I need to run a 220 line about 100ft from my breaker box to where the sauna be located.
The manual says for an 8 kw heater, 8-2 w/ground copper; 40 amp non GFCI breaker But it also says for distances “up” to 30 ft.....
So what if it’s 100?
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Having run a few of these calculations, the break point is usually a little past 100' if you want to hold voltage drop below 3%. However, for a resisitive heater, there is no reason to hold voltage drop below 3%. It is a waste of money.
Regardless, 8000 watts happens at 33.33 amps on US 240V* voltage. The UL-approved product instructions call for 8 AWG wire. Southwire's voltage drop calculator tells me those conditions at 100' run yield 1.92% voltage drop on 8 AWG.
If the hot tub failed on 2% power drop, there is no way UL, CSA or TUV would list it.
You're all set.
* power was 110/220 when AC power was first mass marketed in 1915-ish, and the term stuck. Today, US voltage is 120/240. Except in Puerto Rico, where it's 0!