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What Size Breaker Do I Need for a Tankless Water Heater?

Fact checked by Stephen Conklin

what size breaker do I need for a tankless water heater

Have you ever asked yourself “what size breaker do I need for a tankless water heater?” Knowing the correct circuit breaker capacity can help prevent electrical accidents, ensuring you retain your home’s integrity and your family’s safety.

Determining the correct tankless water heater breaker size is easy if you still have the tankless water heater’s user manual. Still, worry not if you no longer have this document because I will help you figure out the correct circuit breaker size for your tankless water heater.

Table of Contents

What Size of Circuit Breaker Do You Need for an Electric Tankless Water Heater?


As mentioned, your tankless water heater comes with a user manual that specifies its electrical requirements, including the recommended hot water heater breaker size.

For example, EcoSmart tankless water heaters require one 30-amp breaker for its POU 3.5 and POU 6 tankless water heater models, while other variants like the ECO8 need 40-amp breakers. On the other hand, most Rinnai tankless water heaters can run on a 20-amp breaker.

You can also check the water heater breaker size chart for your appliance’s brand, model, and serial number from online resources.

In general, tankless water heaters require 40 amps to 50 amps for each wire, corresponding to the appliance’s number of heating elements. Hence, if your water heater has three heating elements, you can expect to use three wires and three circuit breakers.

For example, my water heater demands 40 amps per wire, a total of 120 amps for the tankless water heater (40 amps x 3 wires or heating elements = 120 amps in three 40-amp circuit breakers).

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

The National Electric Code requires electricians to account for the electrical load to ensure safety when sizing a circuit breaker. The rule is to multiply the amperage requirements of appliances by 125% to determine the correct water heater circuit breaker size.

The first thing you should do is to check your tankless water heater’s voltage and wattage ratings. For example, suppose your tankless water heater runs on 220 volts and 5,500 watts. In that case, you need to divide 5,500 watts by 220 volts to get 25 amperes.

Unfortunately, your calculation does not end here. You must also account for the safety factor as required by the NEC. Multiplying 25 amperes by 125% (or 1.25) will give you 31.25 amperes. Hence, your tankless water heater will need a 35-amp breaker.

While you can install a 25-amp circuit breaker for the water heater in this example, you risk damaging the appliance because of frequent tripping.

What Size Breaker Should I Get for a Gas-powered Tankless Water Heater?

A gas tankless water heater has lower electrical requirements than an electric tankless water heater.

Most gas tankless water heaters need a 120-volt dedicated circuit with a single-pole 15-amp circuit breaker. That is all you need for your gas tankless water heater, regardless of its BTU rating.

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


Using the correct electrical wire is also essential when making your tankless water heater’s electrical connections. Experts recommend either a non-metallic (NM) or metal-clad (MC) electric wire to connect the tankless water heater to the circuit breaker.

Most electricians prefer MC cables because they are more resistant to ripping, cutting, and other physical forces. On the other hand, non-metallic wires are easier to handle and are cheaper.

As for the electric wire gauge, it would be best to check your tankless water heater’s recommendations. As a rule, the higher the amperage or wattage rating is, the greater the electric wire’s diameter should be (the lower the gauge number).

For example, you can use an 8-gauge electric wire for a 7.6-kW tankless water heater with a 40-amp breaker. On the other hand, an 11-kW appliance connected to a 60-amp breaker must use a 6-gauge wire.

Do I Need a Separate Circuit Breaker for My Tankless Water Heater?

Many people ask if they need to install a separate circuit breaker for their tankless water heaters. Some homeowners think that gas-powered tankless water heaters do not need electricity since these appliances run on propane or natural gas to heat the water.

However, gas-powered tankless water heaters have electronic ignition systems, electronic thermostats, and a PC board. These technologies still require electricity to operate. Otherwise, you cannot heat water, even with an adequate gas supply.

Similarly, electric tankless water heaters always require electricity to run. Moreover, these systems need a dedicated 240-volt circuit to operate safely and efficiently. Tankless water heaters heat the water on demand, requiring large amounts of electrical energy within a short period.

If the tankless water heater shares its electrical power with other household appliances connected to one circuit breaker, you increase the risk of electrical overload. It is essential to understand that on-demand water heaters can cause electrical surges or voltage fluctuations whenever they go on and off.

That is why power-hungry and critical-use appliances always require a separate or dedicated circuit breaker. After all, it is a crucial regulation under the National Electric Code.


Answering the question “What size breaker do I need for a tankless water heater?” is easy if you have the user manual.

If not, you can calculate the correct breaker size by dividing the tankless water heater’s wattage rating by its voltage and multiplying the result by 1.25.

On average, an electric tankless water heater will need a 30-amp breaker. On the other hand, a gas-powered tankless water heater will run on a 15-amp single-pole breaker.

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