How Many Amps Does a Space Heater Draw?

The average space heater uses 1500 watts, which means it draws 12.5 amps when plugged into a 120-volt outlet. But this draw changes if the unit is plugged into a 240-volt outlet or if the heater uses more or fewer watts.

Below, we’ll look at how to calculate the amperage of a space heater using three different methods. We’ll also talk about how to use amperage draw to understand if your breaker has enough power to safely run your electric heater.

How Do You Calculate the Amps a Heater Draws?

Ohm's Law pie chart
Matt Rider, CC BY-SA 3.0, via Wikimedia Commons

Calculating the amperage of a space heater (or any other appliance, for that matter) requires knowledge of how many watts the unit uses and how many volts are supplied by the power source.

The average space heater uses 1500 watts, but this can vary by product. The most energy-efficient heaters use far less, while very powerful heaters use more.

The standard voltage of an outlet in an American home is 120 volts. Most homes also include at least one 240-volt outlet (usually for the dryer). Each breaker single pole in your breaker box feeds a run of 120-volt outlets, while 240-volt outlets require a double breaker pole.

To calculate the amperage of your space heater, you’ll need to know:

  • The wattage of the unit. This can be found printed on the bottom of the heater, on the nameplate, or in the user’s manual.
  • The voltage of the outlet being used. The shape of the plug will determine what kind of outlet you must use. Standard electrical plugs are compatible with 120-volt outlets.

Option 1: Manual Amps Calculation 

Once you know these two values, the calculation to get amps is very straightforward. This is calculated using a well known formula called Ohm’s Law

Simply divide the wattage by the voltage:

Amps = Watts / Volts

For example, a 1500-watt space heater plugged into a 120-volt outlet, will draw 12.5 amps.

A more energy-efficient space heater that only uses 1200 watts (plugged into a 120-volt outlet) will only draw 10 amps.

Units plugged into a 240-volt outlet will draw fewer amps compared to the same-sized unit plugged into a 120-volt outlet. For example, a 1500-watt space heater plugged into a 240-volt outlet will only draw 6.25 amps.

However, most 1500-watt (and lower) space heaters are designed to be used with 120-volt outlets. Heaters over 1500 watts are generally designed to be used with 240-volt outlets (or, more often, to be hardwired into double pole breakers).

Option 2: Space Heater Amps Calculator 

An easier way to calculate the amperage of a space heater is to use our handy Amperage Calculator.

Simply enter the wattage of your space heater in the box then choose the voltage. The calculator will automatically calculate the amps drawn by the unit.

This amperage calculator can also be used for other home appliances and electronic equipment.

Option 3: Heater Amp Draw Chart 

If you want to skip the math altogether, you can simply use the table below to figure out how many amps your heater will use.

Use the column on the left to find the wattage of your space heater. Then find the amperage drawn based on the voltage of the outlet it will be used with. The amperage in the middle column is for 120v outlets while the right column represents 240v outlets.

Space Heater Size120 volts240 volts
100 watts0.8 amps0.4 amps
200 watts1.7 amps0.8 amps
300 watts2.5 amps1.3 amps
400 watts3.3 amps1.7 amps
500 watts4.2 amps2.1 amps
600 watts5.0 amps2.5 amps
700 watts5.8 amps2.9 amps
800 watts6.7 amps3.3 amps
900 watts7.5 amps3.8 amps
1000 watts8.3 amps4.2 amps
1100 watts9.2 amps4.6 amps
1200 watts10.0 amps5.0 amps
1300 watts10.8 amps5.4 amps
1400 watts11.7 amps5.8 amps
1500 watts12.5 amps6.3 amps
2000 watts16.7 amps8.3 amps
3000 watts25.0 amps12.5 amps
4000 watts33.3 amps16.7 amps
5000 watts41.7 amps20.8 amps
6000 watts50.0 amps25.0 amps
7000 watts58.3 amps29.2 amps
8000 watts66.7 amps33.3 amps
9000 watts75.0 amps37.5 amps
10000 watts83.3 amps41.7 amps
11000 watts91.7 amps45.8 amps
12000 watts100.0 amps50.0 amps
13000 watts108.3 amps54.2 amps
14000 watts116.7 amps58.3 amps
15000 watts125.0 amps62.5 amps
16000 watts133.3 amps66.7 amps
17000 watts141.7 amps70.8 amps
18000 watts150.0 amps75.0 amps
19000 watts158.3 amps79.2 amps
20000 watts166.7 amps83.3 amps

We calculated each data point in the table below using the standard amperage equation (ohm’s law) featured in section 1. The amperage of each watt value was calculated first for 120-volt outlets, then for 240-volt outlets. As you can see, 240-volt outlets will use half as many amps as 120-volt outlets for each wattage size.

