In the United States, the most common residential electric water heaters are 4500 to 5500 watts operating on a 240-volt dedicated circuit. The right size of wire for this water heater is 10 AWG copper wire with a 30-amp double pole breaker. This typical American water heater draws around 18.8 Amps (4500 watts / 240 volts).

Do note that most states require a permit for a replacement of the water heater, which begins with the inspection of the electrical and plumbing system by a certified expert. Therefore, water heaters are more easily replaced by homeowners than some of the other equipment types.

Sometimes, a water heater wire will require a 6 AWG bonding wire to connect pipes with hot and cold water within the metal plumbing system. This can be required by local electrical codes.

Tankless vs. Standard Water Heaters

Compared to standard water heaters, tankless water heaters draw much more power, and require two 40 amp, two-pole 220-volt breakers. For this reason, they will use a higher gauge copper wire, like 8 AWG or even 6 AWG. However, this article will primarily focus on standard water heaters.

Can I Use 8 AWG Wire For a Standard Water Heater?

It is possible to size up to 8 AWG wires for a standard water heater in case you want extra safety beyond what is required by NEC and local codes. If there is a long run, and voltage drop might be a concern, then using an 8 AWG copper wire is a plus. In most situations, however, an 8 AWG water heater wire would be an overkill. Besides, it is less flexible and more expensive. 

Note that you would need a 40-amp double pole breaker to sustain an 8 AWG wire.

Can I Use 12 AWG Wire For a Standard Water Heater?

No, choosing a 12 AWG copper wire for a standard 4500 to 5500 watts water heater is unsafe, as it can only handle up to 20 Amps, and a pole breaker for a residential heater should be at least 30 Amps.

A rare occasion when you can use 12 AWG wire with water heaters is if you have a tiny water heater (like a point-of-use under-sink model) that specifically requires less than 20 amps. 

Types of Water Heater Wiring

A water heater is usually installed with 10/2 WG (with ground) wire. This provides a safe buffer as NEC dictates that the circuit breaker should be rated for at least 125% of the continuous load.

The wires following the water heater are usually covered with a flexible conduit unless the cables are made with flexible conduits themselves, such as MC metal cable. Here are the common types of cables used for water heaters:

NM-B Romex

10/2 NM-B Romex has two insulated conductors and a ground wire, perfect for a water heater cabling setup. This cable is best for dry, indoor environments where the cable has no exposure to mechanical damage. For this reason, this is an excellent option for residential buildings when connecting the water heater to the electrical panel, provided it's within a protected area like a basement or utility room.

THHN/THWN Cable in Conduit

THHN (Thermoplastic High Heat-resistant Nylon-coated) is a nylon wire used inside conduits. In the case of a water heater, the conduit required is flexible. Types of conduit that can be used with 10 AWG THHN/THWN in water heater installations are either flexible metal conduit ( FMC) or liquid-tight flexible metal conduit (LFMC).

THHN/THWN will typically be used in industrial or commercial installations where physical damage can happen. It can also be used for water heaters located outside or with outside connections.

MC Metal Clad Cable

MC cable consists of insulated conductors enclosed within a metallic sheath. It protects against environmental factors and physical damage. 10/2 MC metal cable is the most popular type of water heater cable as it is flexible in itself and offers protection against mechanical damage. This is the all-around perfect cable for water heaters, especially in areas where cables might be exposed to mechanical damage and moisture. Compared to THHN/THWn, MC is installed more quickly.