Water Heater Breaker and Wire Size Chart

NEC 422.13 defines a storage type water heater with a capacity of 120 gallons or less as a continuous load, meaning it is expected to draw current for three hours or more at a stretch. Continuous loads get a 25 percent safety margin under the NEC so the circuit does not run at its full rated capacity for hours on end. Draw comes from the element wattage and the 240 volt supply, design load is draw times 1.25, and the breaker is the next standard size at or above that design load.

NEC Table 310.16 lists a higher ampacity for 14, 12 and 10 AWG copper than the breaker they are actually allowed to sit behind, because NEC 240.4(D) caps those three sizes at a lower overcurrent protection rating regardless of the insulation's temperature rating. Above 10 AWG, the breaker is sized to the conductor's listed ampacity instead.

No, not for the compressor circuit. A heat pump water heater's compressor is a motor load, not a simple resistive element, so its nameplate lists a Minimum Circuit Ampacity and a Maximum Overcurrent Protection rating directly, and those two figures are what size the breaker and wire, not the draw-times-1.25 arithmetic above. A hybrid heat pump unit's electric backup resistance element, when it has one, does follow this chart's math on its own rating, separately from the compressor circuit.

Minimum Circuit Ampacity and Maximum Overcurrent Protection are standard labeling for motor-driven and mixed-load appliances precisely because a compressor's starting current, and the wiring margin a manufacturer builds in around it, do not reduce to a single wattage figure the way a resistive element does. Reading those two values straight off the data plate, rather than reconstructing them from the compressor's running wattage, is the correct method for that class of appliance.

Whole-home electric tankless units draw far more total current than a single tank element, often more than one standard residential breaker is rated to carry, so manufacturers split the total load across two or three separate circuits inside the unit rather than running it on one oversized breaker and cable. The table below shows the total draw at 240 volts for common tankless ratings; the specific circuit count, the amperage of each individual circuit, and the wire size for each come from that model's installation manual, not from a single lookup.

Frequently, yes. A single 36 kW unit alone draws 150 amps total, which is close to the entire capacity of many older 200 amp residential panels once every other circuit in the house is already accounted for. Because the load is continuous under the same NEC 422.13 logic that applies to a tank element, the panel's available capacity has to cover the full 125 percent design figure for whichever circuits the manufacturer splits the unit across, not just the raw draw. This is one of the more common reasons a whole-home electric tankless retrofit turns into a panel or service upgrade rather than a same-day swap.

Confirm the element or unit's nameplate wattage and voltage, whether it is a single tank element or a multi circuit tankless unit, and whether the existing panel has room for the breaker size the table above calls for. None of that replaces a load calculation for the whole panel, which accounts for every other circuit already installed.

A panel that is already close to its rated capacity, common in an older house that has added central air conditioning, an electric range or a car charger over time, can technically have an open slot for a new breaker while still lacking the actual current-carrying headroom for a large water heater or tankless circuit. That distinction, an open slot versus available capacity, is exactly what a panel load calculation checks and a slot count alone does not.

A time-of-use switch or timer for an electric water heater is wired into the same circuit sized below, not a separate one, and it has to be rated for the full load it is switching. A timer does not change the breaker or wire size the element itself requires; it simply interrupts that same circuit on a schedule.