEC 240.4(D)). These follow the 2023 National Electrical Code; your city may use a different edition or add its own rules.
What heater makers actually list
The manual for your exact heater is the rule your electrician and inspector follow. Two we've read:
- HUUM Drop 6 kW: the manual lists a 30-amp breaker and 10 AWG wire (25 amps at 240 volts). That's smaller than the 35 amps the 125% math above gives, so follow the manual and ask your electrician.
- Harvia Cilindro PC90E (9 kW): the manual gives 37.5 amps on one circuit, or 25 amps per circuit if split into two, but doesn't list a breaker size.
Also check before you buy
- Room in your electrical panel for a 2-pole breaker of that size, and enough total service. A 9 kW heater is roughly what an electric dryer and an oven use together.
- Heat-rated wire inside the sauna room. Many manuals require high-temperature wire for the last run to the heater; ordinary cable isn't allowed in the hot room.
- Hardwired, not plugged in. Most heaters of 4.5 kW and up are wired in; Harvia's Cilindro manual says plugs and pigtails are not allowed.
- A permit. Most cities require one for a new 240-volt circuit.