Replacing an electric water heater requires a permit in most areas. This ensures that both the electrical and plumbing connections are done properly and safely and meet the local code requirements. Here we will look at the basic electrical requirements only, and these apply to a standard tank-style electric water heater, not an on-demand water heater. If you're planning to replace an existing electric hot water heater, now is the time to make sure the original water heater wiring is up to snuff.
Turn Off the Power
Before you do any investigating or touching of water heater wiring or electrical connections, turn off the power to the circuit that supplies the water heater. In most cases, the circuit is served by a 30-amp, double-pole breaker. Switch off the appropriate breaker in the breaker box, then use an electrical tester to be sure that the circuit is off by testing at the water heater.
The electrical connection for a water heater is made at a built-in junction box on the top of the water heater tank. This is enclosed by a cover plate. Typically, the wiring to the heater is enclosed in flexible metal conduit or is made with flexible metal cable, such as metal-clad (MC) cable. A flexible connection provides a little wiggle room, making it easier to replace the water heater, and it is a required in many earthquake areas. The metal protects the high-voltage wiring from damage.
Water Heater Wiring
Electric water heaters require a 240-volt dedicated circuit, which serves only the water heater and no other appliances or devices. The circuit wiring typically includes a 30-amp double-pole breaker and 10-2 non-metallic (NM) or MC cable. At the water heater, the black circuit wire connects to the black heater lead, and the white circuit wire connects to the white heater lead. The white circuit wire should be wrapped with black or red electrical tape near the connection at both ends of the circuit, to indicate that it is a "hot" wire, not a neutral wire. The circuit ground wire connects to the ground screw on the water heater or the heater's ground lead, as applicable.
The Bonding Question
Some building authorities require a bonding wire, or bonding jumper, between the hot water and cold water pipes serving the water heater. This is a question for your local building authority, as rules and recommendations vary widely. A bonding jumper may be required to ensure a reliable bond in a metal water piping system. Some experts believe a bonding jumper helps water heaters last longer by reducing corrosion in the tank caused by electrolysis.
In any case, if you need a bonding wire, it usually consists of a 6 AWG bare copper wire connected to a ground clamp on each of the hot and cold water pipes. Each clamp should be on a smooth part of the pipe and not too close to any fittings; the pressure of the clamp may stress soldered joints and valve connections.