Prevent nuisance trips—calculate the true inrush current of your equipment and select the right breaker to keep your circuits running smoothly.
A compressor clicks, the lights dip, and the breaker opens—yet the nameplate says 15 A and you installed a 20 A breaker. The failure is not steady-state overload; it is inrush: a millisecond surge when magnetizing windings and charging capacitors. Breakers are dual creatures: thermal (seconds) and magnetic (instantaneous).
Why inrush management matters
Undersized protection causes:
- Nuisance trips on every start
- Technicians bumping breakers without checking wire ampacity
- Shop owners blaming “weak utility power” when the curve was wrong
Oversized protection without engineering causes fire risk—this tool targets right-sized breakers with the right curve, not “go bigger until it stops tripping.”
The planning math
I_run = P ÷ V
I_peak = I_run × inrush factor
Breaker handle must satisfy both:
- Continuous load — often 125% of I_run for motor branch rules (planning)
- Magnetic inrush — peak below instantaneous trip band for curve type
| Curve | Typical magnetic band (planning) |
|---|---|
| Type B | ~3–5× rated current |
| Type C | ~5–10× rated current |
| Type D | ~10–20× rated current |
Enter manufacturer inrush factor when available; otherwise use 5–7× for small motors, higher for heavy tools.
Wire vs. breaker
Size copper for I_run, not I_peak. Inrush is short—breakers tolerate it; wire insulation does not tolerate sustained overcurrent.
Pair with Residential Voltage Drop on long branch runs.
Smooth startups need coordinated curve + load type + sometimes soft-start. Model the spike first—the breaker stops being a mystery.