Maintained by BuildSolver · Updated
Calculator · Electrical
Water heater breaker size calculator
A 4,500 W water heater on 240 V draws 18.75 A. The breaker and the conductors are sized at no less than 125 percent of that, 23.44 A, so the smallest breaker is 25 A. With no maximum on the plate, the code allows up to 150 percent rounded up, 30 A, and both take 10 AWG copper. This page gives that range and the wire for any tank heater, and checks the breaker on a circuit now. For the wire in more detail, the AC wire size calculator adds terminal ratings and derating.
- Pick the supply voltage and the voltage on the plate, and enter the heater watts. Add the largest breaker the plate prints, if it prints one.
- To check an existing circuit or a quote, enter the breaker on it now. Leave it blank to size a new circuit.
- The result shows the current, the smallest breaker at 125 percent, the largest breaker the code allows, and the copper and aluminum wire for each.
Example result for the default inputs
4,500 W storage water heater rated 240 V, supplied at 240 V, single phase, on its own circuit. NEC 422.13 and 422.11(E).
Current
18.8 A
Smallest breaker
25 A
Largest breaker
30 A
| Current | 4,500 W / 240 V = 18.75 A |
|---|---|
| 125 percent, 422.13 | 1.25 x 18.75 A = 23.44 A. The breaker and the conductors are sized at no less than this for a storage heater of 120 gallons or less. |
| Smallest breaker | 25 A, the smallest standard size at or above 23.44 A (Table 240.6(A)). |
| Largest breaker | 1.5 x 18.75 A = 28.13 A, and the next standard size up is 30 A (422.11(E)(3), with no rating on the plate). |
| Wire on 25 A | copper 10 AWG, aluminum 10 AWG. At 86°F with two current-carrying conductors and 90°C (194°F) insulation. NM cable (Romex) is held to the 60°C column. A water heater is not in Table 240.4(G), so 240.4(D) caps a breaker on small conductors: 15 A on 14 AWG copper, 20 A on 12 AWG and 30 A on 10 AWG. |
| Wire on 30 A | copper 10 AWG, aluminum 8 AWG. A larger breaker needs wire rated for it (240.4), not only for the heater. |
| Heat input | 4,500 W x 3.412 = 15,354 BTU/h into the water |
For estimation purposes only. Consult a licensed engineer for final designs.
Sign up with your email. Back on this page in the same browser, your inputs are filled in. No card.
Free without signup
How the breaker range and the wire are worked
An electric water heater element is a resistor, so its current follows from the watts and the voltage. The NEC then gives two limits for the breaker: no less than 125 percent of the heater's rating, and no more than the rating on the plate or 150 percent of the current, rounded up to the next standard size. Any standard breaker between the two is allowed, as long as the wire is rated for the breaker you pick.
- Rated current
I = W / VrThe watts and the voltage on the plate. A 4,500 W heater rated 240 V is 18.75 A. NEC 422.13 sizes from "the rating of the water heater" and 422.11(E) from "the appliance rated current", so this page uses the plate rating even when the supply voltage is lower.- Smallest breaker
smallest standard size at or above 1.25 x INEC 422.13 sizes the breaker and the conductors of a storage water heater of 120 gallons or less at no less than 125 percent of its rating. For 4,500 W at 240 V that is 23.44 A, so 25 A. Standard sizes are from NEC Table 240.6(A).- Largest breaker
plate rating, or 1.5 x I and the next size up, or 20 A at 13.3 A or lessNEC 422.11(E) caps the breaker of a single non-motor appliance. With no rating on the plate, 1.5 x 18.75 A is 28.13 A, and the next standard size is 30 A, the answer in Mike Holt's EC&M example. A heater of 13.3 A or less is capped at 20 A.- Wire
ampacity at least 1.25 x I, and rated for the breaker (240.4)The conductor has to carry 125 percent of the heater current and be protected by the breaker. NEC 240.4(D) caps 12 AWG copper at 20 A and 10 AWG at 30 A, so a 4,500 W heater takes 10 AWG copper on 25 A and on 30 A.- 240 V heater on 208 V
W208 = W240 x (208 / 240)^2 = 0.751 x W240The element resistance stays the same, so the watts fall with the square of the voltage. A 4,500 W element makes 3,380 W on 208 V and draws 16.25 A. The breaker is still sized from the plate rating, 18.75 A. The baseboard heater calculator sizes a 240 V heater on 208 V from the current it draws, because NEC 424.4(B) sizes from the load, while 422.13 for a water heater names its rating.