The numbers in bold represent the most likely scenarios in terms of the voltage a heater has been built to utilize. This is based on the fact that most heaters at or below 1500 watts are made to use 120-volt outlets. Anything over this size is typically hardwired or plugged into a 240-volt connection.

Amps Draw vs Amperage of Circuit Breaker

circuit breakers in a home

Knowing how many amps your heater draws is important to determine if you can use it without overwhelming your circuit breaker.

Each breaker circuit in your breaker box can only handle up to a certain amperage before overheating. The breaker system is designed to cut off power if this level is exceeded.

Most single-pole breaker circuits are rated for 15 or 20 amps. Some can be rated for up to 30 amps, but this is less likely in a residential home. Double pole breakers can be rated for 20 to up to 60 amps.

For safety reasons, you should never surpass 80% of the maximum amp rating for each circuit (according to NEC guidelines). You can calculate what 80% of your breaker rating is by taking the amperage and multiplying it by 0.8. Or you can reference the numbers below.

  • 80% of 15 amps = 12 amps
  • 80% of 20 amps = 16 amps
  • 80% of 30 amps = 24 amps
  • 80% of 60 amps = 48 amps

NOTE: Keep in mind that the maximum amperage for each circuit includes all electrical devices plugged into that circuit. If only the heater is plugged into the circuit, then you only need to consider that amperage draw. But if there are other outlets being used on the same circuit, you need to add them to the amps of the heater to assure the total power used is less than 80% of the breaker’s rating.

The table below expresses the minimum circuit amp rating required for different-sized space heaters. Keep in mind, these calculations assume the heater is the only item drawing power on the circuit.

Green = Minimum of 15 amp breaker

Blue = Minimum of 20 amp breaker

Purple = Minimum of 30 amp breaker

Red = Minimum of 60 amp breaker

Black = Requires specialty electrical

Space Heater Size120 volts240 volts
100 watts0.8 amps0.4 amps
200 watts1.7 amps0.8 amps
300 watts2.5 amps1.3 amps
400 watts3.3 amps1.7 amps
500 watts4.2 amps2.1 amps
600 watts5.0 amps2.5 amps
700 watts5.8 amps2.9 amps
800 watts6.7 amps3.3 amps
900 watts7.5 amps3.8 amps
1000 watts8.3 amps4.2 amps
1100 watts9.2 amps4.6 amps
1200 watts10.0 amps5.0 amps
1300 watts10.8 amps5.4 amps
1400 watts11.7 amps5.8 amps
1500 watts12.5 amps6.3 amps
2000 watts16.7 amps8.3 amps
3000 watts25.0 amps12.5 amps
4000 watts33.3 amps16.7 amps
5000 watts41.7 amps20.8 amps
6000 watts50.0 amps25.0 amps
7000 watts58.3 amps29.2 amps
8000 watts66.7 amps33.3 amps
9000 watts75.0 amps37.5 amps
10000 watts83.3 amps41.7 amps
11000 watts91.7 amps45.8 amps
12000 watts100.0 amps50.0 amps
13000 watts108.3 amps54.2 amps
14000 watts116.7 amps58.3 amps
15000 watts125.0 amps62.5 amps
16000 watts133.3 amps66.7 amps
17000 watts141.7 amps70.8 amps
18000 watts150.0 amps75.0 amps
19000 watts158.3 amps79.2 amps
20000 watts166.7 amps83.3 amps

As you can see, a 1500-watt space heater, the most common size, technically requires a 20-amp breaker. If the heater is the only item drawing power from a 15 amp breaker, it is possible to run it without tripping the circuit. However, this goes against NEC safety guidelines and is not recommended.

But keep in mind, the 1500-watt rating on these heaters represents the maximum amperage the heater will draw. This means that you can safely run these heaters on a 15 amp breaker so long as you only use the low setting.

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Author
Aaron Green
Aaron is the founder of and Essential Home and Garden. He likes to spend his spare time with his family, and doing DIY projects in the home and garden.

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