Sizing A Circuit Breaker: Breaker Size Calculator + Amp Chart

Sizing a circuit breaker is never easy. But it’s also not all that difficult. Everybody knows that we need an adequately sized circuit breaker that allows for sufficient electric current. If we undersize a breaker, the breaker will likely catch on flame. No pressure here.

how to size a circuit breaker
Inadequately sizing a circuit breaker can lead to a fire.

How do you go about picking the correct circuit breaker size? Do you need a 10A, 15A, 20A, 30A, 40A, 50A breaker, etc?

Sizing a circuit breaker is actually quite easy. You just need to know a couple of rules. These are:

  1. 80% NEC breaker rule. This is the most basic NEC (National Electric Code) rule that states that you can’t push the current over 80% of its specified ampacity. Example: If you have a 20 amp breaker, you can only allow for 16A current. 16A is 80% of the max. the specified ampacity of the circuit breaker. This is a safety measure; you better have a bit of overhead to prevent the circuit from frying. You can read the full Article 240.4(B) in NEC 2014 on this here.
  2. Knowing how to calculate amps from wattage. You usually have devices with known wattage; let’s say a 3,000-watt air conditioner. Sizing circuit breakers is all about amps. You will have to know how to convert 3,000W to amps. Namely, if you run a 3,000W unit on a 220V circuit, the current is calculated like this: 3,000W / 220V = 13.63 amps.
    measuring what size circuit breaker you need
    You can calculate the amp draw from wattage and voltage.

If you know how to calculate the amps and account for the 80% breaker rule, you can calculate the size of the breaker yourself.

To help everybody sizing these breakers out, we will explain how to determine the right size of a breaker. On top of that, we include a Circuit Breaker Size Calculator further on (just insert watts and volts, and you get the correct breaker size).

At the end, we also included the ‘just tell me the breaker size I need’ Breaker Size Chart that tells you what breaker size you need for devices with different wattages (from 50W units to big 20,000W devices).

Let’s look at how breaker size can be calculated manually (you can also use the calculator or/and chart below):

How To Calculate Size Of A Circuit Breaker?

This is the easiest to explain with an example.

Let’s say that we have a simple 1,500-watt space heater running on a standard 120V circuit. What size amp breaker do you need for a 1,500-watt space heater?

First, you need to calculate how many amps does this heater draw like this:

Current (Amps) = Power (Watts) / Voltage (Volt)

In our situation this is:

Current = 1,500W / 120V = 12.5 Amps

Now we know that the 1,500W space heater draws 12.5 amps. We have to account for the 80% breaker rule. This means that these 12.5 amps should represent 80% of the breaker amps. To calculate the size of the circuit breaker needed, we have to multiply the amp draw by 1.25 factor like this:

Minimum Circuit Breaker Size = 12.5A × 1.25 = 15.63 Amps

We can’t use a 15A breaker because the breaker ampacity should be at least 15.63A. The next breaker size is 20 amps; that means we need to use a 20A breaker for a 1,500W space heater running on 120V standard circuit.

sizing a circuit breaker size in amps
A standard 1,500W space heater on a 120V circuit requires a 20A breaker.

Here is the basic step-by-step procedure we did to determine the size of a circuit breaker:

  1. Calculate the amp draw. We use the basic electric power equation for this. If we know the wattage and voltage, we can quite easily calculate the amp draw.
  2. Multiply amp draw by 1.25 to account for the 80% breaker rule. The resulting amps are the minimum ampacity a correctly sized circuit breaker should have.
  3. Choose a circuit breaker size. We usually pick between 10A, 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A circuit breakers, and so on.

This is how breaker sizing is done manually. The easiest way is to use a dynamic calculator. You simply input that wattage and the voltage, and the calculator will tell you what is the minimum size of a circuit breaker you need. You can use this calculator here:

Circuit Breaker Size Calculator

Here is how this breaker sizing calculator works:

Let’s say you have a big 5,000W air conditioner (this is usually a 5-ton unit). It runs on a 220V circuit. What size circuit breaker do you need?

