How do you size wire for a 20kw aug heater for a heat pump?

Using the formula: Amps = Watts/Volts look up the wire size needed in the national electric code book for the amperage of the device. This formula only works for single phase current. If you are using 3 phase, look in the book. In general, 15 Amps or less: 14 Ga Copper wire. 20 Amps or less 12 Ga. 30 Amps or less 10 Ga. Anything more than 30 amps, look it up in the book.