Water Heater Circuit Calculator
Enter your electric water heater's nameplate rating and get the branch-circuit breaker and minimum conductor. NEC 422.13 treats a fixed storage-type water heater of 120 gallons or less as a continuous load, so the circuit is sized at 125% of the rating, and 422.11(E) caps the breaker at 150%, giving the valid breaker window.
How to use this calculator
- Pick a common water heater from the list, or read the nameplate and enter its rating. Most tanks are marked in watts (for example 4500 W); switch the unit to amps if the label gives amperes instead.
- Set the nominal voltage. Residential tanks are 240 V; commercial units are often 208 V. Small point-of-use heaters may be 120 V.
- Adjust the conductor material, insulation and termination ratings, ambient temperature, and conductor count if your install differs from the standard basis.
- Read the minimum breaker and conductor. The results also show the maximum breaker permitted by 422.11(E), so you can see the full compliant range.
- Size the equipment grounding conductor and check voltage drop separately using the linked tools below.
NEC reference
Sizing follows NEC 2020. Section 422.13 classifies a fixed storage-type water heater of 120 gallons (450 L) or less as a continuous load for the purpose of sizing branch circuits, and requires its conductors and overcurrent device to be sized at not less than 125% of the rating (the general continuous rule of 422.10(A) and 210.20(A)). Section 422.11(E) sets the overcurrent maximum for a single non-motor-operated appliance that is not marked with a protective-device rating: not over 20 A when the appliance is rated 13.3 A or less, or not over 150% of the rating (next higher standard permitted) when it is rated over 13.3 A. The breaker is a 240.6(A) standard rating, and the conductor comes from Table 310.16 with 310.15 derating, the 110.14(C) termination cap, and the 240.4(B)/(D) conductor-protection limits. These rules are unchanged across the 2017, 2020, and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Common water heater circuits: copper, standard conditions
| Nameplate | Current | Min breaker (125%) | Copper conductor | Max breaker (150%) |
|---|---|---|---|---|
| 3000 W @ 240 V | 12.5 A | 20 A | 12 AWG | 20 A |
| 3800 W @ 240 V | 15.8 A | 20 A | 12 AWG | 25 A |
| 4500 W @ 240 V | 18.75 A | 25 A | 10 AWG | 30 A |
| 5500 W @ 240 V | 22.9 A | 30 A | 10 AWG | 35 A |
| 4500 W @ 208 V | 21.6 A | 30 A | 10 AWG | 35 A |
Standard conditions (86°F, 90°C insulation on 75°C terminations, 2 current-carrying conductors, copper). The 4500 W / 240 V tank is the most common residential unit. Aluminum, hot ambients, and long runs (voltage drop) change the conductor; use the calculator above with your actual conditions.
The 125% continuous-load rule for water heaters
The rule that governs a water heater circuit is that the load is continuous. NEC 422.13 makes that determination for you: a fixed storage-type water heater of 120 gallons or less is a continuous load, full stop, because the heating elements can run well past 3 hours on a cold tank or a long recovery. NEC 210.20(A) then applies the continuous-duty requirement, the 125% factor.
So a 4500 W / 240 V heater is not on a 20 A circuit. It draws 18.75 A, needs 18.75 x 1.25 = 23.44 A of protection and conductor ampacity, and lands on a 25 A minimum breaker with 10 AWG copper. Sizing a water heater to the raw current instead of 125% is the most common mistake, and it produces a circuit that nuisance-trips or overheats the terminations under a full heating cycle.
The breaker window: 125% minimum, 150% maximum
A water heater is a single non-motor-operated appliance, so NEC 422.11(E) also caps the breaker from above. When the nameplate does not mark a maximum overcurrent rating, the breaker may not exceed 150% of the appliance rating, with the next higher standard size permitted where 150% is not itself a standard rating. For an 18.75 A heater that is 28.1 A, rounded up to 30 A.
That gives a valid window rather than a single number: a 4500 W heater accepts any breaker from 25 A (the 125% minimum) to 30 A (the 150% maximum), and 10 AWG copper carries the load for either. This is why field practice often lands on a 30 A breaker for a 4500 W tank even though 25 A is the code minimum. If you pick a breaker above the minimum, confirm the conductor is still protected. When the heater is rated 13.3 A or less, 422.11(E)(2) caps the breaker at 20 A instead. If the nameplate marks a maximum overcurrent rating, 422.11(E)(1) makes that marked value govern.
Storage tanks only: tankless and dual-element notes
- Storage type, 120 gallons or less: the case 422.13 covers, and what this tool sizes. The vast majority of residential and light-commercial electric tanks fall here.
- Tankless / instantaneous: not classified continuous by 422.13, usually much higher current (often 40 A to 150 A total), and frequently split across multiple circuits. Size to the manufacturer instructions with the general Breaker Size and Wire Size calculators.
- Dual-element tanks: the upper and lower elements are interlocked and run one at a time (non-simultaneous). Size from the single-element nameplate rating; do not add the two elements. Simultaneous-element heaters are a special order and are marked as such.
Frequently asked questions
What breaker for a 4500 watt water heater?
18.75 A at 240 V x 1.25 = 23.44 A, so a 25 A minimum breaker on 10 AWG copper. NEC 422.11(E) allows up to 30 A, so a 30 A breaker on the same 10 AWG is also compliant. Enter 4500 W above to see the full window and the aluminum equivalent.
Is 10 AWG or 12 AWG right for a water heater?
It depends on the wattage. A 4500 W or 5500 W tank (18.75 A or 22.9 A) needs 10 AWG copper. A lower-watt 3800 W tank (15.8 A) fits 12 AWG on a 20 A breaker. Enter your exact nameplate to be sure, and never assume from the breaker alone.
Does this size the ground wire?
No. The equipment grounding conductor is sized from the breaker rating under NEC Table 250.122, not the 125% circuit ampacity. Use the Equipment Grounding Conductor Size Calculator linked below with the breaker size this tool returns.
What about voltage drop on a long run?
A water heater in a detached structure or at the far end of a long run can push the conductor larger than the ampacity minimum to keep voltage drop within the recommended 3%. This calculator sizes for ampacity and overcurrent only; run the distance through the Voltage Drop Calculator before finalizing.
Related calculators
- Breaker Size Calculator: the general continuous/noncontinuous load sizing this tool is built on.
- EV Charger Circuit Calculator: the same continuous-load 125% rule for an EVSE branch circuit.
- Equipment Grounding Conductor Size Calculator: size the EGC from the breaker rating (Table 250.122).
- Voltage Drop Calculator: check the run length before finalizing the conductor.
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