I'm installing a instant on demand water heater that requires a 220v - 40 amp circuit and would like to know what size wire is required for a 50 foot run. ??????
8 AWG copper wire or 6 AWG aluminum. For the ground wire, 10 AWG copper.
Most on-demand heaters need 2 wires + ground, but check your installation instructions to be sure it doesn't need 3.
Keep in mind a 40A on-demand is a pretty small heater and you may need to reduce flow to get hot enough water.