Circuit Breaker Selection Guide
A circuit breaker protects cables, equipment, and people from excessive current. Selection starts with the working load, but it should not stop there. A good calculation also checks phase type, voltage, power factor, efficiency, demand, continuous duty, spare capacity, ambient derating, grouping derating, and short circuit duty.
Why correct sizing matters
An undersized breaker trips during normal operation. This creates downtime and unsafe bypass habits. An oversized breaker can allow a cable to overheat before the breaker opens. Correct sizing keeps the load supplied while preserving protection. It also helps designers compare practical standard ratings before ordering panels or distribution gear.
Load current and design current
The calculator first estimates base current from entered power or known amperes. For three phase systems it uses line voltage and the square root of three. For single phase systems it uses voltage directly. Power factor and efficiency adjust the result for real equipment behavior. The demand factor then represents expected simultaneous use. Continuous duty applies a higher allowance when the load may run for three hours or more.
Derating and spare margin
Breakers and conductors may carry less current in hot panels, crowded raceways, or grouped enclosures. Derating factors reduce the usable rating. The calculator divides the design current by the combined derating factor, then selects the next standard breaker size. A spare margin can be added for planned growth, but it should never exceed conductor or equipment limits.
Interrupting capacity
A breaker must safely interrupt the available fault current at its installation point. This rating is often called AIC, SCCR, or breaking capacity. The calculator compares the available short circuit current with a selected safety margin and suggests the next common interrupting rating. Final values should match local rules and manufacturer data.
Practical use
Use this tool for planning, teaching, and quick review. Always verify the final design with applicable electrical codes, panel ratings, cable ampacity, coordination studies, and qualified engineering judgment before installation.
The result section lists base current, adjusted current, selected rating, usable derated capacity, loading percentage, pole guidance, trip curve advice, and interrupting capacity. The CSV and PDF buttons help store evidence for estimates, review notes, and maintenance files during early project and audit decisions.