Transformer short-circuit current calculator

Preliminary estimate

Inputs

Calculation note

This is a transformer-only short-circuit estimate. Utility source impedance, conductor impedance, motor contribution, and protective-device coordination are outside this screen.

Method

Three-phase FLA = kVA x 1000 / (sqrt(3) x secondary volts)

Available current = full-load current / (impedance percent / 100)

Single-phase FLA = kVA x 1000 / secondary volts

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Formula variables

  • Transformer kVA is the transformer rating entered by the user.
  • Secondary voltage is the transformer secondary nominal voltage.
  • Impedance percent is the transformer nameplate impedance value.
  • Available current is a transformer-only approximation before system fault-current study adjustments.

Assumptions

  • The estimate uses transformer full-load current divided by impedance in per-unit.
  • The result is transformer-only available current at the secondary terminals.
  • Utility source impedance, motor contribution, conductor impedance, and protective-device clearing are not included.
  • Use transformer nameplate impedance and project fault-current study values for equipment rating review.

Examples

75 kVA at 600 V. A 75 kVA three-phase transformer at 600 V with 5.75 percent impedance gives about 72.2 A full-load current and about 1.26 kA transformer-only short-circuit current.

Study boundary. If equipment rating is being selected, include utility source impedance, conductor impedance, motor contribution, and the actual protective-device arrangement.