What Size Breaker Does a Water Heater Need?

hybrid water heaters

Your electric water heater and gas tankless water heater require circuit breakers to operate. However, the wrong sizing of the breaker can lead to reduced safety and potentially damage the water heater. The additional electrical flow through the breaker puts the wires at risk of overheating, causing a fire, and damaging your water heater. 

The National Electrical Code outlines guidelines that govern the sizing of circuit breakers and wiring for electrical safety. According to the NEC Code, you should always size your circuit breaker at 125% of the load. This means, for safety purposes, it is recommended that you invest in a higher capacity circuit breaker than the electrical load of your electric gadgets – in this case, your water heater. 

Electric water heaters require a dedicated 240-volt dedicated 30-amp circuit and a 10-2 non-metallic (NM) or MC cable. This means the breaker only powers the water heater and no other appliances. A 30-amp circuit breaker can power all 4500watt water heaters regardless of gallon size.

Occasionally, I’ll see a 20-amp circuit breaker on a 4500-watt water heater. While this is technically correctly sized, it leaves little room for overcurrent and does not meet the NEC 125% load guideline.

The breaker size is determined by the wattage of the heating elements and not the tank size. More on that shortly.

A water heater with a 4500-watt heating element requires a 10/2 wire with a 30 amp (240volt) circuit breaker. The 3800-watt heating element can be wired with a smaller 12/2 wire and a 20 amp circuit breaker.

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Sizing Water Heater Breakers 

The key to getting the right size for a water heater is in understanding how the heater works. 

Electric water heaters use anything between 220 to 250 volts of alternating current. You’ll need a double circuit breaker that is correctly rated for the power draw of the water heater. We’re looking at between 20 Amp and 30 Amp for most electric water heaters. 

When looking at the sizing from the power requirements, you also need to consider the wire gauge size. The amperage drawn by the heater determines the wire gauge size.

Element WattsBreaker Size 120vBreaker Size 220vWire Gauge
150015 amp15 amp14
200020 amp15 amp14
250030 amp15 amp14
300030 amp15 amp12
3500NA20 amp12
4000NA25 amp10
4500NA30 amp10
5000NA30 amp10
5500NA40 amp8

How Do You Calculate Breaker Sizing?

Check your water heater’s wattage and voltage ratings. A water heater’s wattage and voltage rating come labeled on the sticker on the thermostat panel. Some heaters have this information near the base. 

While most residential water heater units have 4500-watt capacity, Commercial water heaters can reach 5500 watts.

Now, let’s get into the math of calculating breaker sizes. 

4500/240 = 18.75 amps. 18.75X125% = 23.4375 rounded to 25 amps

If you have a 4500-watt water heater with 240 volts, using the standard division formula you should end up with 18.75 amperes. However, remember an electric water heater runs on a continuous load. So adjust 18.75 by 125%, factoring in that a water heater runs a continuous load. 

Multiply 18.75 amperes by 1.25 to get 23.4375 amperes. When we round off that number, we get 25 amp. You’ll need a 25 or 30 amp circuit breaker.  The 30 amp is universally used because the minimum rating of 220 volts exceeds the 25 amp breaker.  

4500/220 = 20.45 amps. 20.45 ampsX125% =25.568 rounded to 30 amps

Why not 20? After all, it IS closer to 18.75. 

Well, 18.75 is only about 94% of the breaker’s amperage.  But going by the NEC 125% rule, a 20 amp is a risk of frequent tripping and could damage the water heater. 

A 25 amp breaker is sufficient; however, most electricians will install a 30 amp circuit to provide room for electrical fluctuations as outlined above.

What Size Circuit Breaker for Gas Tankless Water Heaters?

Gas tankless water heaters use less power draw and less voltage than electric tank water heaters. Usually, it’s about 12 amps and less than 120 volts. 

Gas tankless water heaters need a minimal amount of electricity to power the electronic ignition system. A gas tankless water heater only needs a 15-amp, 120 volt dedicated single-pole breaker for this purpose.

Should I Have Electric Water Heater or Gas Water Heater?

This choice depends on your locality and the utility costs of your area. However, gas units generally cost more upfront yet cost less to run than electric water heaters. 

On the brighter side, electric water heaters are more energy-efficient than gas water heaters. Gas costs 40% of the cost of electricity. Therefore, it would be cheaper to use gas. However, when you look at time savings over time, electric water heaters are more economical. 

Getting the right size circuit breaker can get technical. If you’re looking to make upgrades to accommodate electric, gas, or tankless water heating systems, you should contact a professional. 

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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