Transformer short circuit fault current calculator

Calculates the short circuit fault current level of a transformer on an infinite bus.

Transfomer rating (kVA)

Secondary transformer voltage (V)

Transfomer impedance (%)


  • Transformer rating (S): Rating in kVA.
  • Voltage rating (V): Voltage rating of the secondary windings.
  • Impedance (\(Z_{\%}\)): Per-unit impedance of the transformer in %. Can be found on the nameplate. Typically 4% to 10%.


  • The transformer fault level calculator assumes that the transformer is supplied from an infinite bus. In other words,¬†the fault level on the primary side of the transformer is considered to be infinite. Note that this¬†assumption will give the worse case fault level on the secondary side of the transformer, which is sufficient for sizing of equipment according to fault rating. However, this assumption is not applicable for motor start-up calculations where the fault level is used as an input to the motor start-up calculation.
  • The transformer short circuit current is calculated as follows:
    • \(I_{fault-actual}= \dfrac{S_{base} \times 100}{\sqrt{3} \times V_{base} \times Z_{\%}}\)
    • where \(I_{fault-actual}\) is in kA, \(S_{base}\) is in kVA, \(V_{base}\) is in V, and \(Z_{\%}\) is in 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:
      • \(I_{nominal-base}= \dfrac{S}{\sqrt{3} \times V}\)
    • Step 3: Calculate the actual fault current:
      • \(I_{fault-actual}= I_{fault-pu} \times I_{nominal-base}\)

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