Circuit Breaker Sizing Calculator
General Use
Motor Load
Continuous Load (>3 hours)
Your recommended circuit breaker size will appear here.
Understanding Circuit Breaker Sizing
Properly sizing a circuit breaker is crucial for electrical safety and system reliability. A circuit breaker's primary function is to protect electrical wiring from overcurrents, which can be caused by short circuits or overloads. If an overcurrent occurs, the breaker "trips" and interrupts the flow of electricity, preventing damage to the wiring, connected appliances, and reducing the risk of fire.
Key Concepts:
- Amperage (A): The measure of electrical current. Circuit breakers are rated in amperes, indicating the maximum current they can safely handle.
- Voltage (V): The electrical potential difference. While voltage doesn't directly determine breaker size, it's a factor in calculating total power (Watts) and is essential for selecting the correct breaker type.
- Continuous Load: An electrical load that is expected to operate for three hours or more at a time (e.g., lighting in a commercial space). Electrical codes often require circuits supplying continuous loads to be sized at 125% of the continuous load.
- Motor Load: Motors have unique starting characteristics where they draw significantly more current when initially starting up (inrush current). Sizing for motor circuits requires specific considerations beyond just the running current.
- Power Factor (PF): In AC circuits, the power factor represents the ratio of real power (used to do work) to apparent power (total power delivered). A lower power factor means more apparent power is needed for the same amount of real work, which can influence calculations for complex AC loads. For DC circuits, the power factor is always 1.
How This Calculator Works
This calculator assists in determining a suitable circuit breaker size based on the provided electrical load and circuit type. The general principles are guided by electrical codes (like the NEC in the US), which mandate certain safety margins.
Calculation Logic:
The calculator applies standard electrical code principles:
- General Use & Continuous Loads: For continuous loads or general use circuits, the breaker size is typically at least 125% of the continuous load (measured in Amps). This ensures the breaker doesn't trip under normal, sustained operation, while still providing protection.
Formula:Breaker Amps = Continuous Load (A) * 1.25 - Motor Loads: Sizing for motor circuits is more complex and often involves specific tables and calculations based on motor horsepower, full-load amps (FLA), and starting characteristics. This simplified calculator will provide a basic recommendation based on the provided load and a general factor, but for precise motor sizing, consulting the motor's nameplate data and relevant electrical codes is essential.
Simplified Formula Used Here:Breaker Amps = Motor Load (A) * 1.25 (for running load) + Starting Current Consideration (often requires higher rating)Note: For motor loads, this calculator uses a general 125% multiplier for the running load and suggests a common next standard breaker size. Actual sizing may require more detailed analysis. - Power Factor Adjustment: If a power factor is provided for AC circuits, it can be used to calculate the apparent power (VA). However, most residential and commercial circuits are sized based on current (Amps) directly, especially when the voltage is known. For simplicity and common practice in breaker sizing, this calculator focuses on Amperage input. If power calculation is needed:
Watts = Volts * Amps * Power Factor, andApparent Power (VA) = Volts * Amps. The breaker is primarily sized based on the *Amps* of the load.
After calculating the minimum required amperage, the calculator selects the next standard available circuit breaker size (e.g., 15A, 20A, 30A, etc.) to ensure code compliance and safety.
When to Use This Calculator:
- Planning new electrical circuits for appliances, lighting, or machinery.
- Upgrading existing circuits or panels.
- Ensuring compliance with electrical safety regulations.
- Understanding the load requirements for specific electrical devices.
Disclaimer: This calculator is intended for informational purposes only and should not replace professional advice from a qualified electrician. Always consult with a licensed electrician to ensure your electrical installations comply with local building codes and safety standards.