Generator short circuit fault current calculator

Calculates the short circuit fault current of a 3-phase AC generator.

The generator short circuit fault current calculator uses a simplified method to calculate the fault current from the following parameters:

  • Rated (Ur). The rated phase-to-phase voltage of the generator in V.
  • Rating (Sr). The rating power of the generator in kVA.
  • Impedance (Zk). The short circuit impedance of the generator as a percentage. The short circuit impedance can be specified for the sub-transient, transient or steady state phase of the generator fault. For simplification the resistance can be ignored and only the reactance can be considered. Typical reactance values are shown below.
Phase Equation Typical reactance (%) Typical time
Sub-transient 10 – 20% 10ms
Transient 15 – 25% 250ms
Steady state 200 – 350% > 250ms

The generator short circuit current is calculated as follows:

\(I_{fault-actual}= \dfrac{S_{r} \times 100}{\sqrt{3} \times U_{r} \times Z_{\%}}\)


  • \(I_{fault-actual}\) is the fault current in kA.
  • \(S_{r}\) is the generator power rating in kVA.
  • \(U_{r}\) is the generator rated voltage in V.
  • \(Z_{\%}\) is the generator impedance in base percentage.

This is basically a combination of the steps in the per-unit calculation method:

Step 1: Calculate the per unit fault current:

\(I_{fault-pu}= \dfrac{1}{Z_{pu}}\)

Note that \(Z_{pu} = \dfrac {Z_{\%} }{100}\)

Step 2: Calculate the nominal base current:

\(I_{nominal-base}= \dfrac{S_r}{\sqrt{3} \times U_r}\)

Step 3: Calculate the actual fault current:

\(I_{fault-actual}= I_{fault-pu} \times I_{nominal-base}\)