What breaker does a 4500 watt water heater need?

A 4,500 watt element at 240 volts draws 18.75 amps. NEC 422.13 treats a storage water heater of 120 gallons or less as a continuous load, so the circuit is sized at 125 percent, which is 23.4 amps. That calls for a 25 or 30 amp double pole breaker and 10 AWG copper conductors. A 30 amp breaker with 10 AWG is the usual installation.

Why is the breaker bigger than the current draw?

Because NEC 422.13 classes a storage water heater as a continuous load, meaning it can run for three hours or more at a time. Continuous loads are sized at 125 percent of the nameplate so the breaker is not operating at its limit for hours on end. The extra capacity is a thermal margin for the breaker, not extra current for the heater.

Why does an 18 kW tankless need more than one circuit?

Because 18 kilowatts at 240 volts is 75 amps, and at the continuous load factor that is a 100 amp branch circuit, which is impractical to run and to terminate. Manufacturers split the load internally across two or three separate circuits with their own breakers. The installation manual states how many, and sizing one circuit for the whole load gives a wrong answer.

Can I reuse the existing water heater circuit for a tankless unit?

Almost never. A tank element circuit is typically 30 amps on 10 AWG, while even a modest whole house electric tankless needs 60 to 100 amps across multiple circuits. That is a panel and often a service question rather than a wiring question. Have an electrician do a load calculation before buying the unit, not after.

Install calculator

Water Heater Breaker and Wire Size Calculator

Quick answer

NEC 422.13 makes a storage water heater of 120 gallons or less a continuous load, so the circuit is sized at 125 percent of nameplate. A 4,500 watt element at 240 volts draws 18.8 amps, which becomes 23.4 amps of design load, a 25 or 30 amp breaker and 10 AWG copper.

Result

Breaker per circuit 25 A Published standard

Copper conductor 10 AWG Published standard

Total draw 18.8 A Published standard

Design load per circuit 23.4 A Published standard

Total breaker capacity 25 A Published standard

Turn the breaker off before touching an element, and never energise an element that is not submerged. Dry-firing destroys an element in seconds.

The formula

Amps = watts / volts. Design amps = amps x 1.25 (continuous load). Breaker = next standard size at or above design amps. Conductor = smallest AWG whose 75 C ampacity meets the design load and whose NEC 240.4(D) limit permits that breaker.

The number of CIRCUITS matters and is not a detail. Electric tankless units above roughly 14 kW are internally split across two or three circuits, and the count is published in the installation manual. Sizing one circuit for the whole load gives a wrong and far more expensive answer.

The figures above are computed with these exact inputs: Total power 4.5 kW, Supply voltage 240, Circuits the unit splits across 1.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Nec42213: 1.25	Published standard	NEC 422.13, a storage water heater of 120 gallons or less is a continuous load, so the circuit is sized at 125 percent of the nameplate
NEC Table 310	Published standard	NEC Table 310.16 ampacity, copper, 75 C column
NEC 240	Published standard	NEC 240.4(D), overcurrent limits for 14, 12 and 10 AWG conductors regardless of their table ampacity

Best for: One fixture group, not a house

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Beginner

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Eleven kilowatts on a 60 amp circuit. It will run a shower in Florida and will not run one in Minnesota, and that is the whole electric tankless story.

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Frequently asked questions

What breaker does a 4500 watt water heater need?: A 4,500 watt element at 240 volts draws 18.75 amps. NEC 422.13 treats a storage water heater of 120 gallons or less as a continuous load, so the circuit is sized at 125 percent, which is 23.4 amps. That calls for a 25 or 30 amp double pole breaker and 10 AWG copper conductors. A 30 amp breaker with 10 AWG is the usual installation.
Why is the breaker bigger than the current draw?: Because NEC 422.13 classes a storage water heater as a continuous load, meaning it can run for three hours or more at a time. Continuous loads are sized at 125 percent of the nameplate so the breaker is not operating at its limit for hours on end. The extra capacity is a thermal margin for the breaker, not extra current for the heater.
Why does an 18 kW tankless need more than one circuit?: Because 18 kilowatts at 240 volts is 75 amps, and at the continuous load factor that is a 100 amp branch circuit, which is impractical to run and to terminate. Manufacturers split the load internally across two or three separate circuits with their own breakers. The installation manual states how many, and sizing one circuit for the whole load gives a wrong answer.
Can I reuse the existing water heater circuit for a tankless unit?: Almost never. A tank element circuit is typically 30 amps on 10 AWG, while even a modest whole house electric tankless needs 60 to 100 amps across multiple circuits. That is a panel and often a service question rather than a wiring question. Have an electrician do a load calculation before buying the unit, not after.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.