Informational Guide

What Water Heater Breaker Size Do I Need?

It’s important to have the right breakers, So if you’ve ever asked yourself, what water heater breaker size do I need, here’s the answer.

by Josh Mitchell

Electrical tankless water heaters generally require special electrical installation because they have higher load requirements than other kinds of electrical devices. 

It's important to have the right kind of breakers to prevent electric overloads and blackouts. So if you have ever asked yourself the question, what water heater breaker size do I need, this article is for you.  

Water heater breakers are measured in amperes. Amperes are a measure of current—how much electricity is flowing through a circuit. The current is equal to the voltage of the circuit divided by the total resistance of the circuit.  

Breakers exist to interrupt current flow if a fault in the electrical system is detected. They prevent damage but also need to handle a specific current without shutting off. The average amperes for a water heater is about 19.9 amps. A tankless electric heater requires about 100 to 120 amps to run itself. 


Considerations When Sizing Breakers for a Water Heater 

Power Rating Of Your Heater 

Breakers need to handle the current of your water heater system. The larger your tankless heater setup, the more current your breakers need to be able to handle. The typical water heater system requires a 30 amp breaker or higher.  

Maximum current drawn 

The total current draw is the amount that your circuit breakers need to handle. For example, if your water heater draws 20 amps of current, then you need a circuit breaker that can handle at least 20 amps of power. You also need to make sure that your electrical system is modified to provide the appropriate amount of current, or else it won't work properly.  

Current rating of your breaker 

Most homes are outfitted with 15-20 amp breakers and are designed to handle about 80% of their overall amperage. So a 15 amp breaker can handle about 12 amps and a 20 amp breaker 16 amps. Most likely, you will have to modify your home’s electrical grid to draw more current.   

What Water Heater Breaker Size Do I Need

What Water Heater Breaker Size Do I Need? (How to Calculate) 

Calculating the breaker size for your heater is relatively straightforward. You first find the power rating (measured in watts or kilowatts) and the required voltage for the machine. Since current is just power divided by voltage, you can calculate the required breaker current size for your heater just by dividing these two numbers.  

For example, say you have a standard 1,500-watt water heater that works on 120-volt connections. 1,500/120 = 12.5, so you will need at least 12.5 amps of current to get the thing going.  

However, most breakers are designed to work with only 80% load capacity. So for our 12.5 current requirements, we would need breakers that can handle at least 15 amps of current. Ideally, you should find breakers that can handle at least 125% of your water heaters’ requirements.  

A 50-gallon tank requires about 4,500 watts of power on a 120- volt circuit. Doing the math, that comes out to about 37.5 require amps. Going by the 125% rule, we need breakers that can handle at least 47 amps. So look for breakers with 40-50 amp loads.  

Water Heater Breaker Size Chart

Water Heater Breaker Size Chart 

Here is a chart marking the required sizes of circuit breakers and copper wire size for tankless water heater units. Keep in mind that these are just estimations. You should always follow the exact instructions listed in your user’s manual.  

Total Water Heater Wattage 

Circuit Breaker 

Copper Wire Size 

Cell

208V 

240V 

208V 

240V 

3800 

25 

20

10 

8

4500 

30 

25 

10 

10 

5500 

35 

30 

10 

10 

6000 

40 

35 

8 

8 

People also Ask (FAQs)

Can I replace a 30 amp breaker with a 60 amp breaker? 

You can only do this if you also replace the wire for the higher current. If you change the 30 amp breaker for the 60 amp breaker without also changing the wire, you might cause a fire.  

What size wire should you use for a 60 amp breaker? 

You need to use at least a 6 gauge (4.1mm) wire to run a 60 amp breaker without risk of damage—the more current, the thicker wire you will need.  

Can I use a 40 amp breaker for a water heater?

For some water heaters, yes, you can use a 40 amp breaker. 40 amp breakers can run some tankless electric water heater units as well.

How many amps can a 30 amp breaker handle? 

A 30 amp breaker can handle up to 30 amps but is only recommended for operation at around 25-26. Generally, a breaker should only be used at about 80% of their max amperage. 


Conclusion

Electric hot water heaters require more electrical power than other appliances, and so you need to make sure you have the right breakers and wires. So make sure that you have the right breakers for your water heater system.  

Josh enjoys researching, testing and diving into home improvement & DIY products. He has a passion for tools, learning new skills and fixing the everyday problems that arise around the house.

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