An electric water heater rated at 4,500 watts or 5,500 watts requires 10/2 NM-B wire on a dedicated 240-volt, 30-amp double-pole circuit. The NEC calculation starts with the heater wattage divided by voltage to get amperage: 4,500 watts divided by 240 volts equals 18.75 amps. Because water heaters are a continuous load under NEC Section 210.19(A)(1), the conductor must be rated at 125 percent of that continuous draw: 18.75 amps multiplied by 1.25 equals 23.4 amps. The smallest copper conductor rated above 23.4 amps per NEC Table 310.16 is 10 AWG at 30 amps, which is why every standard residential electric water heater uses 10/2 NM-B on a 30-amp circuit.
The NEC Math: Why Every Standard Water Heater Uses 10 AWG
The 125-percent continuous load multiplier is the key calculation that many DIYers miss. Without it, a 4,500-watt heater drawing 18.75 amps might seem to fit on a 20-amp circuit. With the continuous load multiplier applied, the effective sizing requirement is 23.4 amps, which exceeds the 20-amp circuit capacity and mandates 10 AWG wire on a 30-amp breaker. A 5,500-watt heater draws 22.9 amps, and 22.9 multiplied by 1.25 equals 28.6 amps, still within the 30-amp capacity of 10 AWG. This is why virtually every residential electric water heater from 30 gallons to 80 gallons uses identical wiring: 10/2 NM-B on a 30-amp, 240-volt circuit.
When 8 AWG Wire Is Required
Some extra-large or high-recovery electric water heaters use elements rated above 5,500 watts. A heater with a 6,000-watt element at 240 volts draws 25 amps. Multiplied by 1.25, the minimum conductor requirement is 31.25 amps, which exceeds the 30-amp rating of 10 AWG and requires upsizing to 8 AWG wire on a 40-amp circuit. Always read the water heater nameplate before selecting wire: the nameplate lists rated wattage, voltage, and full-load amperage that determine the correct wire size for that specific model.
Cable Routing and Disconnect Requirements
Route the 10/2 NM-B from the 30-amp double-pole breaker at the main panel through walls or framing to the water heater location. Connect to the heater terminal block through a listed appliance connector or short flexible conduit nipple at the heater top panel. The NEC requires a means of disconnecting the heater within sight of the appliance or capable of being locked in the open position. In most installations, the circuit breaker at the panel serves as the disconnect when it is visible from the heater location. If the panel is not within sight, a local disconnect switch is required within sight of the heater.
Wet Location Considerations
10/2 NM-B is rated for dry indoor locations. If any portion of the water heater circuit runs through a crawl space with standing water, through a concrete block wall, or in a location subject to moisture, use 10/2 UF-B for those wet sections or route 10 AWG THHN conductors through PVC conduit. The conductor sizing remains the same regardless of cable jacket type: 10 AWG on a 30-amp breaker.
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