Wire gauge required for 30 amp heater?

Answer 1

A #10 copper wire with an insulation rating of 90 degree C is rated at 30 amps. If the heaters exact draw is 30 amps then go to the next wire size. That will be a #8 copper wire with an insulation rating of 90 degree C and it is rated for 45 amps.

Answer 2

This will depend upon how many watts your water heater is rated for, as well as its voltage rating. Some smaller ones may be rated at 3800 watts, which means it is drawing about 15.8 amps (assuming a 240 volt circuit). In this case a 20 amp, double pole breaker can be used and #12 copper wire. Larger heaters are rated at 4500 watts, which means it is drawing 18.75 amps (again, assuming a 240 volt circuit). In this case, a 25 or 30 amp, double pole breaker can be used and #10 copper wire. Remember that an electric water heater is considered a "continuous load", which means that it can run for three hours or longer at a time. Since this is the case, one should never load it's circuit more than 80%. This means that a 20 amp circuit should never be loaded with a load greater than 16 amps; a 25 circuit, with more than 20 amps; and a 30 amp circuit, with more than 24 amps. Also, some heaters are rated for 120 volts, which means that your amps will increase, but only a single pole breaker is needed. Always check your heater's nameplate to see what it is rated for. To calculate, divide your watts by your voltage. This will give you the amps being drawn. Also, if your heater has two elements, they are rated the same, but an internal relay switch prevents them from operating at the same time. So, there is no need to add the two together to get your load calculation. You may have two 3800-watt elements, but the heater is just rated at 3800 watts, not 7600 watts.