What size of wire should you use to wire a 8 kw heat strip at 240 volts?

Wire sizes are based on amperage. To find the amperage of a 8 kW or 8000 watt strip heater the following formula is used. I = W/E. Amps = Watts/Volts, 8000/240 = 33 amps. Legally and the electrical code stated that conductors can only be loaded to 80% of their rated capacity.

A #10 copper wire with an insulation rating of 90 degrees C is rated at 30 amps.

A #8 copper wire with an insulation rating of 75 or 90 degrees C is rated at 45 amps.

Calculating a #8 wire, the ampacity is 45 amps x .80 = 36 amps. The load only draws 33 amps so it looks like your feeder size for the 8 kW heater will be a #8 with an insulation factor of 75 or 90 degrees C.