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

Standard breaker sizes are 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 450A, 500A, 600A, 700A, 800A, 1000A, 1200A, 1600A, 2000A, 2500A, 3000A, 4000A, 5000A, and 6000A.

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

**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 a 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.**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.

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 Ã—

**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.

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

**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.**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.**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.

Thank you, this is very help full.

very much help full

Excelente information. Thank you.

The most amazing easy method. very Helpfull.

Thank you, Imran, we try to simplify it as much as possible. It’s nice to see a bit of recognition.

Helpful.

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?

how can I use 120 v circuit.

Hi Masum, alright, 1500W heater on 120V draws 1500W/120V = 12.5 amps. To apply the 80% rule, you have to multiply this current by 125% like this: 12.5A Ã— 1.25 = 15.63 amps. So, a 15A breaker won’t cut it, but a 20A breaker will be great for a 1500-watt space heater on a 120V circuit. Hope this helps.

Hi I have a 1200 watt generator , the breaker has gone bad and there is no size printed on the breaker. Im not sure if you can calculate the size the same way you would for a AC unit. What size would you suggest ?

Thanks

Hi Renald, the size of the breaker you need for a 1,200 watt generator really depends on the amps it will give out. You can calculate the amps if you know the voltage. Generators can have 12V DC, 24V DC, 110-120V AC, 240V AC voltages.

Example: Let’s say your 1200W generator has a 24V DC voltage. You can calculate the amps like this: 1200W/24V = 50 Amps. This is the current in the wire. You can calculate the minimum required breaker size for this generator like this: 50 Amp Ã— 1.25 = 62.5 Amps. In this case you can go for 70A breaker, or 3x30A breakers, since their capacity if north of 62.5 amps.

Hope this helps.

This article is flawless thank you!

Great info! Much appreciated but Iâ€™m trying to determine what breaker or breakers I need for my bass boat. 3x 12V in series for my 36V trolling motor and 1x 12 V for my electronics and starting battery which also powers the various pumps and aerators. Any help would be greatly appreciated!

Hi Tom, the wattage you need is the key here. Electronics will have lower wattage but pumps and aerators… that should like high wattage stuff. Example: If you would put in 60A amp breaker on 36V voltage, and apply the 80% breaker rule, you will get 1,728W of power. It’s really hard to advise here without wattages but from the looks of it – 36V and energy-demanding pumps – the first presumption would be that you need big breakers, at least 60A. Hope this helps.