5 Star Plumbing | Tankless Water Heater Breaker Size: How do I choose the best size?
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Tankless Water Heater Breaker Size: How do I choose the best size?

5 Star Plumbing > Blog > Tankless Water Heater Breaker Size: How do I choose the best size?

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How do electric tankless water heaters work?

An electric tankless water heater is a water heating system that heats water as it flows through the unit rather than storing heated water in a tank. For example, when you turn on the hot water faucet, cold water enters the tankless heater and is heated by either a heating element or a gas burner. The heated water then flows out to the tap, replacing the cold water that was just drawn off. Because there is no storage tank, you can use as much or as little hot water as you need at any given time without running out. And since there’s no standby heat loss from a storage tank, you’ll save energy dollars too.

The installation process for electric tankless water heaters can be tricky, so it is essential to hire a professional to do the job. The first step is to identify where you will be installing the unit. The best place is usually near the water main, ensuring the water is heated quickly.

Once you have identified the installation location, you will need to cut a hole in the wall and run the appropriate wiring. Again, follow the manufacturer’s instructions carefully, as each unit has specific requirements.

Once the wiring is in place, you can install the tankless water heater. Be sure to seal any gaps around the unit with caulk or insulation, as this will help keep your energy bill low.

Picking the Perfect Breaker Size

  1. Refer to the manual

The best way to discover the right size for your water heater amp circuit breaker is by checking your tankless water heater’s manual. You can look at what kind of wiring it has and how many breakers are required for optimal performance.

The model you have will determine whether it is a 3 x 50 or 4 x 40. Remember that more circuit breakers with higher amperage means more significant electrical current. For example, electric tankless heaters can draw a maximum of 42 to 167 amps. However, if you can’t find the manufacturer’s manual, you can always use the mathematician’s method.

  1. Use the Power formula.

Use the Power equation, P = I x V, from which you can derive the formula to get the specific amp breaker size: I = P ÷ V (I = Amps, P = Watts, and V = Volts). ETWHs operate on 240 Volts, so you’ll need to find the wattage of your device either in its spec sheet or inside the water heater itself.

You will need several variables to calculate the breaker size for a tankless water heater. Here is an example: Let’s say that a heating element needs 7600 watts from a 240-volt circuit. To find the amperage, divide 7,600W by 240V:


I = 7,600 Watts ÷ 240 Volts

I = 31.7 amps


The result reveals that 31.7 amps are required for this heating element to operate appropriately. However, using the same calculated amperage as your final breaker size for a tankless water heater is dangerous. Using the same amp amount could lead to accidents if you do not have protection against power surges.

The National Electric Code (NEC) has established that an amp circuit breaker shouldn’t be used for more than 80% of its total capacity to avoid potential fires. To discover what size breaker for electric tankless water heater you require, multiply 1.25 or 125% by the rated amps of your device – in this example, 31.7 amps.

CB Size = 125% x 31.7 = 39.62 Amps

ased on the final answer of 39.62 amps, get the closest breaker size possible. So, for a safe maximum continuous load on our sample heating element, buy a 40-amp breaker.

This example only featured one circuit breaker for one heating element. However, as I’ve mentioned earlier, there are water heaters that need multiple circuit breakers for each heating element they have. So, make sure to take into account other heating elements as well.


Can my breaker panel handle an electric tankless water heater?

Before buying a new electric tankless water heater, ensure you have enough space in your main pole breaker panel. Each heating element has its own double circuit breaker, so it’s essential to have a spacious and safe panelboard. To determine if you need an upgrade, ask yourself: “Do I need a panelboard upgrade?”

100-amp panels might have been enough in the past for energy-consuming appliances like dishwashers and air conditioners. However, technology has rapidly changed, making many devices outdated. The latest ETWH cannot work under that minimal amperage due to insufficient power.

Three breakers power three heating elements, each carrying 40 to 50 amps for 120+amps. Therefore, your 100-amp panel is useless when an emergency thermal shutoff occurs. Calculate 125% of the heater’s continuous load to size your main pole breaker panel.


We’ll use our Power formula once again to explore this idea. Note that electric tankless boilers can utilize anywhere from 10,000 watts up to 40,000 watts. For a realistic example of a residential ETWH, we’ll say 30,000 Watts. Therefore:

I = 30,000W ÷ 240V

I = 125 Amps

Continuous load = 125% x 125A = 156.25 Amps

The largest panelboard is 200 amps, but remember that the average 200-amp breaker should be at most 160 amps at a time. Your heating device might need this size amp, so it’s best to consult with a contractor who can check the overall power input of your ETWH and other electrical devices. In doing so, you’ll figure out what panel size works best for you.

