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How Many Amps Does a Tankless Water Heater Use?

Written by Edwin Jones / Fact checked by Andrew Wright


Setting up a tankless water heater can be nerve-wracking. One wrong move and the appliance may not function properly. Now, you’re wondering, how many amps does a tankless water heater use?

Generally, tankless water heaters draw about 42 to 167 amps. However, the specific amperage can depend on different factors, such as voltage and wattage ratings.


Here, you’ll learn how to determine the right amperage for a tankless water heater. This post also discusses the factors influencing amp usage, so keep reading!

Amperage Rating For Tankless Water Heater

The average tankless water heater amperage is about 100 amps. But, as mentioned earlier, the definitive ampacity can depend on the unit’s voltage and wattage ratings.

Next, use the following power equation to determine the amp draw of a tankless water heater:

\[ I = \frac{P}{V} \]

In that formula, “I” is the current in amps, “P” is the wattage rating, and “V” is the voltage. Note that a tankless water heater typically has a power requirement range of 10,000 to 40,000 watts (10 to 40 kilowatts).

So, assuming your tankless hot water heater uses 40,000 watts at 240 volts, the appliance’s amperage is 166.67 amps or 167 amps when rounded to the closest whole number.

Also, here’s a quick rundown of the typical amp draws of tankless water heaters based on their wattage ratings as they’re running on a 240-volt circuit:

Water Heater Size (in Watts) Current (in Amps)
10,000 watts 42 amps
15,000 watts 63 amps
20,000 watts 83 amps
25,000 watts 104 amps
30,000 watts 125 amps
35,000 watts 146 amps
40,000 watts 167 amps

Don’t forget to follow NEC requirements when setting up your tankless water heater. For example, NEC 220.30 recommends a 28,000-watt tankless water heater to have a service of 200 amps.

Factors Influencing Amperage Usage

Other factors can affect a tankless water heater’s amperage, besides the voltage and wattage ratings. These elements include the following:

1. Type


Tankless water heaters typically fall under two types: gas and electric. Each type has different wiring requirements. So, each unit has different amperage ratings.

A gas tankless water heater, in particular, uses less amps than its electric counterpart. Generally, this type of water heater draws approximately 12 amps at 115 volts.

On the other hand, an electric tankless water heater uses about 100 amps on average, as mentioned previously. Note that this model often requires a 120-amp circuit breaker to run properly and safely.

2. Capacity


A tankless water heater’s size can also affect its amperage draw. Large tankless heaters, such as a 40-gallon water heater, generally have high flow rates. That means these units have high power consumption, leading to increases in electricity usage.

3. Energy Efficiency Rating


Tankless water heaters with high energy efficiency ratings can also increase amperage draws. However, the increase can also lead to long-term savings, thanks to the appliance’s overall operational efficiency.


In summary, understanding how many amps does a tankless water heater use is essential for a successful setup. Remember, tankless water heaters usually draw about 100 amps.

Using the correct math formula, you can manually calculate a heater’s amp rating. Alternatively, follow the guidelines imposed by the NEC or consider factors, like the water heater’s type, capacity, and energy efficiency, before finalizing your decision.

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