Breaker Size Calculator
Calculate the correct circuit breaker size for your electrical load
Kilowatts (kW)
Volts (V) - typically 120V, 240V, or 480V
0.1 to 1.0 (1.0 for resistive loads, 0.8-0.9 for motors)
How to Use This Calculator
Enter Power Load
Enter the total power consumption of your electrical load in kilowatts (kW). This is the power rating of the device or circuit.
Enter Voltage
Enter the system voltage in volts (V). Common values are 120V for residential, 240V for residential/light commercial, and 480V for industrial.
Set Power Factor (Optional)
Enter the power factor (0-1). For resistive loads (heaters, lights), use 1.0. For motors, use 0.8-0.9. Default is 1.0.
Get Results
Click calculate to see the load current and recommended breaker size. The breaker size includes a 25% safety margin per NEC standards.
Formula
I = P / (V × PF)
Breaker Size = I × 1.25
Where:
- I = Current (Amperes)
- P = Power (Watts) = Power (kW) × 1000
- V = Voltage (Volts)
- PF = Power Factor (0 to 1)
Example 1:
Power = 2.5 kW, Voltage = 240 V, Power Factor = 1.0
I = (2.5 × 1000) / (240 × 1.0) = 2500 / 240 = 10.42 A
Breaker Size = 10.42 × 1.25 = 13.03 A (round up to 15 A)
Example 2:
Power = 5 kW, Voltage = 120 V, Power Factor = 0.9
I = (5 × 1000) / (120 × 0.9) = 5000 / 108 = 46.30 A
Breaker Size = 46.30 × 1.25 = 57.87 A (round up to 60 A)
About Breaker Size Calculator
The Breaker Size Calculator is a free online tool that helps you determine the correct circuit breaker size for electrical circuits. Proper breaker sizing is critical for electrical safety and code compliance. This calculator follows NEC (National Electrical Code) standards, which require breakers to be sized at 125% of continuous load current. This tool is essential for electricians, electrical engineers, and DIY enthusiasts working on electrical projects.
When to Use This Calculator
- Residential Wiring: Size breakers for new circuits, outlets, or appliances
- Commercial Electrical: Calculate breaker sizes for commercial equipment and machinery
- Motor Protection: Determine appropriate breaker size for motor circuits
- Load Analysis: Verify existing breaker sizes are adequate for connected loads
- Code Compliance: Ensure breaker sizing meets NEC or local electrical codes
Why Use Our Calculator?
- ✅ NEC Compliant: Follows National Electrical Code standards (125% continuous load rule)
- ✅ Safety First: Prevents overloading and potential fire hazards
- ✅ Accurate Calculations: Accounts for power factor and provides proper sizing
- ✅ Standard Sizes: Suggests next standard breaker size available
- ✅ Easy to Use: Simple interface requiring minimal inputs
- ✅ Free Tool: No registration or payment required
Common Applications
Home Electrical Projects: When adding new circuits for appliances like electric ovens (typically 2.5-5 kW at 240V), water heaters, or air conditioners, you need to size the breaker correctly to prevent tripping and ensure safety. A 2.5 kW load at 240V requires approximately a 15A breaker.
Workshop Equipment: Industrial equipment like table saws, welders, or compressors require proper breaker sizing. A 3 kW table saw at 240V needs a 20A breaker, while a 5 kW welder needs a 30A breaker.
Motor Circuits: Motors have lower power factors (0.8-0.9) and require higher current for the same power. This calculator accounts for power factor, ensuring motors are properly protected.
Code Inspections: Electrical inspectors verify that breaker sizes match the calculated requirements. Using this calculator helps ensure your installation passes inspection.
Tips for Accurate Results
- Always round up to the next standard breaker size (15A, 20A, 30A, 40A, 50A, etc.)
- Use power factor of 1.0 for resistive loads (heaters, incandescent lights)
- Use power factor of 0.8-0.9 for motors and inductive loads
- For continuous loads (on for 3+ hours), always use 125% sizing rule
- Consider voltage drop in long wire runs - may need larger breaker
- Consult local electrical codes - some jurisdictions may have different requirements
Frequently Asked Questions
Why is the breaker size 125% of the load current?
NEC requires breakers to be sized at 125% of continuous load current to prevent nuisance tripping and ensure the breaker can handle sustained loads. This safety margin accounts for temperature variations and prevents premature breaker failure.
What is power factor and when should I adjust it?
Power factor is the ratio of real power to apparent power. For resistive loads (heaters, lights), use 1.0. For motors and inductive loads, use 0.8-0.9. Lower power factor means higher current for the same power, requiring a larger breaker.
Can I use a smaller breaker than calculated?
No, you should never use a smaller breaker than calculated. This could cause the breaker to trip frequently or fail to protect the circuit properly, creating a fire hazard. Always round up to the next standard size.
What are standard breaker sizes?
Common standard breaker sizes are: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 125A, 150A, 200A, and higher. Always use the next standard size equal to or greater than your calculated value.
Does this work for three-phase systems?
This calculator uses single-phase formulas. For three-phase systems, use: I = P / (√3 × V × PF). You'll need to multiply the result by √3 (approximately 1.732) for three-phase calculations.