What Size Breaker for a Water Heater?

How to size a water heater breaker (the 125% rule)

Sizing a water heater circuit is a two-step job. Find the amps, then add 25 percent.

Start with the running current. Divide the heater's wattage by its voltage. A 4,500-watt heater on 240 volts pulls 4,500 ÷ 240, which comes out to 18.75 amps.

Now add the 25 percent. The NEC treats a storage water heater as a continuous load. Section 422.13 puts it plainly: "a fixed storage-type water heater that has a capacity of 120 gallons or less shall be considered a continuous load for the purposes of sizing branch circuits." Continuous loads get sized at 125 percent of their current (NEC 210.20(A)), so 18.75 × 1.25 works out to 23.4 amps.

The breaker has to be at least 23.4 amps, so you round up to the next standard size, which is a 30-amp breaker (NEC 240.6(A)). The wire has to handle it too. Ten AWG copper is good for 30 amps (NEC 240.4(D)(7)), while 12 AWG stops at 20. So a standard water heater ends up on a 30-amp breaker with 10 AWG copper.

How many amps does a water heater use?

A standard 4,500-watt, 240-volt heater pulls 18.75 amps while it's heating. The circuit is sized at 23.4 amps because of the 125 percent rule, but the current the heater actually draws is 18.75 amps. Step up to a 5,500-watt heater and it's 22.9 amps. Drop to a 3,800-watt unit and it's 15.8 amps. The math is always the same: watts divided by volts.

What size wire for a water heater? 10/2 or 10/3?

Go with 10/2 with ground for a standard 4,500-watt, 240-volt heater. You don't need 10/3. A 240-volt heater is a simple resistive load. It connects to two hot legs and a ground, with no neutral. In 10/2 cable, the black and the white are both used as hots (you re-identify the white with tape at each end), and the bare wire is the ground. Running 10/3 won't hurt anything, but that extra neutral just sits there. If you change the breaker size, check the wire again with the Wire Size Calculator.

Why water heaters use a double-pole breaker

A 240-volt heater needs both hot legs of your panel, so it takes a double-pole (2-pole) breaker. That's a single breaker that fills two slots and switches both legs together. A single-pole breaker only gives you 120 volts, which won't run the heater. When people ask about a "double 30-amp," this is what they mean: one 2-pole 30-amp breaker, not two separate ones.

Two heating elements, one load

A tank usually has two 4,500-watt elements, but they never run at the same time. The thermostat hands off between the upper and lower element, so only one is ever heating. That is the reason you size the circuit for a single element at 18.75 amps instead of 9,000 watts, and it is why one 30-amp circuit covers the whole tank.

Breaker size by tank size (40, 50, 80 gallon)

Gallons don't set the breaker size. Wattage does. Gallon charts only work because most tanks, big or small, use the same 4,500-watt elements. A 40-gallon heater is almost always 4,500 watts, so it's a 30-amp breaker on 10 AWG. A 50-gallon is the same. An 80-gallon is usually 4,500 watts as well, sometimes 5,500, and it still lands on 30 amps with 10 AWG. Read the wattage on the nameplate instead of guessing from the tank size.

Heat pump (hybrid) water heater breaker size

A standard heat pump (hybrid) water heater takes the same 30-amp double-pole breaker and 10 AWG copper as a regular tank. Units like the Rheem ProTerra run on 240 volts and carry a 4,500-watt backup element, so the sizing lines up. The newer 120-volt plug-in hybrids are the exception. They're built to run off a normal 15-amp, 120-volt outlet, so you can add one without pulling a new 240-volt circuit. That outlet does bring GFCI rules into play, which we cover just below.

Tankless water heater breaker size

Electric tankless heaters pull a lot more current, and most need several large circuits. An 18,000-watt unit draws 75 amps, or about 94 amps once you add the 125 percent, and it's usually wired as two 50-amp circuits. A 27,000-watt unit draws 112.5 amps and often needs three. Loads this big frequently push you into a service or panel upgrade. Size a tankless heater from its own nameplate, and run the whole house through the Electrical Load Calculator first.

Do water heaters need GFCI protection? (2026 NEC)

A hardwired 240-volt storage water heater does not need GFCI protection. The GFCI rule (NEC 210.8) covers receptacle outlets on circuits rated 150 volts or less to ground, and a hardwired 240-volt heater is neither on a receptacle nor in that voltage range. A 120-volt cord-and-plug water heater is a different story. If it's in a garage, a basement, or another spot listed in 210.8(A), the outlet feeding it has to be GFCI-protected. This mainly comes up with the new 120-volt plug-in heat pump models. Local amendments vary, so check yours.

Does a water heater need a disconnect?

Yes. An electric water heater needs a disconnecting means under NEC 422.31(B), because it's a permanently connected appliance over 300 volt-amperes. The breaker can do the job itself if it's within sight of the heater (visible, and within about 50 feet) or if it can be locked in the open position. If the panel is in another room, out of sight, and the breaker won't lock off, put a local disconnect switch within sight of the heater.

Long wire runs: check voltage drop

If the heater is a long way from the panel, check voltage drop before you commit to a wire size. On a long run, the wire's own resistance drops the voltage reaching the heater, which cuts its output and wastes energy. The NEC suggests keeping branch-circuit voltage drop under 3 percent. Once you're past about 100 feet, you may need to bump up a wire size to stay there. The Voltage Drop Calculator will tell you.

Gas water heater: does it need a breaker?

If your water heater runs on gas, most of this doesn't apply to you. A standard atmospheric-vent gas heater uses a pilot light or a thermocouple and draws no household power at all. A power-vent or direct-vent gas heater has a blower and a control board that plug into a regular 120-volt, 15-amp outlet, not a dedicated 240-volt breaker. So a gas unit either needs nothing or a normal outlet circuit, never the 30-amp setup above.

Common water heater wiring mistakes

A handful of mistakes show up over and over. The most common is undersizing to a 20-amp breaker on a 4,500-watt heater, which ignores the 125 percent rule. Close behind is running 10/3 when 10/2 is all you need. Pairing 10 AWG wire with a 40-amp breaker is a worse one, because it leaves the wire under-protected, and that's a real fire risk. Skipping the disconnect when the panel is out of sight is another that inspectors catch. And sizing off the tank's gallons instead of the nameplate wattage will put you one size off more often than not.

How to size a water heater breaker yourself

1. Read the wattage and voltage off the nameplate (for example, 4,500 W, 240 V).
2. Divide watts by volts to get running amps (4,500 ÷ 240 = 18.75 A).
3. Multiply by 1.25 for the continuous-load rule (18.75 × 1.25 = 23.4 A).
4. Round up to the next standard breaker size (30 A). It can't go above 150 percent of the rating.
5. Pick wire rated at or above that amperage and matched to the breaker (10 AWG copper for 30 A). Use a double-pole breaker for 240 V.
6. Check it against your local code and inspector. When in doubt, hire a licensed electrician.

Frequently asked questions

Dr. Artie Vance

Technical reviewer · Ph.D. in Physics, MIT · 14 years' experience · Cross-checked against NEC 2026 & 2026

This guide is for general information based on the National Electrical Code. Local codes and your authority having jurisdiction (AHJ) may differ. Electrical work can be dangerous, so when in doubt, hire a licensed electrician.