What Size Wire is Needed for a Water Heater?

When it comes to wiring a water heater, there are a few things you need to know. What size wire is needed for a water heater?

Checkout this video:


An electric water heater requires a 240-volt circuit to function and must be wired with wire rated to handle the electrical load. The size of the wire dictates the amount of current that can safely pass through the wire. Wires are rated by the American Wire Gauge, or AWG, scale. This scale runs in reverse, so the larger the gauge number, the smaller the diameter of the wire, and vice versa. A wire with a smaller diameter can carry more current than a wire with a larger diameter.

What size wire is needed for a water heater?

Size of Wire needed for a Water Heater – (What Size Wire is Needed for a Water Heater?)
You need to use 12-2 with ground romex wire for this application. The circuit breaker used must match the amperage rating on the water heater that you are going to install.

2.1 Wire size and amperage

If you have a 3-wire service, then you need two conductors for the 240-volt circuit and one conductor for the 120-volt circuit. Obviously, the larger the amperage of your hot water heater, the larger the breaker and wire size will be. The following wire and breaker sizes are based on a 240-volt, 4500-watt water heater:

Wire Size – 8 AWG
Amps – 30
Breaker Size – 40 amps

2.2 Voltage drop

The National Electrical Code (NEC) has a rule that limits the voltage drop for the main feeder wires to the water heater to 3%. This is known as the 2% rule and is based on the wire size (gauge) and the distance in feet from the service panel to the water heater.
For example, let’s say you have a 40-gallon electric water heater that is 40 feet from the service panel. The wire size would be #8 THWN copper wire. The NEC permits a maximum of 2.2 volts to be dropped for this circuit.
The voltage drop formula for 3% would be as follows:
Volts drop = I x 2 x L x .03
Where I = current in amperes, L = one-way length of conductor in feet, and .03 is constant for 3%.

2.3 Other factors to consider

There are other factors to consider when choosing the size of wire for your water heater. The National Electrical Code (NEC) requires that the wire size be increased by 20% if the conductor is to be run in conduit. This is because the heat generated in a conductor running in conduit is greater than that generated in a conductor running in air.


In conclusion, the size of wire needed for a water heater is determined by the size of the breaker used to supply power to the unit. A 20-amp breaker requires 12-gauge wire, a 30-amp breaker requires 10-gauge wire, and a 40- or 50-amp breaker requires 8-gauge wire.