How to Wire a New Water Heater to Old Electricity

Turn off the main breaker in the breaker panel. Use a screwdriver to remove the screws in the cover of the panel and lift the cover off the panel to expose the wiring inside. Locate the breaker that supplies the water heater.

Read the amp rating on the breaker. This is printed on the end of the switch lever. If the breaker is already rated correctly for your new water heater, you will not have to upgrade the wire or the breaker. If the amp load is 20 and your new appliance requires a 30 or larger, you will need to install new wire in the circuit. The 12-gauge wire will need to be replaced with 10 gauge. Most water heaters do not need higher than 10-gauge wire. The next three steps will describe how to replace the breaker and wire if it is needed.

Grip the old breaker and pull it from the power bus where it is connected. Use a screwdriver to loosen the two screws on the breaker and pull the wires out of the slots underneath the screws. Locate the white wire associated with these black wires and loosen the screw that holds it to the neutral bus. Pull the wire from the slot under the screw. Pull all of these wires out of the breaker panel.

Push the end of the new wire into the breaker box until about 18 inches of wire are inside the box. Remove 12 inches of the wire's outside insulation with wire strippers to uncover the four inside wires. Use the wire strippers to remove one inch of insulation from the ends of the red, black, and white wires. Push the ends of the white and bare wires under the screw on the neutral bus and tighten the screw. Push the ends of the red and black wires into the two slots under the screws on the new breaker and tighten the screws.

Snap the breaker onto the power bus. Follow the path of the old wire with the new wire until you arrive at the location of the water heater. Strip six inches of the outer insulation from the new wire with the wire strippers. Remove one inch of insulation from the inside wires in the same way as you did inside the breaker panel. You are now ready to wire the water heater.

Remove the cover on top of the water heater where the main wiring is located. This is usually a round or rectangular metal plate with one or two screws holding it in place. Underneath this cover you will see two wires, one black and one red, and a green screw. The manufacturer should have included two wire nuts. One will be on the end of each of the two wires. Remove these nuts and set them nearby.

Loosen the green screw with the screwdriver. Bend the end of the white wire into a small hook and hook it around the screw. Tighten the screw to hold the white wire. Bend the bare wire back along the large wire and out of the way. You will not need it. Hold the end of the two red wires next to each other and screw one of the wire nuts down onto them to fasten them together. Repeat this process for the two black wires.

Push all of the wires down into the top of the water heater and replace the cover and screws. Open the valve to the water heater and let it fill will water. You may need to open a hot-water faucet at one of the sinks in the house so that the air in the tank can bleed off while the tank is filling. When the tank has finished filling, turn off the open faucet and turn on the main breaker and the breaker to the water heater. If these are turned on before the tank is filled, the heating elements will burn up and will need to be replaced. In one to two hours you will have hot water for use in your house.