How to use the AC Inrush Current Limit Calculator
The Formula
Motors, transformers, and SMPS draw brief startup current many times higher than running amps. Thermal breaker curves allow short magnetic overload—if inrush exceeds the magnetic trip band, the breaker opens even though steady-state amps are fine.
Practical Example
I_run = P ÷ V. I_peak = I_run × inrush factor. Breaker ≥ max(1.25 × I_run, I_peak ÷ magnetic multiple for curve type). Type B ~5×, Type C ~7.5×, Type D ~12.5× rated current for instantaneous region (planning).
About AC Inrush Current Limit Calculator
Why inrush trips breakers
Motors, transformers, and SMPS draw brief startup current many times higher than running amps. Thermal breaker curves allow short magnetic overload—if inrush exceeds the magnetic trip band, the breaker opens even though steady-state amps are fine.
Formulas
I_run = P ÷ V. I_peak = I_run × inrush factor. Breaker ≥ max(1.25 × I_run, I_peak ÷ magnetic multiple for curve type). Type B ~5×, Type C ~7.5×, Type D ~12.5× rated current for instantaneous region (planning).
Curve selection
Resistive loads: Type B. Small motors and compressors: Type C. High inrush tools and large motors: Type D. Always verify local code, manufacturer data, and coordinated protection.
Frequently asked questions
Q: Duration of inrush? A: Often 50–200 ms—this tool sizes breaker class, not time-current curves. Q: Power factor? A: Use nameplate running watts/amps when available. Q: Soft-start? A: Lowers effective inrush factor—enter reduced multiplier.