What Size Breaker for a Water Heater?

Most residential electric water heaters need a 30 amp breaker on 10 AWG copper wire. The standard 4,500-watt, 240-volt tank draws 18.75 amps. Because a water heater is a continuous load, the NEC requires sizing the circuit at 125% of that draw: 18.75 × 1.25 = 23.4 amps. The next practical breaker is a 30 amp breaker, wired with 10 AWG copper. The calculator above computes this for any wattage and also shows the load current, required wire ampacity, and recommended wire gauge.

The reason you do not use a 25A breaker (the next standard size above 23.4A) is that 25A breakers and appliance connections are uncommon; the electrical industry standardizes water-heater circuits at 30A/10 AWG, which safely covers the 23.4A requirement with margin. Larger 5,500W heaters draw 22.9 amps and still fit the same 30A/10 AWG circuit, which is why 30A is the near-universal answer for standard tanks.

Water heaters follow the continuous-load rule, which sizes the circuit above the actual draw:

The 1.25 factor is the same NEC 80% rule seen from the other direction: sizing at 125% of the load is identical to keeping the load at 80% of the breaker. NEC 422.13 specifically classifies a storage-type water heater of 120 gallons or less as a continuous load, so this factor is mandatory.

Once the required ampacity is known, both the breaker and the conductor must be rated at least that high. A 30A breaker pairs with 10 AWG copper; the two must always match, because the breaker's job is to protect the wire. Confirm the conductor independently with the wire size calculator, and if you are sizing a different appliance, use the general breaker size calculator.

Example 1 — standard 4,500W / 240V tank: Load = 4,500 ÷ 240 = 18.75 A. Required = 18.75 × 1.25 = 23.44 A. Next practical breaker = 30 A on 10 AWG copper. This is the most common residential water-heater circuit in North America.

Example 2 — 5,500W / 240V high-recovery tank: Load = 22.9 A. Required = 28.6 A. Still fits a 30 A breaker on 10 AWG, because 30A ≥ 28.6A. This heater is at the top of what a 30A circuit can serve.

Example 3 — 6,000W / 240V tank: Load = 25 A. Required = 31.25 A, which exceeds 30A, so it must move up to a 40 A breaker on 8 AWG copper. Many homeowners incorrectly leave these on 30A circuits — always recompute when replacing a heater with a higher-wattage model.

Example 4 — tankless 24 kW / 240V unit: Load = 100 A. Required = 125 A. Tankless electric heaters draw enormous current and often need two or three 40–50A double-pole breakers (for example, 3 × 40A) and correspondingly heavy wiring — a major reason a service upgrade is common when switching from a tank to electric tankless. Verify the panel capacity with the electrical load calculator for house.

The breaker and wire are inseparable. Because the standard 4,500W heater needs a 30A breaker, it must be wired with 10 AWG copper, which is rated for 30 amps at the 60°C column used for most branch circuits. Using 12 AWG (rated 20A) on a 30A water-heater breaker is a dangerous and common code violation.

Water heaters are hardwired (not plugged in), typically with a two-pole 240V breaker feeding two hot conductors and a ground — no neutral is needed because the heating elements run purely on 240V. A 10/2 NM cable (two 10 AWG conductors plus ground) is the usual choice for a 30A circuit in dry indoor locations; in a garage or exterior run, conduit with THHN conductors is common.

If you upsize to a 6,000W or larger heater, you must also upsize the wire: a 40A circuit requires 8 AWG, and a 50A circuit requires 6 AWG. Never reuse the old 10 AWG wire with a larger breaker. The recommended wire gauge shown by the calculator tracks the breaker automatically, but always confirm ampacity, temperature rating, and any derating with the wire size calculator.

1. Enter the heater power in watts. Find it on the rating plate near the thermostat access panel. Standard residential tanks are 3,800W, 4,500W, or 5,500W.
2. Select the voltage. Almost all residential electric water heaters are 240V; some commercial or multifamily units are 208V, which draws slightly more current for the same wattage.
3. Read the recommended breaker. The primary result applies the 125% continuous rule and rounds to the appropriate breaker. The secondary outputs show the actual load current, the minimum wire ampacity, and the recommended copper wire gauge.

