What breaker does a 36 kW water heater need?

At 240 volts it draws 150 amps, which becomes 187.5 amps once the NEC continuous load factor is applied. Units this size are split across 4 circuits, each needing a 50 amp breaker and 8 AWG copper. The installation manual is the authority for the circuit count.

Why is the breaker larger 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 nameplate so the breaker is not operating at its limit for hours on end. The extra capacity is thermal margin for the breaker, not extra current for the heater.

Does it need a double pole breaker?

Yes, on a 240 volt circuit. Both legs are live and both must be switched, which is what a double pole breaker does. A single pole device leaves one leg energised at the element even when it appears to be off, which is genuinely dangerous during service work and is why timers for this application are specified as double pole.

Can I use aluminium conductors instead?

It is permitted with the correct terminations and anti-oxidant treatment, but the ampacity is lower for the same size, so the conductor goes up at least one gauge. The figures on this page are for copper at the 75 C column of NEC Table 310.16. If aluminium is on the table, it is a conversation to have with the electrician rather than a substitution to make yourself.

What does NEC 240.4(D) change?

It caps the overcurrent protection on small conductors regardless of their table ampacity: 15 amps on 14 AWG, 20 on 12 AWG and 30 on 10 AWG. So even though 12 AWG copper shows 25 amps of ampacity at 75 C, it cannot sit under a 25 amp breaker in this application. That rule is why a 4,500 watt element ends up on 10 AWG rather than 12.

Electrical

What Breaker and Wire Size for a 36 kW Water Heater at 240 V?

Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated 2026-09-17.

Quick answer

A 36 kW water heater at 240 volts draws 150 amps. NEC 422.13 makes a storage water heater of 120 gallons or less a continuous load, so the circuit is sized at 125 percent, giving 187.5 amps of design load. A unit this size is normally split across 4 circuits, each with a 50 amp breaker and 8 AWG copper.

Electric water heater circuits are sized on a rule people often miss: NEC 422.13 classes a storage water heater of 120 gallons or less 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 deliberately larger than the current draw.

At 36 kW and 240 volts that is 150 amps of draw and 187.5 amps of design load. Because a single circuit that size is impractical to run and terminate, manufacturers split units at this rating across 4 separate circuits.

The sizing, step by step

Circuit sizing for a 36 kW load at 240 V
Step	Figure	Where it comes from
Nameplate power	36 kW	The unit data plate
Current draw	150 A	Watts divided by volts
Continuous load factor	125%	NEC 422.13
Design load	187.5 A	Draw times 1.25
Circuits	4	Manufacturers split units this size internally
Breaker per circuit	50 A	Next standard size at or above the per circuit design load
Copper conductor	8 AWG	NEC Table 310.16 at 75 C, limited by NEC 240.4(D)

Published standard Source: NEC 422.13, Table 310.16 at 75 C copper, and 240.4(D) small conductor limits. The circuit count is from the manufacturer installation manual. Sizing a single circuit for the whole load gives a wrong and much more expensive answer.

Where this sits against other ratings

Breaker and conductor by heater power at 240 V, single circuit
Power	Draw	Design load	Breaker	Copper
2.5 kW	10.4 A	13 A	15 A	14 AWG
3.5 kW	14.6 A	18.2 A	20 A	12 AWG
4.5 kW	18.8 A	23.4 A	25 A	10 AWG
5.5 kW	22.9 A	28.6 A	30 A	10 AWG
6.5 kW	27.1 A	33.9 A	35 A	8 AWG
7.2 kW	30 A	37.5 A	40 A	8 AWG
9 kW	37.5 A	46.9 A	50 A	8 AWG
11 kW	45.8 A	57.3 A	60 A	6 AWG
12 kW	50 A	62.5 A	70 A	6 AWG
13 kW	54.2 A	67.7 A	70 A	4 AWG

Published standard Source: NEC 422.13 continuous load factor with Table 310.16 ampacity at 75 C copper. Ratings above about 13 kW are split across multiple circuits by the manufacturer, so a single circuit row would be misleading and is left out.

Why this is a service question before it is a wiring question

A 36 kW unit is 150 amps of draw and 200 amps of total breaker capacity across 4 circuits. On a 200 amp service that is most of the house, and a load calculation will usually decide it before the panel does.

Get an electrician to run the load calculation before buying the unit, not after. The service upgrade, where one is needed, costs considerably more than the heater does, and it is the item that most often turns an electric tankless plan into a heat pump water heater plan instead.

Turn the breaker off before touching an element. Both legs of a 240 volt circuit are live, so switching one of them is not enough, and testing with a meter before touching anything is the habit that matters. Never energise an element that is not submerged: dry firing destroys an element in seconds. This page is a planning figure and not a substitute for an electrician.

Electric units and controls at this rating

Beginner

Rheem

Rheem RTEX-13 13kW

$278.00

Thirteen kilowatts at 240 volts is about 54 amps, which many existing panels can take. In a warm inlet climate it supports roughly one shower at a time.

Best for: A single bathroom in a warm inlet climate

Check price on Amazon

Intermediate

Rheem

Rheem RTEX-18 18kW

$438.00

Eighteen kilowatts draws 75 amps across two circuits. This is the largest electric tankless a typical 200 amp service absorbs without a load calculation fight.

Best for: The realistic ceiling on a 200 amp service

Check price on Amazon

Intermediate

Intermatic

Intermatic WH21 Water Heater Timer

$69.26

Built for this appliance rather than adapted to it, rated 208 to 277 volts at 6,250 watts, which covers a standard 4,500 watt element with headroom.

Best for: A 240 volt electric tank on a time of use rate

Check price on Amazon

Frequently asked questions

What breaker does a 36 kW water heater need?: At 240 volts it draws 150 amps, which becomes 187.5 amps once the NEC continuous load factor is applied. Units this size are split across 4 circuits, each needing a 50 amp breaker and 8 AWG copper. The installation manual is the authority for the circuit count.
Why is the breaker larger 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 nameplate so the breaker is not operating at its limit for hours on end. The extra capacity is thermal margin for the breaker, not extra current for the heater.
Does it need a double pole breaker?: Yes, on a 240 volt circuit. Both legs are live and both must be switched, which is what a double pole breaker does. A single pole device leaves one leg energised at the element even when it appears to be off, which is genuinely dangerous during service work and is why timers for this application are specified as double pole.
Can I use aluminium conductors instead?: It is permitted with the correct terminations and anti-oxidant treatment, but the ampacity is lower for the same size, so the conductor goes up at least one gauge. The figures on this page are for copper at the 75 C column of NEC Table 310.16. If aluminium is on the table, it is a conversation to have with the electrician rather than a substitution to make yourself.
What does NEC 240.4(D) change?: It caps the overcurrent protection on small conductors regardless of their table ampacity: 15 amps on 14 AWG, 20 on 12 AWG and 30 on 10 AWG. So even though 12 AWG copper shows 25 amps of ampacity at 75 C, it cannot sit under a 25 amp breaker in this application. That rule is why a 4,500 watt element ends up on 10 AWG rather than 12.

Turn the breaker off before touching an element. Both legs of a 240 volt circuit are live, so switching one of them off is not enough, and testing with a meter before you touch anything is the habit that matters. Never energise an element that is not submerged: dry firing destroys an element in seconds and can crack the tank lining around it.