What gauge wire will you need for a 400 watt amp?

To answer this question a voltage needs to be stated. The formula needed to find the amperage is I = W/E. Amps = Watts/Voltage. Once the amperage is found, the proper size conductor to handle that current can be established. Without knowing the amperage on 400 watts a good guess would be a #14 wire. This wire size is good up to 1440 watts at 120 volts.