Most home hot water heaters are powered by a 240-volt circuit. While all voltages are dangerous, it is this particular voltage which is most notorious for causing severe injury or death. It is extremely important that the circuit be OFF when wiring an electric water heater.
That being said, the actual job of wiring an electric hot water heater is pretty straight forward and relatively easy. Here is how to wire an electric water heater.
Wiring an Electric Water Heater
If you are replacing an existing hot water heater, then the circuit will already be present at the site. But, if you are changing over from a gas-heated unit, then you will have to run the circuit yourself. If this is the case, then you will need a two-pole, 30-amp breaker and 10-2 NM cable to run for your circuit.
Important: Do not fill up the tank with water until the tank is wired up and then, do not turn the circuit on until the tank is filled with water!
On the top or the side of the hot water heater (depending on the type), you will see the junction box cover where the connections will be made. Remove the screw and the cover. Inside you should see two lead wires and a ground screw.
Before you begin, use the voltage tester to ensure that the circuit is OFF. Next, use the needlenose pliers and remove the knockout on the junction box. If this is a new installation, strip the plastic outer sheath of the NM cable so that about six-inches of wire is exposed. Then use the wire strippers to remove about 3/4-inch of insulation from each of the individual wires.
Fish the wires through the ROMEX® connector and tighten the clamp screws so that the clamp tightens over the sheathed section of the NM cable, NOT the exposed wires. Next, fish the wires through the knockout and secure the ROMEX® connector to the junction box using the included lock nut.
Wrap the ground wire around the ground screw and tighten it down securely. Next, take the black wire from the feed circuit and one wire from the water heater (it doesn't matter which wire) and twist them together. Secure them with a wire connector and wrap electrical tape around the connection for additional safety. With the white wire coming from the panel, wrap electrical tape over its insulation to identify it as a current-carrying line (this should also be done at the breaker as well). Then, connect it to the other wire from the water heater just as you did the black wire.
Note: Since the hot water heater is a 240-volt piece of equipment, it does not require a neutral wire. Thus, the white wire is actually a "hot" line in this application.
Replace the junction box cover. Turn the water supply back on and fill up the water heater with water. Once fully filled with water, turn the circuit breaker back on.