The breaker sees a fault. The motor sees Monday morning. Inrush current is the brief overlap where those two stories disagree.
What creates inrush
| Load type | Mechanism | Typical multiplier |
|---|---|---|
| Induction motor | Flux must build; rotor at standstill looks like low impedance | 5–8× FLA |
| Transformer | Magnetizing inrush charging core | 8–12× for ms |
| SMPS / drives | Bulk capacitors charge | 10–50× for µs–ms |
| Incandescent | Cold filament resistance | 10–12× briefly |
Duration matters: motor inrush may be 50–300 ms; breaker magnetic zones allow a few cycles of overload if the curve matches.
Why thermal rating is not enough
A 20 A breaker does not mean “anything under 20 A forever.” It means:
- Thermal element — trips on sustained overload (~minutes)
- Magnetic element — trips on sudden high current (~milliseconds)
You can be under 20 A average and still trip if I_peak crosses the magnetic threshold for your curve class.
Using the calculator to avoid nuisance trips
- Enter nameplate running watts (or measured run amps × V).
- Enter line voltage at the equipment (120, 208, 230, 240).
- Enter inrush factor from manual or table (6× default for motors).
- Read peak amps and recommended breaker + curve.
- If installed breaker is smaller or Type B with high peak—upgrade curve (C/D) or add soft-start.
Do not “fix” trips by jumping two wire sizes on breaker alone without matching conductor.
Field symptoms vs. root cause
| Symptom | Often actually |
|---|---|
| Trips only on cold start | Magnetic inrush |
| Trips after 10+ minutes | Thermal overload or bad bearing |
| Random trip under load | Loose terminal, not inrush |
| GFCI/AFCI trip | Leakage or inverter noise—different diagnosis |
Inrush is physics, not personality. Measure run and peak, match the curve, and breakers stop arguing with your motors.