Tankless Water Heater Wire Size Calculator: What Gauge Wire (AWG)?

The main electric requirements for electric tankless water heaters are wire size and amp breaker size. We have explained how to easily calculate the amp breaker size here, and now we are going to look at what size wire you need for a tankless water heater.

The same as for the amp breakers, you will get a Tankless Water Heater Wire Size Calculator as well as wire sizes for 3kW to 36kW 240V tankless water heaters (summarized in a neat chart).

There seems to be a lot of needless confusion about these wire sizes. You might see questions like these posed on the electricity or HVAC forums:

“Is 8 AWG wire enough for a tankless water heater?”

“Should I be using 8 AWG wire for the on-demand electric heater, or 10 AWG? My neighbor has 10 AWG has he says he has no problem.”

“How many 8 AWG wires do I need for a tankless heater?”

Well, it’s true that these tankless water heaters have big power requirements (up to 36 kilowatts of electricity), but you don’t need to guess what size wire you need. You take the guesswork out of the equation by adequately sizing a wire for an electric tankless water heater wire in one of these two ways (we are going to focus on the 2nd one):

• Check the electricity requirements technical sheet. Again, we can look for an example at this Stiebel Eltron tech sheet; you can see that the 24kW Trend unit requires two 8/2 AWG copper wires. That’s because 24kW on a 240V circuit draws 100 amps, and one 8 AWG wire can handle 50 amps (8/2 AWG just means you have 1 hot wire that you are using and the 2nd wire is ground wire). This brings the combined allowable ampacity up to 100A, which is exactly how much we need.
• Simply calculate the tankless water heater wire size. We know the power draw (kW) and voltage (usually 240V); based on this, it’s incredibly simple to calculate the amp draw. When we have the amp draw, we consult this copper wire ampacity chart, and just pick the wire gauge that can handle our amp draw.

Quick Example: Let’s say that we have the biggest residential 36kW 240V tankless water heater. Here is the equation for amp draw:

Amp Draw = 36,000W / 240V = 150A

Alright, we know that we need a wire (or several wires) that can handle at least 150A. Now we consult the copper wire ampacity chart (here is the screenshot):

As you can see, for 150A current, we would need three #8 copper wires. This is exactly the same answer that Stiebel Eltron gives in the tech sheet (3 × 8/2 AWG copper wires).

To make this calculation easier, we have constructed a calculator that determines the amp draw. Based on that, we can then figure out what size and how many wires we need for electric tankless water heaters. On top of that, we have calculated the wire sizes for 3kW to 36kW on-demand water heaters, and summarized the results in the chart further on. Let’s start with the calculator:

Wire Size Calculator For Tankless Water Heaters

Here is how this calculator works (you can play around with the numbers, of course):

Let’s say you have a 20kW electric tankless heater on a 240V circuit. What size wire do you need? Just slide the 1st wattage slider to ’24’, and the 2nd voltage slider to ‘240’, and you get the result:

20kW 240V tankless electric heater requires two 8/2 AWG copper wires. Basically, the amp draw is 83.33 amps, and two 8 AWG copper wires have a combined ampacity of 100 amps (more than 83.33A).

Here is the full chart of what size and how many copper wires you need for 240V electric tankless heaters):

Tankless Water Heater Wire Size Chart (240V):

As you can clearly see, we use 10 AWG and 8 AWG copper wires for wiring electric tankless water heaters. When the amp draw is more than what one 8 AWG wire can handle, we don’t actually go for 6 AWG or 4 AWG wires. Instead, we use multiple 8 AWG or 10 AWG copper wires.

From the chart, you can now easily get the info on what size wire you need for your on-demand heater. Example: If you have a 27kW 240V heater, you can see that you are dealing with a 112.50 amp draw. To facilitate that current, you have to use three 8 AWG copper wires.

We hope that, with the calculator and this chart, you now have all the wire size information you need for any electric tankless water heater. If you require our help, you can just give us some numbers in the comment section below, and we will help you out.

Thank you.