# Breaker Size Calculator

If you want to learn how to calculate breaker sizes, this **circuit breaker size calculator** is for you. Properly sizing your circuit breakers will not only help you avoid constantly resetting your tripped circuit breakers but can also help you protect yourselves from fire hazards.

Keep on reading to learn:

- The importance of circuit breaker sizing calculations;
- How to calculate breaker size; and
- How to use this circuit breaker calculator.

## Importance of circuit breaker sizing calculations

Sizing a breaker depends on how much load we anticipate it to handle. Basically, **a breaker cuts off the flow of electricity when it detects an unsafe amperage flowing through it**. With that said, we rate circuit breakers in amperes.

In a structure, we group our outlets and other electrical loads (e.g., lighting fixtures, ceiling fans, etc.) so that a circuit breaker only handles a section of the structure. A bedroom can have one circuit breaker assigned to its outlets, and the entire floor can share one circuit breaker for all its lighting fixtures.

For parts of the structure that demand high current flow, we can assign more than one circuit breaker for their outlets or use a larger size breaker.

As a rule of thumb, we size a breaker at **125% of the load** we anticipate it to handle. Typically, we see appliance power ratings in watts. With the help of **Watt's Law**, we can find the amperage requirement of each appliance that we anticipate to belong to a circuit. Let's discuss more of the calculation process in the next section of this text.

## How to calculate breaker size

Say, we need to find the advisable breaker size for a kitchen where we anticipate using a `120-W`

refrigerator and `900-W (max)`

microwave oven **simultaneously** at some point. Assuming these appliances function at `0.9 power factor`

and the house has a load distribution of `220-volt single-phase AC`

, let's find each appliance's amperage using the **Watt's Law** shown below:

Where:

- $I$ is the
**amperage requirement**of an appliance; - $W$ is the
**power consumption**or**wattage rating**of an appliance; - $V$ is the
**voltage**of the power source; and - $pf$ is the
**power factor**of the appliance.

To find the amperage of our refrigerator, we plug in our known values to our formula, as shown here:

On the other hand, we calculate the amperage for our microwave oven as follows:

After finding our appliances' current requirements, we add them up to obtain the total load ($I_\text{total}$) and multiply the sum by 125% (or 1.25) to get our minimum breaker size ($I_\text{min}$), as we can see below:

Our last step is to choose the closest larger breaker size from the list of standard sizes of circuit breakers. Here are the standard circuit breaker sizes:

Standard circuit breaker sizes | |||||
---|---|---|---|---|---|

15A | 35A | 60A | 100A | 175A | 300A |

20A | 40A | 70A | 110A | 200A | 350A |

25A | 45A | 80A | 125A | 225A | 400A |

30A | 50A | 90A | 150A | 250A |

✅ Since the closest larger breaker size to our calculated minimum amperage requirement is 15A, we will **use a 15A circuit breaker** for this circuit.

##### Other amperage formulas

Our sample calculation above works for any single-phase AC systems only. If you need to find an appliance's amperage for **three-phase AC** and **DC** systems, here are the formulas you need to use:

For three-phase AC:

For DC:

After finding each appliance's amperage, **find the total amperage** for your considered circuit, **multiply the sum by 125%**, and **select the closest larger circuit breaker size** from the list shown earlier.

If you need to calculate more than one circuit breaker size, you can use our breaker size calculator for convenience. Read on to learn how to use this breaker size calculator.

## How to use this circuit breaker calculator

Using our circuit breaker size calculator is very easy. Here are the steps to follow when using it:

- Pick the
**current type**or your power source. It could be`DC`

,`AC single-phase`

, or`AC three-phase`

. - Enter how much
**voltage**your power source supplies. - Select the
**appliance type**of the appliance you anticipate using in a particular circuit. Choose`Other`

if your appliance type is not on the list. We call your first appliance as Appliance A. - Enter the
**quantity**of Appliance A. - Input Appliance A's
**wattage**rating. - If you anticipate other appliances simultaneously using this circuit, then
**repeat steps 3 to 5**for them. You can enter**up to five devices**in this tool. - If you selected either
`AC single-phase`

or`AC three-phase`

for the**current type**, you can use our tool in`Advanced mode`

to enter or edit the**power factor**values for each of your appliances. Click on the`Advanced mode button`

below our tool to toggle to that mode.

At this point, our tool will already have a recommendation for your breaker size for this circuit.

## Disclaimer

This tool is for informational purposes only. Please consult a certified electrician before buying your circuit breakers.