Transformer Short Circuit Calculator

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Short Circuits, also known as faults, can occur in electrical power distribution systems. When a fault occurs on the load side of a transformer, fault current, or short circuit current, flows through the transformer. Transformers are designed to withstand these fault currents, which are significantly higher than the rated current.

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It is worth noting that transformer impedance is one of the limiting factors for the short circuit current of transformer at the secondary coil. The higher the transformer impedance (% Z), the lower the ratio of fault to rated current. In other words, a Transformer with higher %Z better limits the flow of fault current.

Short Circuit Current of a Transformer is calculated using the formula as shown. Note that UL 1561 transformer has a ±10% of impedance tolerance. For high end worst case, we assume a lower transformer impedance by multiplying a 90% safety factor as shown below*.

Where

IF.L. = Full Load Current (A)

TXKVA = Transformer Rating (kVA)

VL-L = Phase Voltage

Z = Transformer Impedance,

%Z = Transformer % Impedance

IS.C. = Short Circuit Current (kA)

M = Multiplier, or ratio of Short Circuit Current to Full Load Current

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To compute the short circuit current at a distance, L feet away from the first fault current, I S.C. calculated above, over a conductor of C-Values, C and of N number per phase, factor f and Multiplier M can be derived from the formulae below.

A downloadable Excel File for Transformer Short Circuit Current Calculation, and estimated Short Circuit Current at X2, is available below:

Transformer Short Circuit Current Calculator (Excel)