What Gauge Romex for Water Heaters?

Romex

• Romex is a trade name of Southwire, named for the original developer, Rome Wire. Romex-type cables are ubiquitously referred to as “Romex,” even though the manufacturer may not be Southwire. Romex and Romex-type cables are classified as nonmetallic sheathed cable -- NM and NMC in industry-speak -- by the National Electric Code. Romex is almost always used for residential applications because it fulfills all the requirements of safe wiring; the sheath is nonconducting, and it resists both water and fire. Romex is ideal for damp environments, including bathrooms and basements, which makes it the choice for powering water heaters.

Romex Gauges

• Romex-type wiring typically has information printed on the outside of the sheath. Numeric values are part of that information. A typical Romex wire should carry a value such as “10-2.” The first number in the value is the gauge; in this case the wire would be 10-gauge. The second number in the value notes the number of sheathed conductors -- properly called service wires -- inside the external sheath; in this case there should be two, a hot and a neutral, and an unsheathed ground wire.

Amperage

• For supplying appliances that draw 15 amps or less, 14-gauge Romex is adequate. For 20 amps or less, 12-gauge is adequate. For 30 amps or less, 10-gauge is adequate. The outer sheath of 14-gauge Romex-type wire is typically colored gray; 12-gauge is colored yellow; 10-gauge is colored orange. The smaller the number, the thicker the wire; the thicker the wire, the more current it can conduct without becoming hot.

Typical Application

• A typical residential water heater is likely to draw approximately 30 amps. It should therefore be supplied by orange-sheathed 10-gauge Romex. Check with the manufacturer’s installation instructions and the local code enforcement office to confirm this is the case.