Electrical, Plumbing, Water Heaters

Water Heater Breaker Size: Easy Wire & Breaker Size Guide

The water heater breaker size is an important factor to consider when installing a new water heater. Proper wire and circuit breaker size ensure that your new water heater will run safely and efficiently. It also helps prevent excess strain on the electrical circuit, which can cause fires or damage your home.  

Residential water heaters require a 240-volt dedicated 30-amp circuit and a 10 AWG wire size. The 30 amp breaker only powers the water heater and no other appliances. A 30-amp circuit breaker can power all 4500-watt water heaters regardless of gallon size.

Electric Water Heater Breaker Size Chart

Electric Tank Water Heater SizeBreaker Size
10 Gallon Water Heater20 Amp or 30 Amp
20 Gallon Water Heater20 Amp or 30 Amp
30 Gallon Water Heater30 Amp Breaker
40 Gallon Water Heater30 Amp Breaker
50 Gallon Water Heater30 Amp Breaker
60 Gallon Water Heater30 Amp Breaker
70 Gallon Water Heater30 Amp Breaker
80 Gallon Water Heater30 Amp Breaker
90 Gallon Water Heater30 Amp or 40 Amp Breaker
100 Gallon Water Heater30 Amp or 40 Amp Breaker

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 dedicated circuit breaker at 125% of the load. Different water heaters require different wire and breaker sizes.

water heater breaker 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 size of the breaker is determined by the heating elements’ wattage rating, not the size of the tank. More on that shortly.

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

An electric tankless water heater can vary from 120 to 240 volts depending on the size and type. Under sink point-of-use (POS) electric on-demand water heaters use a 120-volt circuit with a 20-amp breaker.

Larger electric tankless water heaters use one 240-volt 40-amp breaker per heating element. Some whole-house electric tankless water heaters use as many as four 240-volt 40-amp breakers.

What Size Breaker for Water Heater Explained

The key to getting the right size breaker for water heaters is understanding how the heater works. 

Electric water heaters use anything between 220 to 240 volts of alternating current flow. You’ll need a double circuit breaker with the correct power rating for the water heater tank size. We’re looking at 20 Amp and 30 amp wire sizes for most electric water heaters. 

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

  • A 20-amp breaker needs a 12-gauge wire size
  • A 25-amp breaker needs a 12-gauge wire size
  • A 30-amp breaker needs a 10-gauge wire size
  • A 40-amp breaker needs an 8-gauge wire size

If your home only supports a 3800-watt water heater with a 12-gauge wire and 25-amp breaker, you’ll need to rewire the circuit with a 10-gauge wire and 30-amp breaker to support a 4500-watt water heater safely.

How Do You Calculate Water Heater Breaker Size?

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

While most residential water heater units have a 4500-watt capacity, commercial water heaters can reach 5500 watts requiring a larger breaker.

It’s important to have the correct circuit breaker. Having the wrong breaker size for water heaters is dangerous. Too small, and the breaker may frequently trip. Too large and the breaker may allow too much current, damaging a new water heater.

Now, let’s get into the math of calculating the breaker size for water heaters. 

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 heater runs on a continuous load. So adjust 18.75 by 125%, factoring 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 amps. 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 voltage than electric tank water heaters. Usually, it’s about 12 amps and less than 120 volts. 

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

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 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. 

Water Heater Breaker Size FAQs

When searching for water heater breaker sizes, you may have many questions about the right size breaker for electric water heaters. Here are some of the frequently asked questions.

What size breaker is needed for a 50-gallon electric water heater?

A 50-gallon water heater will require a 240-volt circuit with a 30-amp double-pole breaker.

Can a water heater be on a 20 amp breaker?

Most electric water heaters require a 30-amp breaker; however, some smaller, on-demand electric water heaters can operate on a 20-amp breaker. Consult with a professional electrician regarding your specific water heater breaker needs.

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

No, you should not use a 40 amp breaker for a residential electric water heater. The National Electric Code recommends using a 30-amp double pole breaker.

How many amps does a 40-gallon water heater use?

A 40-gallon water heater will use up to 4500 watts and between 18.75 and 23.5 amps.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.