Water heater installation involves hardwiring a 240V circuit and must comply with local code — many jurisdictions require a permit and a licensed electrician. Use this tool to plan and verify, then confirm the panel can accept the new circuit with the electrical load calculator for house.

• Forgetting the 125% continuous factor. Sizing a 4,500W heater to its bare 18.75A draw and using a 20A breaker will cause nuisance trips — it must be 30A.
• Reusing undersized wire after an upgrade. Moving from a 4,500W to a 6,000W heater requires going from 10 AWG/30A to 8 AWG/40A; keeping the old wire is a fire risk.
• Wiring 208V as if it were 240V. At 208V a heater draws more current and produces less heat; recompute the breaker for the actual voltage.
• Using a single-pole breaker. A 240V water heater needs a two-pole (double) breaker feeding both hot legs.
• Undersizing for tankless units. Electric tankless heaters draw 4–5× a tank's current and often need multiple large breakers and a service upgrade.

A standard residential storage water heater actually contains two heating elements — an upper element near the top of the tank and a lower element near the bottom — yet the whole appliance runs on a single 30A circuit rated for just one element's worth of current. The reason is a clever control scheme called non-simultaneous or interlocked operation. The two elements are wired so that only one can be energized at any instant; they never draw power together.

When you draw a lot of hot water, cold water enters the bottom of the tank. The upper thermostat has priority: on a cold start it energizes the upper element first to guarantee a quick supply of hot water at the top of the tank, where the outlet is. Once the upper portion reaches temperature, control passes to the lower thermostat, which then runs the lower element to heat the remainder of the tank. Because the transfer is mechanical and mutually exclusive, the maximum current the circuit ever sees is that of a single 4,500-watt element — 18.75 amps — not the 37.5 amps two elements would draw together.

This is why the calculator sizes from the single-element wattage on the nameplate and why a 30A/10 AWG circuit is sufficient for the common 4,500-watt heater despite it containing two elements. It also explains a frequent field observation: a water heater recovers hot water fastest right after the elements switch, and its energy draw, though substantial, is intermittent as the thermostats cycle. If a heater were ever miswired so both elements could energize at once, the circuit would be badly overloaded — a reason to leave water-heater wiring to a qualified electrician and to test element operation after any repair.

The interlock is specific to conventional resistive tanks. Point-of-use and tankless electric heaters have no storage buffer and must heat water instantaneously, so they energize very large elements continuously and cannot rely on this trick — which is exactly why their circuit and service requirements are so much heavier. Heat-pump hybrid tanks, by contrast, spend most of their time running a small compressor rather than the resistive elements, so their typical draw is far below even a single element, and a modest circuit suffices.

The breaker answer depends heavily on the type of water heater. A standard storage tank (40–50 gallon, 4,500W) is the simplest case: 30A breaker, 10 AWG wire, one two-pole circuit. Because the two elements are interlocked (only one heats at a time), the draw never exceeds a single element's current, so the 30A circuit is sufficient.

Electric tankless heaters are a different world. To heat water on demand they may draw 80–150 amps, requiring multiple 40–60A double-pole breakers, very heavy conductors, and frequently a 200A (or larger) service. Homeowners switching to electric tankless are often surprised by the electrical work involved; a gas tankless or a heat-pump (hybrid) tank avoids it. Heat-pump water heaters are the most efficient electric option and typically run on a modest 30A/240V or even 15A/240V circuit because the compressor draws far less than resistive elements.

Whatever the type, always size from the nameplate, apply the 125% continuous factor, match the wire to the breaker, and verify the whole-house load. Pair this calculator with the breaker size calculator for other appliances and the circuit load calculator to check circuit capacity.

Finally, remember that a water heater circuit is a safety-critical, high-current, hardwired installation, not a plug-in convenience. It should have a means of disconnect within sight of the unit (a breaker lock or a local disconnect switch), a properly bonded ground, and connections torqued to the manufacturer's specification, since loose lugs on a high-current appliance are a leading cause of overheating and panel damage. Most jurisdictions require a permit and inspection for a new water-heater circuit. Use this tool to plan the breaker and wire, then have the actual installation performed or verified by a licensed electrician who can confirm the disconnect, grounding, and local amendments to the code.