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What Size Breaker for Electric Tankless Water Heater?

Written by Edwin Jones / Fact checked by Andrew Wright

what size breaker for electric tankless water heater

Isn’t it quite troublesome and tiring when you’ve bought a breaker and an appliance, but they don’t match? Sometimes, it gets confusing and repetitive to search what size breaker for Electric Tankless Water Heater (ETWH) to use.

It is also challenging as you’ll need to buy a circuit breaker for each terminal/ heating element. So, to find out the best breaker size, you should know your water heater’s watts, volts, and amps.

Picking the Perfect Breaker Size


1. Refer to the manual

Now, to get the perfect breaker size for a water heater, the most reliable and accurate method is to check the manual that goes along with your electric tankless water heater. You can view what type of wiring it has, which is equivalent to the number of breakers needed for the device.

Is it a 3 x 50 (this means there are three heating elements protected by three 50-amp breakers) or a 4 x 40 (four 40-amp circuit breakers)? It always depends on the model you have. Remember that the more breakers and the higher amperage they have, the higher the electrical current.

Electric tankless heaters can draw amps ranging from 42-amp to 167-amp max. However, if you can’t find the manufacturer’s manual, you can always proceed to the mathematician’s method.

2. Use the Power formula

Use the Power equation, P= I x V, from which we have this formula to get the amp size specifically: I= P ÷ V (I=Amps, P= Watts, and V= Volts). The standard Volts for an ETWH is already 240 Volts, so you’ll only need to find the amount of power input or wattage of your device located inside the device’s specs sheet or in the water heater itself.

Once you’ve gotten all the needed variables for a tankless water heater breaker size, proceed to the calculation. Let me give you an example. Let’s say a heating element needs 7600 watts from a 240-Volt circuit. To get the amperage, divide 7,600W by 240V:

I = 7,600 Watts ÷ 240 Volts

I = 31.7 amps

The final answer concludes that in order for this heating element to work, 31.7 amps are required. However, using the exact amperage as the breaker size is a terrible idea. Do not use the same calculated amperage as your final size breaker for the tankless water heater. Otherwise, you’ll risk accidents because there’ll be no protection for a power surge.

The National Electric Code (NEC) states that a circuit breaker should only handle 80% or less of its total capacity. This safety method is necessary because 100% continuous loads generate heat dangerous enough to cause a fire at the wiring terminals of the circuit breaker.

We’ll use the example earlier, which is 31.7 amps. To find the breaker size: multiply 1.25 or 125 % by the rated amp of your device, which, in this case, is 31.7 amps.

CB Size = 125% x 31.7 = 39.62 Amps

Finally, get the closest breaker size based on the final answer (39.62 amps), so for a safe maximum continuous load on our sample heating element, the breaker you should buy is 40-amps.

Take note that this example only featured one circuit breaker for one heating element. 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.

Electric Tankless Water Heater: How It Works

In addition to the electric water heater breaker size, you may want to know how this machine works, its pros and cons, and overall power.

Now, if you have an old tank water heater, you may already have a grasp of a water heater’s mechanics. Well, tankless and tank water heaters are not that far from each other, but you’ll see a huge difference in the levels of electrical consumption.

While the traditional heaters contain the hot water inside as it boils up, your ETWH uses electrical elements to heat the water inside the pipelines. These elements use energy and are protected by multiple breakers (one breaker per element) rated 40-50 amps (the typical rating).

Different manufacturers install extra and premium features for every ETWH they make. However, every product shares the same concept. It is simple. The device senses the flow of water once a faucet opens, and that’s when the heating activates. Once the flowing stops, the heat elements halt their operations as well.

The size breaker for an electric water heater is always based on the power input and the voltage required for the heater to operate. Find out the heater’s wattage (ranging between 10 kW to 40 kW) from the manual. Moreover, an electric tankless heater draws its power from a 240 Volt circuit.

Many homeowners switch to this heater due to the convenience of a never-ending hot water supply. Other pros include instantaneous warm water, space saving, durability, and low-cost monthly fees for small residential homes.

On the other hand, there are cons like compulsory add-on equipment, high maintenance, and inconsistent water temperatures.

Is Your Main Breaker Panel Suitable for Your Water Heater


Due to the multiple circuit breakers serving each heating element, electric tankless water heater installation requirements include a spacious main breaker panel for safety. So, before you purchase a brand new heating device, ask yourself: “Do I need a panelboard upgrade?”

100-amp panels are enough for large appliances such as dishwashers and air-conditioners, considering our fast-paced technology today. The latest ETWH today cannot work under that minimal amperage. How and why, you ask?

Supposedly, you have three breakers rated 40-50 amps feeding three heating elements. In total, that’s already 120+ amps, so your 100A panel is totally useless when an ETWH comes in.

If you don’t know how to size your main breaker panel, you can calculate 125% of the heater’s continuous load first.


Let’s use our Power formula again. Note that electric tankless boilers can use 10,000 Watts up to 40,000 Watts. We’ll use 30,000 Watts for a realistic example of a residential ETWH. Therefore:

I= 30,000W ÷ 240V

I= 125 Amps

Continuous load = 125% x 125A = 156.25 Amps

If your heating device needs this amp size, the best panelboard is the largest, which is 200-amps. Note that the typical 200-amp breaker panel shouldn’t handle more than 160 amps all at once.

However, your trusted contractor knows best, so it’s better to consult him and let him check the overall power input of your ETWH and other electrical devices. You’ll get to know the perfect panel size afterward.


Responsibility and careful computations play a huge part in knowing what size breaker for electric tankless water heater one should use. Before installing this heating device, you should pay extra attention to the size of your fail-safe device or devices that will safeguard your house from electrical and fire accidents.

The best option is to call a professional electrician to determine the most suitable breaker amp size. However, if you plan to go DIY, use the simple mathematical method, which is the Power formula—find the amperage by dividing Wattage by Voltage.

Lastly, compute the continuous load by multiplying 125% by the final amperage.

So, please take a look at other guides to calculate breaker amps for:

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