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What Size Breaker Do I Need for a Furnace – Detailed Answer

Written by Edwin Jones / Fact checked by Andrew Wright

what size breaker for electric furnace

Are you planning to install a furnace in your house? Do you know the basic furnace installation requirements, like What Size Breaker Do I Need for a Furnace?

Like any other electrical appliance or unit in your house, it is good to consider the basics of furnace installation. To know your furnace’s ideal circuit breaker size, consider first the electricity loads needed by knowing the amp or wattage of your unit.

Check out this article to know what furnace breaker size is needed for your installation.

What Size Breaker for Electric Furnace


Typically, an electric furnace uses 80 to 100-amp circuit breakers. However, the size breaker for furnace may vary depending on the output voltage rating and its rated power consumption. You also need to consider the 80 percent rule of thumb in your circuit breaker, since it will be used continuously.

To get the needed amp capacity of your furnace, divide its watt rating with your current power supply voltage. Typically, most US households would have a 220-volt supply just for their large appliances. However, do not assume this. If you’re not sure of the voltage your furnace uses, check.

For example, if you have a 17,600-watt furnace, divide it by 220 volts to get its current requirement. That gives you a value of 80 amps. Considering the 80%-rule for continuous loads, divide the amp value you get by 0.8 to get 100 amps. Additionally, the wire size for this 100-amp breaker is a 2 AWG non-metal cable, as per most state electricity codes.

This formula of watt capacity divided by voltage applies to all kinds of electrical appliances, whether it’s a large furnace or a small lamp. Furthermore, the wire gauge requirement will also vary, depending on the breaker you install.

For a better explanation, you can watch this video by CEU Solution, where he explains what breaker and wire sizing to use for a furnace.

Also, note that some professional electricians use two circuit breakers for large furnaces that demand a higher electrical load. If you’re not sure about the maximum load of your furnace, it’s better to ask a licensed electrical engineer about this.

What Size Breaker for Gas Furnace

If you’re running a gas furnace, you might be surprised to know that it still uses electricity. However, its load requirement is typically 600 watts or less. The electrical load is used to ignite the flame and power the fans that circulate the warm air inside your house.

For this reason, you may use a 15-amp breaker and a 14 gauge wire size. However, if your gas furnace needs much higher electricity, you can use a 20 amp circuit breaker with a 12 gauge wire size for a more efficient supply.

Pro Tip: A gas furnace can also be powered by most portable generators. So even if your home loses electrical service, you can hook it up to a generator as an emergency electrical system, ensuring you’re still warm despite the loss of electricity.


Knowing how many amps your furnace requires will make it easier for you to replace your breaker. Whether you’re choosing an electrical or gas heater, you would know the proper breaker to install for it.

So, does this article answer your question of what size breaker do I need for a furnace? Can you share which furnace you want to install in your house and what circuit breaker size you use? Write it in the comments section below!

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