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Water Heater Breaker Size Calculator

Standard breaker rating from the load, checked against the NEC 310.16 ampacity of the conductor and the 240.4(D) small-conductor limits.

Inputs

The load

The current the circuit will carry, from the nameplate or the load calculation. A 4,500 W element at 240 V draws 18.75 A.

A continuous load is one expected to run three hours or more. NEC 422.13 treats a fixed storage-type water heater of 120 gallons or less as a continuous load, so continuous is the right answer for a tank heater.

Used only for the load in watts and volt-amperes.

The conductor

The size you intend to install. Sizes run from 14 AWG to 500 kcmil.

Copper or aluminum. Aluminum needs two sizes larger for the same ampacity and cannot be used at 14 AWG.

The column you may use is set by the weakest termination on the circuit, not by the insulation. 60 C for older equipment and most 14/12 AWG circuits, 75 C for most modern equipment rated 100 A and below, 90 C only when every termination is rated 90 C (NEC 110.14(C)).

Percent of the table ampacity after ambient correction (310.15(B)) and the conductor-count adjustment (310.15(C)(1)). Leave 100 if the circuit is in free air or you have already applied the factors.

Result

Load after the continuous factor
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Next standard breaker rating
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Corrected conductor ampacity
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Largest breaker the conductor allows
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Table 310.16 ampacity before adjustment
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Load in watts
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—
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Continuous-load treatment
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Where this stops applying
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Calculation details
Adjustment applied
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Ampacity headroom over the breaker
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Conductor circular mils
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Continuous-load multiplier
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240.4(D) limit for this size
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What to check

  • Nameplate amperage or wattage of the water heater
  • The conductor size, material and termination temperature rating
  • The continuous-load treatment, which NEC 422.13 makes mandatory for a storage water heater up to 120 gallons

Standards referenced

  • NFPA 70 National Electrical Code (NEC) — 2023 edition · Branch circuits, conductors, protection, raceways, and services. Local AHJ adoptions vary; the enforcing edition is set by the jurisdiction.

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