Eight water heaters through the same formula
The first row is the EC&M Code Q&A example and the fourth is the 208 V heater from an EC&M column, and the page gives the same breakers. The 240 V heaters on 208 V draw less than their rating, and the breaker is still sized from the plate. Both elements at once and a 120 V tank close the table.
| Heater | Rated current, A | Smallest breaker, A | Largest breaker, A | Copper on smallest, AWG |
|---|---|---|---|---|
| 4,500 W, 240 V (EC&M Code Q&A example) | 18.75 | 25 | 30 | 10 |
| 3,500 W, 240 V | 14.58 | 20 | 25 | 12 |
| 6,000 W, 240 V | 25.00 | 35 | 40 | 8 |
| 6,000 W, 208 V (EC&M LeBlanc example) | 28.85 | 40 | 45 | 8 |
| 4,500 W, 240 V heater on 208 V | 18.75 | 25 | 30 | 10 |
| 6,000 W, 240 V heater on 208 V | 25.00 | 35 | 40 | 8 |
| 9,000 W, 240 V, both elements at once | 37.50 | 50 | 60 | 6 |
| 1,500 W, 120 V point-of-use tank | 12.50 | 20 | 20 | 12 |
Copper wire is for 60°C (140°F) terminals and NM cable. 30 A on 10 AWG covers the 4,500 W and 5,500 W heaters at 240 V. A 6,000 W heater at 240 V needs 35 A, and the same rating at 208 V needs 40 A, so 10 AWG on 30 A is too small for both.
Standards and data sources
- NEC 2023, 422.13, 422.11(E), Table 240.6(A), Table 310.16 and 240.4(D) - the 125 percent rule for the breaker and conductors of a storage water heater of 120 gallons or less, the largest breaker for a single non-motor appliance, the standard breaker sizes and the small-conductor limits. The NEC text was read as EC&M and the Mike Holt forum print it, not in NFPA's viewer.
- Mike Holt in EC&M Code Q&A, Overcurrent Protection Requirements for Appliance Circuits (2020) - the three items of 422.11(E) and the worked example this page opens on: 4,500 W at 240 V is 18.75 A, 150 percent is 28 A, and the largest breaker is 30 A.
- Russ LeBlanc in EC&M, A Continuous Load Conundrum (2024) - the 2020 and 2023 wording of 422.13, the history of the continuous-load wording since 1975, and the 6,000 W, 208 V heater that needs a 40 A circuit, where 30 A on 10 AWG would not comply.
- John Lupacchino in EC&M, Code Quiz on fixed storage type water heaters (2026) - the 2026 edition calling a storage water heater of 120 gallons or less a continuous load again, which also reaches feeders and services.
- Mike Holt forum threads on water heater breakers, 2012 to 2021 - the reading of 125 percent as the least breaker and 150 percent as the most, the 25 A and 30 A answers for 4,500 W, 35 to 40 A for 6,000 W at 240 V, and the rule that a larger breaker needs wire rated for it.
Limitations of this calculator
- This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
- The heater's rating plate and installation instructions come first. A maximum breaker or fuse printed there governs over the 150 percent rule, and listed equipment is installed according to its listing.
- The 125 percent rule of 422.13 is for storage (tank) water heaters of 120 gallons or less. Tankless heaters are not storage type and often need two or more circuits set by their own instructions; larger tanks fall under the general appliance rules. Neither is worked here.
- The NEC text was read as EC&M columns and the Mike Holt forum restate it, not in NFPA's own viewer. The branch-circuit numbers are the same in the 2017, 2020 and 2023 editions. The 2026 edition, as EC&M reports it, calls the heater a continuous load again, which matters for feeders and services but not for the breaker here. Check the edition your AHJ adopted.
- The 2020 and 2023 text of 422.13 sets the 125 percent directly. This page applies it to every heater and uses no exception for a breaker listed for 100 percent operation.
- Wire sizes are for copper and aluminum at 86°F with two current-carrying conductors and 90°C (194°F) insulation. NM cable (Romex) is held to the 60°C column. A hot attic, more conductors in a conduit and a long run change the size. Use the AC wire size calculator for corrections and the voltage drop calculator for a long run.
- The page works one heater on its own circuit, single phase at 120, 208 and 240 V, from 500 to 12,000 W and up to 60 A of current. Three phase, 277 V and 480 V commercial heaters are not covered.
- Heat pump water heaters print a minimum circuit ampacity and a maximum breaker on the plate, like an air conditioner. Use those two numbers, not this page.
- The page does not size the service or the feeder, and it does not apply the demand factors of a dwelling load calculation. Have the service checked by an electrician before adding a heater.
