Motor Locked-Rotor Starting Current

At the instant of starting, an induction motor is a transformer with a shorted secondary. The rotor is not moving, no back-EMF opposes the supply, and the only thing limiting current is the winding impedance — so the motor pulls somewhere between five and eight times its full-load current until it accelerates. Manufacturers state this on the nameplate as a code letter, which stands for a range of locked-rotor kilovolt-amperes per horsepower. Look the letter's kVA/hp value up in your own code book, enter it here, and the arithmetic is straightforward: multiply by the horsepower to get locked-rotor kVA, then divide by √3 V to get amps. A 25 hp, 460 V motor at 6.3 kVA/hp draws about 198 A to start against roughly 30 A running.

What that inrush actually does is sag the voltage — momentarily, but sometimes visibly, and occasionally enough to drop out a contactor elsewhere on the panel or to stall the motor that caused it. The current is also strongly reactive at that instant, with a power factor down around 0.2, so it is nearly pure magnetizing current doing no work at all. This is the number that sizes soft starters, decides whether a wye-delta or autotransformer starter is worth the panel space, and, more than any running load, sizes a generator on a site with big motors.

Two cautions. The kVA/hp figure is a range per letter, and the code table gives a band, not a point — use the top of the band when the consequence of being wrong is a nuisance trip. And do not use this number for overload protection or conductor sizing: those follow the code's own tables and rules, which are built to let the inrush pass while still protecting the machine. Locked-rotor current tells you about the moment of starting; it says nothing about the hour that follows.