Home / Heater sizing
Heater size, breaker and wire
What size sauna heater do you need?
AI-generated illustrationStart from HUUM’s rule of thumb, about 1 kW per 35 ft³ of hot room, adding volume for glass, tile and stone and ×1.5 for log or barrel walls, then pick from the heater’s own room table, which allows up to 44–56 ft³ per kW. A 6×5×7 ft room with a glass door sizes at about 265 ft³: a 6–7.5 kW heater. A 6 kW heater draws 25 A on 240 V and needs a dedicated circuit.
Checked Sep 23, 2026 · how we checkThe size updates as you answer.
Heaters that fit: HUUM Drop 6, HUUM Drop 7.5, Harvia Spirit 6 and 5 more below.
How we got this
| Room volume (6 × 5 × 7 ft) | 210 ft³ |
| Added for glass, tile or stone [Harvia Spirit manual] | +55 ft³ |
| Sizing volume | 265 ft³ |
| About 1 kW per 35 ft³ [HUUM sizing guide] | 7.5 kW |
A starting point, not a design. The heater manual and your electrician or inspector have the final word.
Every heater rated for this roomNo. 2The heaters
Electric heaters rated for 265 ft³
| Heater | kW | Room range (ft³) | Amps | Maker’s breaker | 125% rule minimum | Maker’s wire | Source |
|---|---|---|---|---|---|---|---|
| HUUM Drop 6 | 6 | 177–318 | 25 | 30 A | 35 A check | 10 AWG | [HUUM Drop US manual] |
| HUUM Drop 7.5 | 7.5 | 248–389 | 31.3 | 40 A | 40 A | 8 AWG | [HUUM Drop US manual] |
| Harvia Spirit 6 | 6 | 177–294 | 25 | not stated | 35 A | 10 AWG | [Harvia Spirit manual] |
| Harvia Spirit 8 | 8 | 177–431 | 33.3 | not stated | 45 A | 8 AWG | [Harvia Spirit manual] |
| Finnleo Himalaya SL2 6.8 | 6.8 | 175–350 | 28.3 | not stated | 40 A | #8 | [Finnleo Himalaya manual] |
| Amerec Designer B 6 | 6 | 175–310 | 25 | not stated | 35 A | 10-2 | [Amerec Designer B manual] |
| Amerec Designer B 8 | 8 | 250–425 | 33.3 | not stated | 45 A | 8-2 | [Amerec Designer B manual] |
| Saunum Air 7 | 6.4 | 177–282 | 26.7 | 40 A | 35 A | not stated | [Saunum manual] |
Room ranges, amps, breakers and wire are copied from each maker’s manual. “125% rule minimum” is the smallest standard breaker at 125% of the heater’s current, because the National Electrical Code treats fixed space heating as a continuous load (424.3(B), 210.19–210.20). Where the maker’s breaker is smaller, we flag it rather than override the manual. [NEC continuous-load sizing (EC&M)]
Wood stoves rated for this room
| Stove | Room range (ft³) | Notes | Source |
|---|---|---|---|
| Harvia Legend 150 | 212–459 | 120 kg of stones; 1 m clearance to ceiling. | [Harvia Legend 150] |
| HUUM Hive Wood 13 | 212–459 | 115 mm flue. | [HUUM Hive Wood] |
Ventilation and notes for your electrician
Floor area 30 ft² needs a vent of at least 9.3 in² (UL 875 as quoted in the Finnleo and Amerec manuals), with the outlet about 2 ft above the inlet. Harvia and HUUM call for about 6 air changes an hour. [Finnleo Himalaya manual] [Harvia Spirit manual]
- Supply wire: copper rated at least 90 °C. [Harvia Spirit manual]
- No receptacles inside the hot room. [Harvia Spirit manual]
- US codes require heaters listed to UL 875, a thermostat capping the room at 194 °F, and a high-limit sensor within 6 in of the ceiling. [IRC M1902 sauna heaters]
- Outdoor saunas: newer codes widen GFCI protection outdoors, while Finnleo, HUUM and Saunum advise against it on their heaters. Your inspector decides. [Mike Holt forum: outdoor sauna on GFCI (secondary)]
The heater is one line of the bill.
Add the sauna, freight, the 240 V circuit, the pad and permits for where you live.
No. 3Questions
Breakers, glass doors, barrels.
How many kW sauna heater per cubic foot?
About 1 kW for every 35 ft³ (1 m³) of room, after adjusting for glass, tile or stone, and ×1.5 for log walls. That’s HUUM’s rule of thumb; other guides use as much as 50 ft³ per kW. Size from the maker’s own room table: across the Harvia, HUUM, Saunum, Finnleo and Amerec tables we checked, the maximum room works out to 44–56 ft³ per kW.
What size breaker and wire does a 6 kW sauna heater need?
A 6 kW heater draws 25 A at 240 V. HUUM’s manual specifies a 30 A breaker and 10 AWG copper for the Drop 6. The electrical code treats a sauna heater as a continuous load, so wiring and breaker must handle 125% of it (31.25 A), which points to 35 A. When the manual and the 125% rule disagree, your electrician or inspector decides. The calculator flags every heater where they differ.
Does a glass door change the heater size?
Yes. Makers add room volume for every square foot of glass, tile, stone or concrete: Harvia about 3.9 ft³ per ft² (1.2 m³ per m², glass door included), Saunum 3.9 (door excluded), HUUM 3.3. A standard glass door adds roughly 55 ft³ under Harvia’s rule, often one heater size.
What size heater for a barrel sauna?
No heater maker we checked publishes a barrel rule. Common practice treats single-wall staves like log walls and multiplies volume by 1.5 (Harvia’s log rule). Start with the heater the barrel maker recommends.
Do outdoor saunas need a GFCI?
Your inspector decides. Recent electrical codes widen GFCI protection outdoors (the 2026 NEC reportedly covers circuits up to 60 A, including accessory buildings; secondary source), while Finnleo, HUUM and Saunum advise against GFCIs on their heaters because they nuisance-trip.
The Sanctuary Report