Common mistakes with water heater circuits
- Putting a 4,500 W heater on a 20 A breaker. A 4,500 W heater at 240 V draws 18.75 A, which is under 20 A. At 125 percent it is 23.44 A, so the smallest breaker is 25 A. A 20 A breaker is below the least size, and the 12 AWG copper that goes with it is not enough either.
- Using 10 AWG and 30 A for every heater. 30 A on 10 AWG copper fits a 4,500 W or 5,500 W heater at 240 V. A 6,000 W heater at 240 V draws 25.00 A and needs at least 35 A and 8 AWG copper. At 208 V the same rating draws 28.85 A and needs 40 A, so a 30 A circuit is too small.
- Raising the breaker without the wire. 422.11(E) allows a breaker up to 35 A on a 5,500 W heater at 240 V, but the wire has to be rated for that breaker, not only for the heater. 10 AWG copper carries the 30 A breaker, and the 35 A breaker takes 8 AWG. Swapping the breaker for a bigger one on the old wire breaks 240.4(D).
- Ignoring the maximum breaker on the plate. If the plate prints a maximum breaker, that number caps the circuit. A 6,000 W heater at 240 V with 30 A on its plate would conflict with the 35 A that 125 percent gives, which usually means the watts or the voltage were misread. Read the plate again before choosing a breaker.
- Adding the two elements of a standard tank. Most two-element tanks heat with one element at a time, and the plate shows that element: 4,500 W, not 9,000 W. Sizing for both would give 50 A and 6 AWG copper. Only a tank wired to run both elements at once, with the sum on its plate, needs that.
- Expecting full heat from a 240 V heater on 208 V. A 4,500 W, 240 V element makes 3,380 W on 208 V, about 75 percent, so the tank recovers more slowly. The current falls to 16.25 A, but the breaker is still sized from the plate rating. Some heaters ship with 208 V elements; the plate says which.
Common questions about water heater breakers
What size breaker do I need for a water heater?
Divide the watts on the plate by the voltage, multiply by 1.25 and take the next standard breaker size. That is the smallest breaker. The largest is the size printed on the plate, or 150 percent of the current rounded up. A 4,500 W heater at 240 V draws 18.75 A, so the breaker is 25 A to 30 A.
Is a 30 amp breaker right for a 4,500 watt water heater?
Yes, at 240 V. The heater draws 18.75 A. 125 percent is 23.44 A, so 25 A is the smallest breaker, and 150 percent is 28.13 A, which rounds up to 30 A, the largest. Both take 10 AWG copper. If the plate prints a smaller maximum, the plate governs.
What breaker does a 5,500 watt water heater need?
At 240 V it draws 22.92 A. 125 percent is 28.65 A, so the smallest breaker is 30 A on 10 AWG copper. The largest without a plate rating is 35 A, and that one needs 8 AWG copper.
What size wire for a 6,000 watt water heater?
At 240 V a 6,000 W heater draws 25.00 A and needs a 35 A breaker at least, with 8 AWG copper on 60°C (140°F) terminals. At 208 V a heater rated 6,000 W at 208 V draws 28.85 A and needs 40 A and 8 AWG copper. 10 AWG on 30 A is too small for either.
Is a water heater a continuous load?
The 2017 NEC called a storage water heater of 120 gallons or less a continuous load for sizing branch circuits. The 2020 and 2023 editions say instead that the breaker and conductors are no smaller than 125 percent of its rating, which gives the same branch circuit. EC&M reports that the 2026 edition makes it a continuous load again, which also reaches the feeder and service.
Why can the breaker be bigger than 125 percent?
422.13 sets the least size and 422.11(E) sets the most. For a heater with no maximum on its plate, the breaker may go up to 150 percent of the current, and to the next standard size when 150 percent falls between sizes. Any standard size between the two limits is allowed, with wire rated for the breaker.
What breaker for a small 120 volt water heater?
A 1,500 W, 120 V point-of-use tank draws 12.50 A. 125 percent is 15.63 A, so the breaker is 20 A. A heater of 13.3 A or less with no rating on its plate is held to 20 A, so 20 A is also the largest. A 2,500 W, 240 V heater draws 10.42 A and takes 15 A to 20 A.
Does this work for a tankless water heater?
No. 422.13 is for storage tanks, and a whole-house electric tankless heater often needs two or more circuits, with breakers and wire printed in its own instructions. The tankless water heater size calculator gives the kW and amps for the flow you need.
Does this replace an electrician?
No. This page is for estimating and for checking numbers on a quote. A licensed electrician sizes and installs the circuit under a permit, and the local authority having jurisdiction inspects it.