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What Size Breaker for Furnace? (Read This First!)

Winter can be harsh in some regions, making a household furnace a life savior for people, especially for those living in cold regions. A household furnace works wonders and turns your gloomy shivering days into crispy warm and delightful. 

There are different types of furnaces, they work by turning cold air, gas, or oil into warm air serving your house and saving you from unwanted winter zephyr. 

However, each furnace has a limit to which extent they can raise the temperature. If heat rises above that temperature the furnace may blow off. Circuit breakers help control that.

So, here rises a question. What size breaker is suitable for your existing furnace? Keep reading to find out everything you need to know regarding the different types of furnaces and the suitable breakers for them. 

Furnace breaker size

Usually, 60 and 80 amp breakers are the standard sizes for an electric furnace. However, the size usually depends on the output voltage, power consumption, and the type of furnace. The total wattage is the product of amps and volts which will help you to determine the size of the breaker.

Just like you can get the wattage with the product of amp and volts, most people use the amp to determine the correct circuit breaker. You can find the amp by dividing the wattage by volts. 

There are three different types of furnaces. Although they have the same function, some of them are more suitable for certain conditions. 

The three types of furnaces run on gas, oil, and electricity. The electric furnace is most commonly used by households.  

A circuit breaker works by protecting your furnace from heating up more than it can condone. This is why they have different ratings according to the capacity of each furnace.


Electric furnace: 

The electric furnace uses electricity to heat the elements inside the chamber and then a fan is used to pass the hot air through the vents in your room. This works similarly to a space heater.

An electric heater used in most households in the US has a capacity of 17,600 watts and a volt of 220 V. By dividing wattage by the volt you will find a current of 80 amp. 

So, you will need a breaker of 80 amp which falls in the range of the standard size breakers used. However, if you are not sure about the rating research a bit. 

Gas furnace:

A gas furnace works similarly to an electric furnace but instead of electricity, it uses natural gas. Due to the unavailability of abundant natural gas, they are not as frequently used as an electric furnace. 

The natural gas is ignited inside the furnace and then the warm air is distributed inside the house. A gas furnace usually has a power of 600 watts so it requires a breaker of 15 amp.

Oil furnace:

An oil furnace turns the oil into a spray when the temperature rises in the combustion chamber. The spray is then turned into hot air that is spread through the vents inside the house, heating the room. 

An oil furnace has a power consumption of 2500 watts so you will need a breaker of 30 amp to be precise. 


15kw furnace:

A 15kw furnace has a power consumption of 15000 watts with a voltage of 208/230 V.

Hence it will require a breaker of 30/60 amp depending on the voltage rating. It has a minimum circuit capacity of 50/25 amp. 

20kw furnace:

A 20kw furnace has a power consumption of 20000 watts with a voltage of 208/230 V.

It will require a breaker of 60 amp to be precise. It has a minimum circuit capacity of 50 amp.

25kw furnace:

A 25kw furnace has a power consumption of 25000 watts with a voltage of 208/230 V. It will require a breaker of 100/120 amp depending on the voltage. 

Should my furnace be on a 15 or 20-amp breaker?

Generally, gas furnaces require a breaker of 15 amp due to the power consumption and voltage. However, if your furnace has a greater power consumption or voltage you can use a 20 amp breaker,

You can figure this out by finding the power and voltage of your furnace. You need to divide the power by voltage and you will know which breaker suits your device best. 

If it has a voltage of 40 needs you need a 15 amp breaker and if it has a voltage of 30 you need a 20 amp breaker.

As surprising as it is, your gas furnace does use electricity to some extent even though it is based on natural gas. However, it has a small power consumption of 600 watts compared to electric furnaces.  

How many amps does a furnace use?

An electric furnace uses 60/80 amps, a gas furnace uses 15/20 amps and an oil furnace uses 30 amps. It depends on the type, power consumption, and voltage of the furnace.

An electric furnace has the greatest power consumption so it uses the greatest current of 60/80 amp. The number of amps a furnace uses is directly proportional to the power and voltage of the furnace. 

An oil furnace has a power consumption of 2500 watts so it uses a current of 30 amp. The number of an amp a furnace possesses can be figured out by dividing power by voltage.

A gas furnace has the lowest power consumption as it burns natural gas for warm air production. So, it uses the least current of 15/20 amp.

How do determine a breaker size for a furnace?

Determining the correct breaker size for your furnace is essential as a furnace costs a lot. A minor miscalculation in determining the breaker size can result in a major issue. 

As we know, a breaker usually protects the furnace by breaking, when the power consumption might exceed that of your furnace’s capacity. 

There are a few steps that you can follow to determine the correct size of the breaker for your furnace. The steps are discussed in detail below to help you select the correct breaker for your furnace.

Determining the type of furnace:

The first step is very basic. You need to know whether the furnace is oil, gas, or electric. This will give you a good idea about the range of breakers that you will need for your furnace.

Reading the label:

The second step would be reading the label on your furnace to know about the power consumption of the furnace. 

Usually, the label can be found near the heater housing cord. It is the specification label that has the details

Finding the voltage:      

You need the operating voltage to find the current. If you divide the power by voltage then you can determine the current of the furnace. 

Usually, there’s a specific range of voltage as well. The voltage is usually either 208 or 230 volts. 

Finding the current: 

The next and most important step would be finding the current. You will need the current to determine the size of the breaker.

The current can be found by dividing the power by voltage. In the example given above if the voltage is 208, dividing 15000 by 208 will give you a current of 72.2 and if the voltage is 230 you will get a current of 65.2. 

Exceeding the value:

The next step would be multiplying the value of the current by 125 percent to find the correct breaker. 

If you find the 125 percent of 72.1 it will give you a value of 90 so you will select the closest existing breaker which is 100 amp. 

Which breaker is for the furnace?

Usually, a furnace has a breaker with a double-pole due to its extensive power consumption. Those breakers are usually between 60 and 80 amps. 

However, you need to verify your theory. Here is a list of ways that will help you to determine the solution to your question.

Circuit breaker finder:

There’s a circuit breaker finder available that you can use to trace the correct breaker for your furnace. 

If the furnace isn’t equipped with a dedicated outlet into which you can plug your circuit breaker finder, you’ll need to connect the alligator clips that are provided to exposed wires to trace the circuit.

Performing manually: 

You can manually change the breakers and find out which breaker is suitable to cut off the furnace when the power level exceeds. 

However, it is not recommended as switching the power on and off several times can break the furnace. 

Final thoughts 

To sum up, the correct size of the breaker can be found by dividing the power by voltage and multiplying the result by 125 percent. After that, the next closest breaker available can be selected. The standard breaker sizes are 15, 60, 80, & 100. You can select according to your furnace’s power load.