What Size Wire Do I Need for a 9 kW Sauna Heater?
A 9,000 watt sauna heater at 240 volts draws 37.5 amperes. Sauna heaters run for hours at a time, so they are a continuous load and 210.19(A)(1) requires the branch circuit conductors to have an ampacity of at least 125 percent of that value. Thirty seven point five times 1.25 is 46.9 amperes, which lands on a 50 ampere breaker with 8 AWG copper conductors.
Why 8 AWG copper works
Eight AWG copper THHN or THWN-2 is rated 55 amperes in the 90 degree C column of Table 310.16 and 50 amperes in the 75 degree C column. Section 110.14(C) caps you at the lowest temperature rating in the circuit, and virtually all breakers and heater terminal blocks are 75 degree C rated, so 50 amperes is the usable ampacity. That covers the 46.9 ampere requirement.
Section 240.4(D) does not apply here, because the small conductor rule only reaches down to 10 AWG. There is no artificial cap on 8 AWG, so a 50 ampere breaker on 8 AWG copper is a legitimate pairing when the terminations are 75 degree C rated.
Aluminum changes the answer. Eight AWG aluminum is only 40 amperes at 75 degrees C, which is not enough. Aluminum needs 6 AWG, rated 50 amperes at 75 degrees C, to serve the same load.
Read the heater instructions first
Section 110.3(B) makes the manufacturer's installation instructions enforceable, and sauna heaters are one of the appliance categories where the marked circuit size regularly differs from the pure calculation. Most 9 kW residential sauna heaters ship with a 50 ampere circuit specification, which matches the math above, but some European units are marked for a different supply arrangement and a few are marked for 60 amperes. The marking wins.
Verify the voltage too. A 9 kW heater nameplated at 240 volts installed on a 208 volt commercial supply delivers only about 6.8 kW and draws 32.5 amperes, which changes both the breaker and the sauna performance. Section 424.3(B) treats fixed electric space heating as continuous, and while a sauna heater is more properly an appliance under Article 422, the 125 percent factor applies either way through 210.19(A)(1).
Disconnect, controls, and enclosure conditions
Section 422.31(B) requires a disconnecting means within sight of an appliance rated over 300 volt amperes, or a lockable disconnect per 422.31(B) where it is not in sight. A 9 kW heater qualifies, so plan on a 60 ampere non fused disconnect or a lockable breaker at the panel.
The wiring inside the sauna room itself has to survive the environment. Sauna interiors reach 190 degrees F or higher near the ceiling, and standard THHN is rated 90 degrees C, or 194 degrees F, dry. Manufacturers usually specify high temperature wire for the heater whip and require the junction box and the thermostat sensor to be located per their instructions. Do not improvise this part, since a conductor cooked past its insulation rating is a fire that starts inside a cedar box.
The heater is hard wired, so 210.8 receptacle GFCI rules do not apply to the heater circuit itself. Any receptacle in the sauna changing area or bathroom nearby is a separate question and follows 210.8(A).