Breaker Size Calculator

Created by Kenneth Alambra

Last updated: Aug 06, 2022

Table of contents:

Importance of circuit breaker sizing calculations
How to calculate breaker size
How to use this circuit breaker calculator
Disclaimer

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:

\small I = \frac{W}{V \times pf}

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:

\small \begin{align*} I_\text{ref} &= \frac{W_\text{ref}}{V \times pf_\text{ref}}\\\\ &= \frac{120\ \text{W}}{220\ \text{V} \times 0.9}\\\\ &= \frac{120\ \text{W}}{198\ \text{V}}\\\\ &= 0.606\ \text{A} \end{align*}

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

\small \begin{align*} I_\text{oven} &= \frac{W_\text{oven}}{V \times pf_\text{oven}}\\\\ &= \frac{900\ \text{W}}{220\ \text{V} \times 0.9}\\\\ &= \frac{900\ \text{W}}{198\ \text{V}}\\\\ &= 4.545\ \text{A} \end{align*}

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:

\small \begin{align*} I_\text{min} &= I_\text{total}\times 1.25\\ &= (0.606\ \text{A} + 4.545\ \text{A})\times 1.25\\ &= (5.1515\ \text{A})\times 1.25\\ &= 6.439\ \text{A} \end{align*}

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:

I = \frac{W}{V \times pf \times \sqrt{3}}

For DC:

I = \frac{W}{V}

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.

Kenneth Alambra

Current type

Voltage

Input up to 5 appliances:

Appliance A

Appliance type

Quantity

Wattage

Appliance B (leave empty if unused)