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Switchgear Short Circuit Rating Calculator

Free calculator estimate Switchgear Short Circuit Rating with contribution from upstream transformer and downstream rotating motor load.

Secondary Voltage
Motor Lump Load

Secondary Rating Current
Transformer SC contribution
Motor SC contribution
Switchgear SC Current
Switchgear Short Circuit Rating Calculator

Basis and Assumption for Switchgear Short Circuit Rating Calculator

The calculator is based on following assumption:

  • Neglect power source at primary side of Transformer,
  • All cable or bus duct impedance is consider zero,
  • Motor load contribution is estimated as recommended by IEEE Std 141 (chapter,
  • Voltage factor c as IEC 60909 is ignored.

Clearly, the result from calculator is conservative when all above parameters accounted (i.e. cable impedance will lower short circuit current).

Transformer Impedance

Typical impedance of transformer can be found as below table

Rated Power (kVA)Impedance (%)
25 to 6304
631 to 12505
1251 to 25006
2501 to 63007

Transformer Short Circuit Current

Transformer short circuit current is calculated with data from short circuit impedance:


  • Isc : Short circuit at transformer secondary side
  • Isec : Rating current at transformer secondary side
  • Z : Transformer short circuit impedance (% or pu)

For other related calculator, pls access Transformer Rating Current Calculator or Transformer Efficiency Calculator.