Can I Calculate the Correct Breaker Sizing for My Electric Tankless Water Heater?

Yes, the breaker sizing for your electric tankless water heater can be calculated by using the following formula: 240 volts / amperage = breaker size

For example, there are some breaker tips you can use: for example, if you have a 240-volt tankless water heater with an amperage of 10, your breaker size would be 24.

Do tankless water heaters need a dedicated circuit?

Tankless water heaters, also called demand-type or instantaneous water heaters, don’t require a dedicated circuit. However, suppose the tankless water heater is installed in an area with a high electrical load, such as a bathroom with two showers running simultaneously. In that case, it might be necessary to establish a dedicated circuit for the tankless water heater.

H3 Do I Have to Use the Correct Wire for a Tankless Water Heater Breaker?

  • Using the proper electrical wire is critical when installing the electrical connections for your tankless water heater breakers to avoid electric shock.
  • For the tankless water heater to be properly and safely connected according to the circuit breaker rating, it is recommended by experts that you use either a nonmetallic or metal-clad electrical wire with the proper wire size.
  • Most electricians prefer MC cables over other types because they are stronger and more resistant to tearing, cutting, and other physical forces.
  • Non-metallic wires are easier to work with and more budget-friendly.
  • The ideal way to determine the wire gauge necessary for your tankless water heater is by consulting the manufacturer’s suggestions.
  • As a general rule of thumb, the more amperage or wattage an electric wire has, the thicker its diameter should be.

There are some exceptions to this rule: for example, a 7.6 kW appliance requires an 8-gauge electric line. In contrast, an 11-kW device linked to a 60-amp breaker necessitates a 6-gauge wire.

Dangers of Having the Wrong-Sized Breaker

A breaker is a safety device used to prevent an electrical fire. Breakers are rated by the amount of current they can handle safely. A breaker that is too small will trip when it should not, and a breaker that is too large will not trip when it should. Both of these situations can cause an electrical fire.

If you are still determining the size of your circuit breaker, call a licensed electrician to come and inspect your electrical system. Do not try to change the breaker size yourself. Only a licensed electrician should do this job.

Should I Have an Electric Water Heater or Gas Water Heater?

Electric and gas water heaters are two of the most popular types on the market. However, both have pros and cons, so which one is right for you?

Electric water heaters are more expensive to install than gas water heaters but are much more efficient than gas water heaters. They also last longer than gas water heaters. Gas water heaters are cheaper to install than electric water heaters, but they are less efficient than electric water heaters. They also do not last as long as electric water heaters.

If you want to know which type of water heater is more efficient, electric or gas, the answer is electric. Even though natural gas is cheaper than electricity, an electric heater saves you money on your monthly utility bill—a gas heater actually uses more energy overall and releases waste into the environment. The waste gasses take some heat with them as they escape, making the process less efficient than an electric water heater.

Summing Up

The breaker size for a tankless water heater is essential to consider when choosing a new tankless water heater. You’ll want to select the best size breaker for your home to ensure you have enough power to run the unit. You’ll also want to ensure that the breaker size can handle the load of the tankless water heater.

Sacramento plumbers provide top-notch services and are always willing to go the extra mile for their customers. So if you need plumbing services, be sure to give them a call!


Can my house handle an electric tankless water heater?

The answer to this question depends on several factors, including which size breaker you use in your house, the climate you live in, and how much water you use daily. If you are unsure whether your home can support an electric tankless water heater, it is best to consult a professional before making a purchase.

Do electric tankless water heaters use 110 or 220 volts?

Tankless water heaters can use either 110 or 220 volts, depending on the model. All electric tankless water heaters operate on a 240-volt circuit to reduce amp usage.

What size breaker do I need for a 110-volt water heater?

The Q230 30-Amp Double Pole Type QP Circuit Breaker is ideal for an electrical water heater, typically using a 30-amp circuit breaker.

What size tankless water heater do I need for a family of four?

A family of four typically needs a tankless water heater with a flow rate of 6.6 gallons per minute (GPM).

Mike Joshua

Mike Joshua, a 33-year-old plumber based in California. With a passion for solving complex problems and a commitment to his trade, Mike has dedicated the past 8 years of his life to honing his skills as a professional plumber. After completing technical college Mike quickly secured a job in a plumbing company and has been working steadily ever since. Mike's experience includes not only households but industrial gas and plumbing systems and installation, repair the equipment for different types of business He is known for his attention to detail, strong work ethic, and ability to complete projects on time and within budget. He is always striving to improve his skills and stay up-to-date with the latest industry developments and advancements. In conclusion, Mike enjoys outdoor activities such as hiking and camping. He also has a passion for DIY projects around the home and is always looking for new challenges to tackle.

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