Breaker Size Calculator

Disclaimer: While we strive to ensure the accuracy of our calculator tools, we cannot be held responsible for any damages or financial losses resulting from their use.

Are you worried about selecting the right breaker size for your electrical circuit? Unsure if your current setup can handle the load safely? Choosing the correct breaker prevents overheating, electrical fires, and equipment damage.

With our Breaker Size Calculator, you can easily determine the ideal breaker size for your needs, whether it’s for DC, AC Single-Phase, or AC Three-Phase systems. Just enter your load, voltage, and power factor (if applicable), and let us handle the rest!

How to Select The Right Circuit Breaker Size?

Selecting the right breaker size is necessary for the safety and efficiency of your electrical system. The breaker size must be chosen based on the load current and must comply with electrical codes like the NEC 80% rule for continuous loads.

Below is a step-by-step guide with formulas and examples to help you select the correct breaker size.

Step 1: Determine the Load Current

The first step is to calculate the load current (in amperes) based on the type of circuit (DC, AC Single Phase, or AC Three Phase).

Formulas:

For DC Circuits:

Current (A) = Power (W) / Voltage (V)

For AC Single Phase Circuits:

Current (A) = Power (W) / Voltage (V)×Power Factor (PF)

For AC Three Phase Circuits:

Current (A) = Power (W) / √3×Voltage (V)×Power Factor (PF)

Where:

• √3 ​≈ 1.732 (constant for three-phase systems).
• Power Factor (PF): Typically ranges from 0.8 to 1 (use 0.8 if unknown).

Step 2: Apply the NEC 80% Rule

For continuous loads (loads that run for 3 hours or more), the breaker should only be loaded to 80% of its rated capacity. This means:

Breaker Size (A) = Load Current (A) / 0.8

Step 3: Select the Nearest Standard Breaker Size

Breakers come in standard sizes (e.g., 15A, 20A, 25A, 30A, etc.). After calculating the breaker size, round up to the nearest standard size.

Step 4: Verify Maximum Allowed Load

Ensure the load does not exceed 80% of the breaker’s rated capacity:

Maximum Allowed Load (A) = Breaker Size (A) × 0.8

Example 1: DC Circuit

• Load: 1200W
• Voltage: 12V

Calculate Load Current:

Current = 1200 / 12 ​= 100A

Apply NEC 80% Rule:

Breaker Size = 100 / 0.8 = 125 A

Select Standard Breaker Size: 125A (standard size).

Verify Maximum Allowed Load:

Maximum Allowed Load = 125 × 0.8 = 100 A

Example 2: AC Single-Phase Circuit

Load: 2400W

Voltage: 120V

Power Factor: 0.9

Calculate Load Current:

Current = 2400 / 120 × 0.9 = 22.22 A

Apply NEC 80% Rule:

Breaker Size = 22.22 / 0.8 = 27.78 A

Select Standard Breaker Size: 30A.

Verify Maximum Allowed Load:

Maximum Allowed Load = 30 × 0.8 = 24 A

Example 3: AC Three-Phase Circuit

• Load: 10,000W
• Voltage: 400V
• Power Factor: 0.85

Calculate Load Current:

Current = 10000 / 1.732×400×0.85 = 16.98 A

Apply NEC 80% Rule:

Breaker Size = 16.980.8 = 21.23 A

Select Standard Breaker Size: 25A

Verify Maximum Allowed Load:

Maximum Allowed Load = 25 × 0.8 = 20 A