Electric Water Heater Breaker & Wire Size
Free online tool • 2026 NEC Compliant
Electric water heaters are continuous loads under NEC Article 422. A standard 4,500-watt, 240V residential water heater draws 18.75 amps and, sized at 125% for continuous loads, requires a 30 amp double-pole breaker with 10 AWG copper wire. Use the calculator below to verify sizing for your specific water heater.
Recommended Sizing
NEC Standard SizeBreaker Type: Standard Inverse-Time
NEC Reference Note: 125% multiplier applied for continuous loads per NEC 210.20(A).
How to Use This Calculator
Find wattage
Check the water heater nameplate for element wattage (typically 4500W).
Calculate draw
Watts ÷ 240V = amps (4500 ÷ 240 = 18.75A).
Apply 125% continuous factor
18.75A × 1.25 = 23.4A minimum breaker.
Select standard size
Round up to 30A from NEC 240.6(A) standard sizes.
Frequently Asked Questions
What size breaker does a water heater need?
A standard 4,500W 240V electric water heater requires a 30 amp double-pole circuit breaker with 10 AWG copper wire. This is sized at 125% of the 18.75A draw because water heaters are continuous loads per NEC 422.13.
What wire size for a 40 gallon water heater?
Most 40 and 50-gallon residential electric water heaters have a 4,500W heating element at 240V, drawing 18.75A. The required wire is 10 AWG copper on a 30A double-pole breaker.
Can I use a 20 amp breaker for a water heater?
No. A standard 4,500W water heater draws nearly 19 amps, which exceeds the 80% continuous load limit of a 20A breaker. A 30A breaker is required.
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