Sauna heater cable size calculator

Find the supply cable size for an electric sauna heater from its power, the length of the run and how the cable is installed. The calculator below starts with a 9 kW three-phase heater on a 15 m run.

Cable size (copper)–

Breaker rating In
–
Capacity after derating Iz
–
Voltage drop
–
Size set by
–

Copper conductors, IEC 60364-5-52 Annex B current-carrying capacities at 30 °C, temperature factors from Table B.52.14 (rounded up to the next 5 °C step) and grouping factors for bunched circuits from Table B.52.17. The breaker is the smallest standard rating at or above the load current, and the cable must carry at least that rating after derating (Ib ≤ In ≤ Iz). Not covered here: buried cables (method D), aluminium, fault-loop disconnection times and short-circuit checks. Guidance only: check the standard, the cable maker's data and your national wiring rules. The installer remains responsible for the design.

Typical sizes for three-phase sauna heaters

400 V three-phase, resistive load (cos φ = 1), 15 m of copper PVC cable at 30 °C on its own, 3% voltage drop. The breaker is the smallest standard rating at or above the heater current.

HeaterCurrent per phaseBreakerClipped direct (C)In an insulated wall (A1)
4.5 kW6.5 A10 A1.5 mm²1.5 mm²
6 kW8.7 A10 A1.5 mm²1.5 mm²
8 kW11.5 A13 A1.5 mm²1.5 mm²
9 kW13.0 A13 A1.5 mm²1.5 mm²
10.5 kW15.2 A16 A1.5 mm²2.5 mm²

The heater manual comes first

Sauna heater manuals list the supply cable and fuse for each model, and some ask for a bigger fuse than the smallest standard rating, for example 16 A for a 9 kW heater. If yours does, enter the fuse rating as the load in amps: the cable is then sized to carry the fuse. With 16 A, 15 m in an insulated wall needs 2.5 mm²; clipped direct 1.5 mm².

Inside the hot room

This calculator sizes the supply cable up to the sauna, at normal room temperature. The cable from the junction box to the heater runs inside the hot room and must be the heat-resistant type the heater manual specifies. Saunas are a special location in IEC 60364-7-703, with extra rules for where equipment may go, and many countries add their own.

Questions

What size cable for a 9 kW sauna heater?

A 9 kW heater on 400 V three-phase draws about 13.0 A per phase. Over 15 m that is 1.5 mm² clipped direct and 1.5 mm² in an insulated wall, on a 13 A breaker. If the manual asks for a 16 A fuse, size for 16 A instead.

How do I work out the current of a sauna heater?

For a three-phase heater, divide the power in watts by √3 × 400 V. A 9 kW heater: 9000 / (1.732 × 400) ≈ 13 A per phase. For single-phase, divide by 230 V.

Can a sauna heater run on single-phase?

Some small heaters can, but most heaters above about 6 kW are designed for three-phase. The heater manual lists the supplies it accepts and the cable for each.

Can I rely on the result without checking?

No. It is guidance based on IEC 60364. The heater manufacturer's instructions and your national wiring rules come first. The installer remains responsible for the design.

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