What size breaker for a water heater?
Short answer: A standard 4,500 W, 240 V tank water heater draws 18.75 A. The NEC treats it as a continuous load, so the circuit must be rated for 125%: 23.4 A, which means a 25 A breaker minimum on 10 AWG copper. Most are installed at 30 A with 10/2 copper, which is also allowed. A 5,500 W heater needs 30 A.
Breaker and wire by element wattage
240 V, sized at 125% (NEC 422.13), copper NM-B cable:
| Element | Amps | 125% | Minimum breaker | Wire |
|---|---|---|---|---|
| 3,000 W | 12.5 | 15.6 | 20 A | 12 AWG |
| 3,500 W | 14.6 | 18.2 | 20 A | 12 AWG |
| 4,500 W | 18.8 | 23.4 | 25 A | 10 AWG |
| 5,500 W | 22.9 | 28.6 | 30 A | 10 AWG |
| 6,000 W | 25.0 | 31.3 | 35 A | 8 AWG |
The wattage is on the label on the side of the tank. Most tanks have two elements but run only one at a time ("non-simultaneous"), so the label wattage is the most it ever draws. A label that says the elements run simultaneously means both can be on together: add them.
Why the 125% rule applies
NEC 422.13 says a fixed storage water heater of 120 gallons or less is a continuous load. Standard breakers are designed to carry 80% of their rating for 3 hours or more without overheating, so the circuit is sized at 125% of the load (1 ÷ 0.8). That is why a 4,500 W heater doesn't go on a 20 A breaker even though 18.75 A is less than 20.
25 A or 30 A?
Both are correct for 4,500 W on 10 AWG copper. 30 A is more common because 25 A double-pole breakers are harder to find in stores, and it lets you swap in a 5,500 W element later without rewiring. If the heater's label lists a maximum breaker size, don't exceed it.
On 208 V
In apartments and some condos the supply is 208 V, not 240 V. A 4,500 W element then delivers only about 3,380 W (power falls with the square of the voltage), so water heats about 25% more slowly. Some heaters ship with 208 V elements for this reason.
Size the breaker for your heater
Safety. A water heater circuit is 240 V with no neutral. Turn the breaker off and test both hot wires before touching the heater's terminals: the thermostat can leave one leg live.