Motor locked-rotor current calculator

Preliminary estimate

Inputs

Calculation note

This is starting-current planning only. Motor nameplate data, starter type, VFD or soft-start behavior, and manufacturer locked-rotor data can override the estimate.

Method

Starting kVA = horsepower x kVA per horsepower

Three-phase locked-rotor amps = starting kVA x 1000 / (sqrt(3) x volts)

Single-phase locked-rotor amps = starting kVA x 1000 / volts

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Formula variables

  • Horsepower is the motor output horsepower.
  • kVA per horsepower is the entered planning factor from nameplate, code-letter, or manufacturer data.
  • Voltage and phase define the current conversion.
  • Locked-rotor current is a starting-current estimate, not full-load current.

Assumptions

  • The kVA-per-horsepower factor is user-entered so the calculator can follow a motor code-letter or manufacturer value.
  • The estimate is starting-current planning only, not a conductor, starter, overload, or breaker selection.
  • Voltage, motor design, starter type, VFD use, and manufacturer data can change actual starting current.
  • Use motor nameplate and manufacturer locked-rotor data when available.

Examples

10 hp at 600 V. A 10 hp three-phase motor with a 5.3 kVA/hp factor gives 53 kVA starting demand and about 51 A locked-rotor current at 600 V.

Generator workflow. Use the starting kVA result when reviewing whether a generator or upstream source can tolerate motor starting.