This is not a preference question. The voltage is decided by the size of the room you want to heat, and the arithmetic that decides it takes about a minute.
A 120V circuit can carry roughly 1.4 kW continuously on 15 amps, or 1.9 kW on 20 amps. That is enough for a small one- or two-person infrared cabin and nothing more. Every traditional Finnish heater sold in the US starts at 3 kW and runs on 240V. If you want steam, stones and 180 °F, the answer is 240V and there is no way around it.
A sauna heater is a continuous load — it runs for three hours or more at a stretch. The NEC handles continuous loads in Article 210 by requiring the circuit to be sized at 125 % of the load, which is the same thing as saying a load may not exceed 80 % of the circuit rating. Run the numbers on a household circuit and you get:
| Circuit | Rating | Continuous ceiling | Watts available |
|---|---|---|---|
| 120V household, 15 A | 15 A | 12 A | 1,440 W |
| 120V household, 20 A | 20 A | 16 A | 1,920 W |
| 240V, 30 A | 30 A | 24 A | 5,760 W |
| 240V, 40 A | 40 A | 32 A | 7,680 W |
| 240V, 50 A | 50 A | 40 A | 9,600 W |
Watts available after the 80 % continuous-load limit. The two 120V bars are every plug-in option you have.
That last column is the whole article. Doubling the voltage doubles the watts a given amperage delivers, which is why 240V is not a luxury upgrade but the only way to get a real heater onto a circuit a house can actually carry.
Small far-infrared cabins. The Dynamic Barcelona, a common one- to two-person model, lists its electrical service as 120V/15 A. These are radiant panel cabins: they warm the person rather than the room, so they operate well below traditional sauna temperatures and take a good while to get there. If that is what you want, a 120V unit is a legitimate purchase and not a compromise.
What 120V will not do is heat a wood-lined room to Finnish temperature. No amount of insulation fixes a 1,440-watt ceiling.
Traditional heaters with stones. The smallest in Harvia's KIP range is 3.0 kW at 240V, rated by its manual for a room of 84–130 cubic feet with a minimum ceiling height of 75 inches. Move up to a 6 kW and the manual's band is 170–300 cubic feet; at 8 kW, 250–425 cubic feet.
Take your interior length, width and height in feet and multiply them. Harvia's published rule is one kilowatt per cubic metre of room volume, so divide your cubic feet by about 35 to get cubic metres, and that number in kilowatts is roughly what the room needs. Then add for glass: every square metre of glass, stone or other uninsulated surface counts as if you had added 1.2 cubic metres of room. A full glass door alone usually adds around two kilowatts of requirement.
If the figure lands above about 1.9, you need 240V. In practice almost every real sauna room does.
Enter your room and it returns the kW figure Harvia’s published rule produces, then shows what the manuals specify for breaker and wire at that size next to what the code formula gives — because the two do not agree.
Open the calculatorA new 240V circuit means a two-pole breaker in your panel, a home run of appropriately sized copper to the sauna, a disconnecting means within sight of the heater, and enough spare capacity in the service to carry the new load. The cost is driven almost entirely by the distance from the panel and how hard that path is to reach, not by the breaker itself — which is why quotes for the same heater vary so widely between houses.
Two things worth knowing before you get quotes. First, your panel may not have room, and finding that out after buying a heater is an expensive order of events. Second, if the run is long, the wire may need to be larger than the heater's manual states, because voltage drop over distance is a separate calculation from the load itself.
Do not size this yourself. Everything above is arithmetic you can check, but the circuit that ends up in your wall has to be designed and installed by a licensed electrician working to the code edition your jurisdiction has adopted, and inspected. Bring them the heater's manual, not a printout of this page.
Every figure above is reproduced from one of these. None of it is our own engineering determination, and where two sources disagree we have said so rather than picking one.