Transformer Short-Circuit Current Calculator
Estimate three-phase fault current at LV transformer terminals from transformer rating and impedance, with an optional upstream source fault level.
Transformer and source parameters
Use nameplate transformer data. Add the upstream short-circuit power when it is known; enter 0 to retain the strong-source assumption.
This remains a symmetrical RMS estimate. It does not include transformer impedance tolerance, voltage factor, X/R-dependent peak current, downstream conductors or motor contribution.
Result
READYMethod
The transformer impedance and optional upstream source impedance are combined on the transformer MVA base.
zsource,pu = Stransformer / Ssc,source
Isc = In / (ztransformer,pu + zsource,pu)
Calculation method
With an infinite source the familiar terminal estimate is Isc ≈ In × 100 / uk%. When upstream short-circuit power is entered, its per-unit source impedance is added to the transformer impedance before current is calculated.
Finite source: Isc = In / [(uk% / 100) + (Stransformer / Ssc,source)]
Worked example
Load the worked example for 1000 kVA, 400 V and uk = 6% with the upstream source left at 0 MVA. Rated current is 1443.4 A and the traditional strong-source terminal fault estimate is 24.06 kA.
Method guide: Transformer Short-Circuit Current: Worked Example shows the same calculation and the effect of finite source strength step by step.
Effect of a finite upstream source
If the same transformer is supplied from a system with 250 MVA short-circuit power, total per-unit impedance becomes 0.064 and the terminal current falls to about 22.55 kA. This illustrates why the infinite-source calculation can overstate the actual available current.
Frequently asked questions
Is this a complete IEC 60909 study?
No. It is a simplified transformer-terminal model. IEC 60909 studies can require voltage factors, network impedances, rotating-machine contribution and peak-current treatment.
What should I enter for upstream fault level?
Use the three-phase short-circuit power at the transformer supply point from the utility/network study. If it is unknown, 0 retains the explicit strong-source assumption rather than inventing a value.
Continue the design check
All calculators · FaultChain · Short-circuit and SCCR guide · Standards library
Technical reference: transformer terminal short-circuit guidance.