Locked-rotor current is typically 6–8× full-load current. A 7.5 kW 400 V motor runs at 14 A and starts at 85–113 A.
At the instant of switch-on the rotor is stationary, so the machine is electrically a transformer with a shorted secondary: very low impedance, enormous current, and a power factor around 0.2. As the rotor picks up speed the slip falls, the impedance rises and the current collapses to its running value. The thermal element of a breaker is happy with a two-second surge; the magnetic element, which trips instantly at 3–5× its rating on a type B and 5–10× on a type C, is not.
The spread is the whole problem, so the nominal figure is not a design value. That same 7.5 kW motor on a 20 A type C holds comfortably at 6× (85 A, below the 100 A window) and sits inside the trip window at 8× (113 A) — and a device inside its window is not required to hold or to operate, so two identical breakers can behave differently. Use the code letter on the plate, not the rule. And going up a breaker size to stop the nuisance tripping raises the fault-clearing threshold and leaves the motor under-protected: change the type, or fit a device made for motors.
Estimate with the rule, then check it against the calculator that models it properly.
Open Motor Starting & Inrush →Locked-rotor current is typically 6–8× full-load current. A 7.5 kW 400 V motor runs at 14 A and starts at 85–113 A. At the instant of switch-on the rotor is stationary, so the machine is electrically a transformer with a shorted secondary: very low impedance, enormous current, and a power factor around 0.2.