Transformer Fault Current Calculator With Calculation Formula

Transformer Fault Current Calculator:

Enter the transformer rating in kVA (kilo Volt-Amp), Enter the secondary terminal voltage in volts and the percentage impedance in %. Then press the calculate button to get the transformer short circuit fault current in kilo Amps (kA). Reset button resets the input values.

Enter Transformer Rating: kVA
Enter Secondary Voltage: V
Enter Percentage Impedance: %
Transformer Secondary Fault Current:

Transformers are the heavy-duty equipment, even though the failure occurs on behalf of the electrical faults. Our calculator is designed to calculate the short circuit fault current of the transformer.

Transformer short circuit fault current Calculations

Short circuit fault current I(fault) in kilo amps is equal to 100 times of transformer’s rating S(kVA) in kVA divided by the multiplication of root 3, transformer’s secondary voltage V(V) in Volts and percentage impedance in percentage.

All the above details will available at the transformer’s nameplate details.

Transformer short circuit fault current

I(fault) = S(kVA) x 100 / (1.732 x V(V) x %Z).

The %Z will lie between 4 to 10%.


A transformer’s nameplate details are 25 kVA, 440V secondary voltage, 5% of percentage impedance, calculate the short circuit fault current.

I(fault) = 25 x 100 / (1.732 x 440 x 5)

 I(fault) = 0.66 kA


We have assumed the transformer is connected with the infinity bus to get the worse case fault level on the secondary side of the transformer. It is enough to find design or sizing the transformer according to fault rating.

Learn More:   Transformer Impedance Calculator


  1. Hello, can you please advise if the fault current calculated should be considered as peak current or rms current ?
    Thanks in advance,
    George V


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