Replacing an electric water heater requires a permit in most areas, whether the installation is performed by a professional or by a homeowner. As part of the permit process, the work will be reviewed by an inspector to ensure that both the electrical and plumbing connections are done properly and safely, and that the installation complies with the local code requirements. Here we will look at the basic electrical requirements only, as they apply to a standard tank-style electric water heater, not an on-demand (tankless) 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.
Turning Off the Power
Before you examine or touch the 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 circuit breaker. Switch off the appropriate breaker in the breaker box, then use an electrical voltage tester to make sure that the circuit is off by testing at the water heater.
Locating the Electrical Connections
The electrical wire connections for a water heater are made at a built-in junction box on the top of the water heater tank. This is enclosed by a cover plate, which you can remove to inspect the wire connections inside. Typically, the wire conductors leading to the heater are enclosed in flexible metal conduit or are made with flexible metal cable, such as metal-clad (MC) cable. This flexibility provides a little wiggle room, making it easier to replace the water heater, and it is a required feature in many earthquake areas.
With the cover plate removed, you can test for power simply by holding a non-contact voltage tester next to the wire connections; if the circuit has been properly shut off, the tester will not light up.
Understanding 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 wire lead on the water heater, and the white circuit wire connects to the white wire lead on the water heater.
The white circuit wire should be wrapped with black or red electrical tape near the connection at both ends of the circuit (at the water heater and at the breaker box), to indicate that it is a "hot" wire, not a neutral wire. Unlike standard 120-volt circuits, a 240-volt circuit carries live current in both the black and white wires. The circuit ground wire connects to the green ground screw on the water heater or to the water heater's ground lead, as applicable.
Heating Element Wiring
Although you won't need to deal with the thermostats or heating elements during a simple replacement of an electric water heater, it's helpful to know that electric water heaters also include inner wiring that runs from the wire connection box down along the side of the tank to two different heating elements, each controlled by its own thermostat. The heating elements, and the thermostats that control them, are contained inside access panels mounted on the side of the water heater tank. Each pair of thermostats and heating elements has screw terminals that are connected to wire leads in the water heater. You will not need to deal with these connections unless you are replacing a thermostat or heating element on an existing water heater.
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. It's important to note that the bonding jumper is not required by the National Electrical Code nor the Uniform Plumbing Code, but it may be required by your local building authority.
The 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. Another function of the bonding wire is to maintain the electrical grounding pathway on the water pipes. Without the jumper, there is a break between the hot water and cold water pipes in the system, which potentially disrupts the continuous grounding pathway of the electrical system.
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. When replacing a water heater, it's a simple matter of making sure the jumper connections are in place after you finish installing the new water heater.