Motor starting voltage dip calculator

Preliminary estimate

Inputs

Calculation note

This is a source-strength ratio screen. It does not model acceleration time, starter type, generator transient response, or utility flicker review.

Method

Voltage dip percent = motor starting kVA / available fault kVA x 100

Minimum voltage = nominal voltage x (1 - dip percent / 100)

Remaining voltage percent = 100 - voltage dip percent

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Formula variables

  • Nominal voltage is the system voltage before starting.
  • Available fault kVA is the entered source strength at the motor bus.
  • Motor starting kVA is the apparent power during starting.
  • Minimum voltage is a preliminary estimate during the starting event.

Assumptions

  • Available fault kVA is entered by the user from a study, utility data, or project estimate.
  • Motor starting kVA is entered from nameplate, code-letter, manufacturer, or project data.
  • The result is a simple ratio screen and does not model dynamic motor acceleration.
  • This calculator does not replace a motor-starting study, generator transient review, utility review, or manufacturer analysis.

Examples

600 V motor start. A 500 kVA starting load on a 10,000 kVA source estimates about 5 percent dip and 570 V minimum.

Generator workflow. Use generator and motor-starting references before treating the ratio as an equipment decision.