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Calculate the right breaker size fast with our circuit breaker sizing calculator. Enter load values, apply the 125% rule, and get a quick breaker size estimate.
Choosing the right breaker size matters. A breaker helps protect a circuit from too much current. The right size can help keep your wiring safe.
Our circuit breaker sizing calculator makes this job easy. It uses your continuous and noncontinuous load. Then, it finds a suitable breaker size from the list in the tool.
You don't need to do the math by hand. Just enter your load values. The calculator does the rest.
The main circuit breaker sizing formula is:
Required Load = (Continuous Load × 1.25) + Noncontinuous Load
This is also the main formula for calculating breaker size in this calculator.
The rule is simple. A continuous load gets a 125% factor. A noncontinuous load gets a 100% factor.
For example, let's say your circuit has:
Continuous Load = 20 A
Noncontinuous Load = 10 A
First, multiply the continuous load by 1.25:
20 A × 1.25 = 25 A
Now add the noncontinuous load:
25 A + 10 A = 35 A
So, the calculated load is 35 A.
The calculator then checks its standard breaker list. It picks the first breaker size that meets or exceeds the calculated load.
Think of it like picking a shoe. You don't pick a shoe that's too small. You pick one that fits the need.
Using our circuit breaker size calculator is quick and easy.
1. Enter the continuous load.
Enter the continuous load current in amps. If you have no continuous load, you can leave this field blank.
2. Enter the noncontinuous load.
Enter the noncontinuous load current in amps. You can leave it blank if you have none.
3. Check your values.
Make sure your values are zero or higher. Don't enter negative numbers.
4. Click the calculate button.
The tool applies the 125% rule to the continuous load. It then adds the noncontinuous load.
5. Check your result.
The calculator shows the required load. It also suggests a breaker size from its standard size list.
This gives you a fast circuit breaker sizing calculation without doing the math by hand.
Let's use a simple example.
Continuous Load = 16 A
Noncontinuous Load = 8 A
Use the circuit breaker size calculation formula:
Required Load = (16 × 1.25) + 8
Required Load = 20 + 8
Required Load = 28 A
Now, the calculator checks its breaker size list.
The next listed size above 28 A is:
Suggested Breaker Size = 30 A
So, the result is:
Calculated Load: 28 A
Suggested Breaker: 30 A
The same method works when you have only one load type.
For a 24 A continuous load:
24 × 1.25 = 30 A
So, the calculated load is 30 A.
For a 42 A noncontinuous load:
42 A = 42 A
The tool then looks for the next available breaker size in its list.
Breaker sizing and cable size go hand in hand. You should not choose a breaker by load alone.
The wire must have enough ampacity for the circuit. You also need to check the voltage, wire type, install method, temperature, and local electrical rules.
This calculator focuses on the load-based breaker size calculation. It does not replace a full electrical design check.
A main breaker sizing calculator may also need more data. The same is true for a motor circuit breaker sizing calculator, DC circuit breaker sizing calculator, or transformer circuit breaker sizing calculator.
These systems can have extra rules. Motor and generator circuits may need special checks. DC circuits also differ from AC circuits.
So, use the result as a starting point. Then check the full circuit design before installation.
A typical breaker list may include:
15 A, 20 A, 25 A, 30 A, 35 A, 40 A, 45 A, 50 A, 60 A, 70 A, 80 A, 90 A, 100 A, 125 A, 150 A, 175 A, 200 A, and higher sizes.
Your local code and equipment may limit which sizes you can use. Always check the rules that apply to your project.
Our circuit breaker sizing calculator takes the guesswork out of a basic breaker load calculation.
The key formula is:
Required Load = (Continuous Load × 1.25) + Noncontinuous Load
Enter your loads. Run the calculation. Then check the suggested breaker size.
For a real installation, don't stop there. Check the cable, equipment, and local electrical code too. When in doubt, ask a qualified electrician.
Use this formula:
Breaker Load = (Continuous Load × 1.25) + Noncontinuous Load
Then choose a suitable breaker that meets the calculated load, subject to the rules that apply to the circuit.
The 125% rule means you multiply the continuous load by 1.25. You then add the full noncontinuous load.
Not by itself. Motor circuits can have special sizing rules. A motor circuit needs more checks than this basic load formula.
Yes. The breaker and cable must work together. The cable must have enough ampacity for the circuit and breaker.
Common sizes include 15 A, 20 A, 25 A, 30 A, 40 A, 50 A, 60 A, and 100 A. Larger systems use larger ratings.
For this calculator, the formula is:
Required Load = (Continuous Load × 1.25) + Noncontinuous Load
The final breaker choice still needs a full code and circuit check.
No. Generator circuits may need extra checks. Generator output, voltage, current, and system design can affect the final breaker choice.