What size breaker does an 8 kW sauna heater need?

An 8 kW heater draws about 33 A at 240 V. Sized at 125 % of that current, the next common breaker is 50 A. The heater manual gives the exact breaker and wire size.

What size breaker does a 6 kW sauna heater need?

A 6 kW heater draws 25 A at 240 V. At 125 % that is about 31 A, so the next common breaker is 40 A. Check the heater manual for the exact value.

Can a sauna heater run on 120 V?

Only very small units. Traditional sauna heaters from about 4.5 kW upward need a 240 V circuit in North America.

Sauna Heater Breaker Size: Amps for 4.5 to 18 kW

Have a licensed electrician do the connection. A sauna heater is one of the largest loads in a house, and the installation must follow your local code and the heater manual.

Current and breaker by heater size

At 240 V, current is power divided by voltage. In the US, fixed electric heating is treated as a continuous load, so the breaker must be at least 125 % of the current (NEC 424.3(B) with 210.20(A)). The table rounds up to the next common two-pole breaker.

Heater	Current at 240 V	At 125 %	Common breaker	400 V three-phase, per phase	Typical fuse
3.6 kW	15.0 A	18.8 A	20 A	5.2 A	3 × 10 A
4.5 kW	18.8 A	23.4 A	25 A	6.5 A	3 × 10 A
6 kW	25.0 A	31.3 A	40 A	8.7 A	3 × 10 A
8 kW	33.3 A	41.7 A	50 A	11.5 A	3 × 16 A
9 kW	37.5 A	46.9 A	50 A	13.0 A	3 × 16 A
10.5 kW	43.8 A	54.7 A	60 A	15.2 A	3 × 16 A
12 kW	50.0 A	62.5 A	70 A	17.3 A	3 × 20 A
15 kW	62.5 A	78.1 A	80 A	21.7 A	3 × 25 A
18 kW	75.0 A	93.8 A	100 A	26.0 A	3 × 35 A

Breakers: 15, 20, 25, 30, 40, 50, 60, 70, 80, 90 and 100 A. Some panels also take 35 and 45 A breakers, which can fit a heater more closely. Fuses: common European sizes 10, 16, 20, 25 and 35 A. Wire size depends on the breaker, the cable type and the run, and is given in the heater manual.

Europe and Finland: three-phase

In Finland and much of Europe, sauna heaters above a few kilowatts are normally connected to three phases at 400 V. The current per phase is then about a third of the single-phase current, which is why a 9 kW heater runs happily on 3 × 16 A fuses. Small heaters can also be connected single-phase at 230 V; the manual lists both options when the heater supports them.

Things the electrician will check

Panel capacity. A 40–60 A circuit is a big addition. Older panels may need a load calculation or an upgrade.
Control unit or contactor. Larger heaters and heaters with a separate control panel switch the power through a contactor. Check whether your heater needs one before ordering.
Cable rated for the heat. The last stretch to the heater runs in a hot room and needs cable rated for it, as the manual specifies.
Dedicated circuit. The heater gets its own circuit. The lights go on a separate one.

Bigger heater, bigger circuit

A larger heater does not cost more to run per session, because it warms the room faster. What it costs is a larger breaker and thicker wire. If your panel is close to its limit, the next size down with a little more warm-up time may be the sensible choice. The calculator shows warm-up time for the recommended size and one size either side.