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What Size Breaker for a Hot Water Heater: A Breaker Size Solution

Writen by Edwin Jones

Fact checked by Andrew Wright

what size breaker for a hot water heater

Does your electrical water heater keep tripping the breaker? Did your heater overload without notice? With just these few questions, it is clear that you’re having a problem matching your appliance with its safety device.

What size breaker for a hot water heater, you say? The standard size for most heaters is 30-amps. However, this breaker ampacity doesn’t necessarily work for every water heater there is.

There is a formula for finding the correct breaker size, but there are other essential factors to talk about later in this article.

Understanding Your Water Heater

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Upon purchasing an electric water heater, one of the most common mistakes of a homeowner is not checking its amperage. Some heater tanks only have a label for their voltage and watt requirements but never their ampacity.

It is best if you look for your tank specifications from the retailer or the brand manufacturer itself. Your heater must be your basis for determining the correct breaker size. To ensure safety, here are the three factors you should consider:

Voltage- The volts of your boiler plays an integral part in a healthy circuit. You can plug an average heater to a 220 or 240 Volts outlet based on your tank size and wattage.

Wattage- Take your heater’s wattage requirement into your list. Knowing its wattage is a great advantage if your device usually pulls an immense amount of power for its tank size and other features. It will help you search for a suitable breaker to handle it.

Wire Size- Lastly, it is essential to know what cable type is compatible with your hot water heater breaker size. An incompatible size or reused faulty wires can cause more risk than welfare for your heater and breaker.

A Breaker Size Solution: What’s the Formula?

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If you’re clueless about the amp size you’ll use, doing a simple calculation is the answer. Since you already know the wattage and voltage of your water heater, all you need is to follow the Ampere formula, which is I (amps) = P (watts)/ V (volts).

For example, I have a heater tank of 3,000 watts that needs 120-volt power. To get its amperage, you need to divide your watts by volts.

Solution: I = P/V

I = 3000 watts / 120 volts

I = 25 amps

Now you may be thinking, a device with 25 amps needs a breaker size with the same value. Well, that’s where overloading and over-tripping start to happen.

There’s a regulation set by the National Electric Code (NEC) wherein you need to stay at 125% of the maximum continuous load. This step is vital to get the safe amount for your electric water heater breaker size.

For the example earlier, apply the continuous load rule: 125% x 25 amps = 31.25 amps. Then, you can get the closest amp size which is 35 amps. For a 35-amp CB, the Square D by Schneider Electric HOM235CP is a highly suggested product.

If you have a 20-amps breaker dedicated to your 25A water heater, expect that your heater will constantly trip it. Giving space for your breaker will allow a power surge without tripping.

To better understand, you can watch this video from House Wiring T.V.: correct way to size your water heater wires and Breaker size!!!! – YouTube

What Size Breaker for 40 and 80-Gallon Water Heater?

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Most families use a tank size of 40 to 80 gallons for a water heater — depending on how much they rely on warm water daily.

To find out how many amp breaker for water heater of these tank sizes, you’ll need their wattage and voltage, then apply the solution earlier.

For a 40-gallon heater, the average capacity is 4500 Watts with 240 Volts. Again, I = P/V; 4500W/240V = 18.75 amps. Then considering the continuous load (125% x 18.75A), we’ll get a total of 23.4 amps.

With our answer, a 40-gallon water heater needs at least a 25-amp breaker, the closest to the amperage value. However, I highly recommend using 30-amps for a safer breaker size.

Meanwhile, an 80-gallon heater powered by 5500 watts and 240 volts, will result in 5500W/240V x (1.25 continual load) = 28.64 amps. Meaning, a 30-amp dedicated circuit breaker is enough.

Pro-tip: Although a 30A breaker sounds like it’s not enough for an 80G tank, installing a 40A won’t protect the device in case of an overload. A breaker amperage that’s much higher than necessary won’t trip when needed.

What’s the Correct Wire Size for a Water Heater?

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The connection of a water heater to its breaker won’t work well if the wiring is incompatible. Make sure that the amperage of your heater matches its wire size. You can refer to this list:

  • 20-amps = 12-gauge wire
  • 25-amps = 10-gauge wire
  • 30/40-amps = 8-gauge wire
  • Other large heaters up to 50-amps = 6-gauge wire

Also, consider your breaker size for water heater. 30-amp circuit breakers, commonly used for a large boiler tank, require a 10-2 non-metallic or MC cable. This type of wire is suitable for start loads of 40-amps and a constant load of 35-amps.

Preventive Measures Before Buying a Circuit Breaker

When choosing the right water heater circuit breaker size, you should be delicate about the amp difference between the breaker and the heater. You must get the right size. A breaker too large or too little can risk an electrical hazard.

If the breaker is too small for the water heater’s ampere requirement, expect that your CB will trip continuously. Not only that, too much on and off of the heater can cause internal damage. Do not have your water heater share a circuit with other appliances, and please upgrade the breaker if necessary.

A breaker with a higher amperage than intended doesn’t immediately mean that it’s safer for the heater. If the tank suddenly has a short circuit or overload, your breaker will disregard the event — no protection at all.

Conclusion

Knowing what size breaker for a hot water heater is not that exhausting if you gather the correct information. Just by understanding the three wire heater elements — voltage, wattage, and wire size, you can get the proper breaker size.

A simple calculation will also do the work, but keep the breaker size near the amount of your answer (the actual amp size of the heater).

An electrical water heater typically uses a 30-amp breaker, so something like the Q230 30-Amp Double Pole Type QP Circuit Breaker will do the work well.

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