Just slide the wattage slider to ‘5000’ and voltage slides to ‘220’ and you get ‘28.41 Amps’. Therefore you need a circuit breaker with at least 28.41A ampacity. 25A breaker is too small; you need a 30A breaker.

You can do this for literally any device running on any voltage. You can also play around with numbers to see how the amps change.

If you want the ‘just tell me the circuit breaker I need’ you can consult this chart:

Breaker Size Chart (For 120V And 220V Circuits)

You just need to know the wattage of the device you need a circuit breaker for and you can check what size breaker you need if you run it on a standard 120V circuit or an upgraded 220V circuit:

Power (Watts): Min. Breaker Amps (At 120V): Breaker Size (At 120V): Min. Breaker Amps (At 220V): Breaker Size (At 220V):
50 Watts 0.52 Amps 5A Breaker 0.28 Amps 5A Breaker
100 Watts 1.04 Amps 5A Breaker 0.57 Amps 5A Breaker
200 Watts 2.08 Amps 5A Breaker 1.14 Amps 5A Breaker
300 Watts 3.13 Amps 5A Breaker 1.70 Amps 5A Breaker
400 Watts 4.17 Amps 5A Breaker 2.27 Amps 5A Breaker
500 Watts 5.21 Amps 10A Breaker 2.84 Amps 5A Breaker
600 Watts 6.25 Amps 10A Breaker 3.41 Amps 5A Breaker
700 Watts 7.29 Amps 10A Breaker 3.98 Amps 5A Breaker
800 Watts 8.33 Amps 10A Breaker 4.55 Amps 5A Breaker
900 Watts 9.38 Amps 10A Breaker 5.11 Amps 10A Breaker
1,000 Watts 10.42 Amps 15A Breaker 5.68 Amps 10A Breaker
1,500 Watts 15.63 Amps 20A Breaker 8.52 Amps 10A Breaker
2,000 Watts 20.83 Amps 25A Breaker 11.36 Amps 15A Breaker
2,500 Watts 26.04 Amps 30A Breaker 14.20 Amps 15A Breaker
3,000 Watts 31.25 Amps 35A Breaker 17.05 Amps 20A Breaker
3,500 Watts 36.46 Amps 40A Breaker 19.89 Amps 20A Breaker
4,000 Watts 41.67 Amps 45A Breaker 22.73 Amps 25A Breaker
4,500 Watts 46.88 Amps 50A Breaker 25.57 Amps 30A Breaker
5,000 Watts 52.08 Amps 55A Breaker 28.41 Amps 30A Breaker
6,000 Watts 62.50 Amps 65A Breaker 34.09 Amps 35A Breaker
7,000 Watts 72.92 Amps 75A Breaker 39.77 Amps 40A Breaker
8,000 Watts 83.33 Amps 85A Breaker 45.45 Amps 50A Breaker
9,000 Watts 93.75 Amps 95A Breaker 51.14 Amps 55A Breaker
10,000 Watts 104.16 Amps 110A Breaker 56.82 Amps 60A Breaker
15 kW 156.24 Amps 160A Breaker 85.23 Amps 90A Breaker
20 kW 208.32 Amps 210A Breaker 113.64 Amps 120A Breaker
25 kW 260.4 Amps 270A Breaker 142.05 Amps 150A Breaker
30 kW 312.48 Amps 320A Breaker 170.46 Amps 180A Breaker
40 kW 416.64 Amps 420A Breaker 227.28 Amps 230A Breaker
50 kW 520.8 Amps 530A Breaker 284.10 Amps 290A Breaker

As you can see, calculating what size breaker you need is not all that hard. Of course, with bigger amp draws, you can connect several 30A or 50A in parallel to increase the total breaker ampacity.

We hope this illustrates how everybody can figure out the size of circuit breaker they need. If you have any questions regarding this breaker sizing, you can use the comments below and we’ll try to help you out.

Thank you.

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