There are two answers to this question and they do not match. Most pages give you one of them without mentioning the other exists. Here is both, with the sources, and the reason they differ.
At 240V: 6 kW takes a 30 A two-pole breaker with 10/2 copper. 8 kW takes 40 A with 8/2. 9 kW takes 45 A with 8/2. Copper with 90 °C insulation, never aluminium, hard-wired to the heater.
Every one of these is a line item on the electrician’s quote, and the last two are where inspections fail.
These figures come from Harvia's installation guide and from Redwood Outdoors' published heater and electrical requirements. Where a size appears in both, they agree.
| Heater | Breaker | Copper wire | Source |
|---|---|---|---|
| 4.5 kW | 30 A, 2-pole | 10/2 | Harvia installation guide |
| 6.0 kW | 30 A, 2-pole | 10/2 — 8/2 on a long run | Harvia; Redwood Outdoors |
| 8.0 kW | 40 A, 2-pole | 8/2 | Harvia; Redwood Outdoors |
| 9.0 kW | 45 A, 2-pole | 8/2 | Redwood Outdoors |
| 10.5 kW | 50 A, 2-pole | 6/2 | Redwood Outdoors |
A sauna heater is a continuous load, and NEC Article 210 sizes continuous loads at 125 % of the load current. Do that arithmetic and you get a different column:
| Heater | Amps at 240V | × 1.25 | Next standard breaker | Manual says |
|---|---|---|---|---|
| 4.5 kW | 18.8 A | 23.4 A | 25 A | 30 A |
| 6.0 kW | 25.0 A | 31.3 A | 35 A | 30 A |
| 8.0 kW | 33.3 A | 41.7 A | 45 A | 40 A |
| 9.0 kW | 37.5 A | 46.9 A | 50 A | 45 A |
| 10.5 kW | 43.8 A | 54.7 A | 60 A | 50 A |
Breaker size at 240V. Solid bars are what the manufacturer manual specifies; faded bars are what the NEC continuous-load formula gives.
They disagree at every size, and that is the most useful thing on this page.
The usual explanation is that a sauna heater is listed equipment. NEC 110.3(B) requires listed equipment to be installed in accordance with its listing, and the breaker size the manufacturer marks on the appliance was established as part of that listing. A marked rating generally governs over a derivation you did yourself. The heater also cycles on a thermostat rather than drawing flat out for the whole session.
We are not going to tell you which number wins in your jurisdiction, because that is your inspector's call and it depends on your specific unit's listing. What we will say is that a contractor who quotes one of these without knowing the other exists has not read the manual.
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 calculatorHarvia's guide and Redwood both specify copper with 90 °C insulation and Redwood states plainly: never aluminium. Gauge is the number everyone asks about; insulation temperature rating is the one that gets skipped, and it matters in a hot, enclosed space.
Harvia's installation guide moves a 4.5 or 6 kW heater from 10/2 up to 8/2 beyond 30 feet of run. Redwood Outdoors puts that same threshold at 90 feet. Both are published, both are in print right now, and they are three times apart.
Neither is really a rule. The underlying issue is voltage drop, which depends on the actual length, the load and the conductor — a calculation your electrician does from your numbers. Treat both published figures as a signal that on any run past about thirty feet the question needs to be asked out loud, and have the exact distance ready when you ask it.
Harvia's installation guide says, in plain words, not to use a GFCI with the heater. Redwood Outdoors specifies a standard non-GFCI two-pole breaker. Meanwhile NEC 210.8 requires GFCI protection in a range of wet and outdoor locations, and an outdoor sauna can fall inside that scope.
These genuinely conflict. The resolution depends on your heater's listing and where the unit physically sits, and it is settled by your local inspector — not by us, and not by a forum thread. Raise it explicitly before anyone pulls wire, because discovering it at inspection means opening a wall.
Harvia's guide is explicit that the heater is hard-wired directly rather than plugged in. Separately, a readily accessible disconnecting means within sight of the unit is standard practice for fixed heating equipment. Neither of these shows up in the breaker-and-wire question people search for, and both show up at inspection.
Conductor temperature derating for the ambient conditions of the run, voltage drop over your actual distance, the feeder and panel capacity this new load lands on, whether your service has room for it at all, grounding and bonding, and the disconnect. Those are the electrician's work, and they are the reason a licensed one has to sign off on this rather than a table on a website.
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.