NEC 422.13 · 310.16 · 250.122
What Size Breaker for an Electric Water Heater?
The breaker, copper wire, and ground for an electric water heater by element wattage, sized at 125% for the continuous load per NEC 422.13. Number first, then the code behind it.
Electric Water Heater Breaker & Wire by Wattage
| Element / nameplate | Running amps | Design (125%) | Min breaker | Copper wire | Ground (Cu) |
|---|---|---|---|---|---|
| 3,000Wpoint-of-use / small tank | 12.5A | 15.63A | 20A | 12 AWG | 12 AWG |
| 3,800Wcompact 30-gal | 15.83A | 19.79A | 20A | 12 AWG | 12 AWG |
| 4,500Wstandard 40-50 gal | 18.75A | 23.44A | 25A | 10 AWG | 10 AWG |
| 5,500Whigh-recovery / 80-gal | 22.92A | 28.65A | 30A | 10 AWG | 10 AWG |
Running amps = watts ÷ volts. A fixed storage water heater 120 gallons or under is a continuous load (NEC 422.13), so the branch circuit and breaker are sized to 125% of that (the design amps). The breaker shown is the code minimum, the smallest standard overcurrent device that covers the design load; if the nameplate prints a specific maximum breaker (NEC 422.11(E)), that governs. Copper conductor at the 60°C column (NEC 310.16, the residential default), ground per NEC Table 250.122.
The 4500W row shows a 25A code-minimum breaker, but 25A breakers are uncommon and most 4500W heaters are installed on a 30A breaker (the typical nameplate maximum); the 10 AWG copper conductor is rated for both, so the wire does not change. On a 208V supply the same element draws more current (a 4500W element is 21.63A at 208V), so confirm the voltage before you size the circuit.
The 4500W Water Heater, Worked
- Running current: 4500W ÷ 240V = 18.75A.
- Continuous load (NEC 422.13): a fixed storage water heater 120 gallons or under is continuous, so the circuit carries 125% of the load: 18.75A × 1.25 = 23.44A design.
- Breaker: the smallest standard overcurrent device covering 23.44A is a 25A breaker (the code minimum); the common install is a 30A breaker, the usual nameplate maximum.
- Conductor (NEC 310.16, 60°C): 10 AWG copper is rated 30A, which covers the 23.44A design load on either breaker.
- Ground (NEC Table 250.122): a 10 AWG copper equipment grounding conductor.
water_heater_circuit tool live, handing back the verdict with its cited source instead of a guess. Then keep the reading as one job in the Intry app.Sharing a circuit, or a 120V unit?
A 240V resistive tank always needs its own dedicated circuit. If the goal is to avoid a new circuit, the newer 120V plug-in heat pump water heater class is the one to look at: some models are listed to share a general-purpose 15 or 20A circuit under NEC 210.23(B)(2). The heat pump water heater circuit tool answers the shared-circuit question and sizes the 240V hybrid class. Adding a water heater to a panel that may be near capacity? Check whether the panel can handle it first.
Frequently Asked Questions
Is the Intry Water Heater Circuit Calculator accurate and NEC compliant?
Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit. The per-tool receipt is public at https://www.intrysys.com/verified.
What size breaker do I need for an electric water heater?
A standard 40 to 50 gallon, 4500W residential electric water heater at 240V draws 18.75A. NEC 422.13 treats a fixed storage water heater of 120 gallons or less as a continuous load, so the circuit and breaker are sized to 125 percent of that, 23.44A. The code minimum overcurrent device is a 25A breaker, but the unit is most commonly installed on a 30A double-pole breaker (the usual nameplate maximum), and both use 10 AWG copper. A 5500W unit needs a 30A breaker; a 3000 to 3800W unit fits a 20A breaker on 12 AWG. The nameplate maximum breaker always governs.
What wire size for a 4500W water heater?
10 AWG copper. The 4500W element draws 18.75A, sized at 125 percent to 23.44A for the continuous load (NEC 422.13). 10 AWG copper is rated 30A at the 60C column (NEC 310.16), which covers the 23.44A design load on either the 25A code-minimum or the common 30A breaker. Use a 10 AWG copper equipment ground (NEC Table 250.122). On 208V the same 4500W element draws more, 21.63A, so confirm the supply voltage.
Does an electric water heater need its own circuit?
A 240V storage water heater needs its own dedicated circuit; it is a large continuous load and cannot share a general-purpose branch circuit. The exception is the newer 120V plug-in heat pump water heater class, some of which are listed to share a 15 or 20A circuit under NEC 210.23(B)(2). If sharing a circuit is the goal, that is the class to look at, not a resistive tank.
Why is a water heater sized at 125 percent?
Because NEC 422.13 classifies a fixed storage water heater of 120 gallons or less as a continuous load, and a continuous load is carried at 125 percent by the branch circuit and its overcurrent device (NEC 210.19 and 210.20). That is why an 18.75A heater is put on a circuit sized for 23.44A rather than 18.75A. It is the same reason an EV charger and other continuous loads get the 125 percent treatment.
Related Calculators
Can My Panel Handle a Tankless Water Heater?
The electric tankless is a far larger load, 46 to 112A. The NEC 220.87 fit threshold by unit size and service.
Heat Pump Water Heater Circuit
The 240V hybrid circuit, or the 120V plug-in class that can share a general-purpose circuit under NEC 210.23(B)(2).
Wire Size for Appliances
The breaker, copper and aluminum wire, and ground for every common household appliance circuit.
Can My Panel Handle It?
Adding any big load: the one NEC 220.87 check, plus the no-service-upgrade workarounds.