Tankless Water Heater Breaker Size Calculator: What Size Breaker?

Figuring out what size breaker you need for an electric tankless water heater is fairly easy. We will show you how to calculate the tankless water heater breaker size adequately.

To make things even easier, we have included Tankless Water Heater Breaker Size Calculator (just insert kW, and get breaker size) as well as a chart with breaker sizes for 3kW to 36kW electric tankless water heaters (corresponding to 1 × 15A breaker and 3 × 50A breakers, respectively).

Here’s why it’s a good idea to size the on-demand water heater breaker size correctly:

• You don’t want the breaker to flip out during a shower. Most people who undersize an electric tankless water heater breaker have a story that goes like this “Everything was fine, hot water, and then immediately the water turned freezing cold, and I had to jump out of the shower.” This is a classic case of an unwanted cold shower as a result of undersizing the on-demand water heater’s breaker.
• You want to follow the electrical code. The NEC code is there to protect your circuit from catching fire. Since the tankless water heater circuit is considered non-continuous (it doesn’t run at 100% all the time), you load the amp breaker to 100% of its capacity. If this were a continuous load, you would have to load the amp breaker only up to 80% of its max. capacity, in accordance with the National Electric Code (NEC) Article 210.20(A).

Alright, there are two ways how to size a tankless water heater breaker adequately:

1. Check the manual. All electric tankless water heater producers – Stiebel Eltron, Rheem, EcoSmart, Eemax, and so on – will specify the ‘Number And Minimum Recommended Size Of Circuit Breakers’ in their technical sheets. Here is an example of this specs sheet for Stiebel Eltron Trend and Plus models ranging from 12kW to 36kW.
2. Simply calculate it. If you know the power (kW) and voltage (usually 240V), you can calculate the required minimum on-demand electric tankless heater breaker size quite easily.

Example: Let’s say you have a 27kW on-demand heater on a 240V circuit. What size breaker do you need? Here is the formula for sizing a breaker:

Min. Breaker Size = 27,000W / 240V = 112.5 Amps

For the 27kW heater, you would thus require amp breakers that can handle at least 112.5 amps. The standard choice here is to use three 40A breakers for a combined breaker size of 120 amps (which is, obviously, more than the 112.5A minimum requirement).

To help you out with this calculation, we have designed a calculator that determines the minimum breaker size for tankless water heaters. You simply input kW and voltage (usually 240V), and you get the breaker amps. We have also calculated the breaker sizes for 3kW to 36kW water heaters, specified how many amp breakers and what size amp breakers you need for each of them, and summarized the results in the neat chart you can freely use:

Tankless Water Heater Breaker Size Calculator

Let’s look at 1 example to see how this calculator works:

What size breaker do you need for a 20kW 240V electric tankless water heater? Slide the 1st slider to ’20’ and the 2nd slider to ‘240’. You get the result: 20kW 240V on-demand water heater requires a min. 83.33A breaker size. In this case, you would use two 50A breakers, for a net amperage of 100 amps, or 30A breakers for net amperage of 90 amps.

This was just one example. Let’s have a look at what size amp breaker you need for different electric tankless water heater sizes:

3kW To 36kW On-Demand Water Heater Breaker Sizes (At 240V)

Now, you can check look at this chart and select the breaker size for your tankless water heater.

For example, if you check the Stiebel Eltron electric requirements sheet we cited above, you can see that the 24kW Trend unit requires 2 × 50A breakers. If you look at the 24kW tankless water heater in this chart, you can see that the min. breaker size is 100.00A and we come to the same conclusion: You would need two 50A breakers.

The same goes for the biggest residential electric tankless water heater – the 36kW unit. For this one, you would need three 50A breakers.

Hopefully, now you have a good idea of how to go about sizing an amp breaker for your tankless water heater. If you have any questions regarding these breaker sizes, you can give us some numbers in the comments below, and we will help you out. For electric water heater recommendations, you can check out this post.