Fault current · transformer terminals

Transformer Short-Circuit Current: Worked Example

Protection guide · Reviewed 22 Sep 2026 · ElectroDesigner

Transformer percentage impedance gives a fast first estimate of the available symmetrical RMS fault current at the LV terminals. The familiar infinite-source calculation is useful for screening, but a finite upstream source reduces the result and downstream cable or busbar impedance reduces it further.

In this article1. Equations 2. Worked example 3. Finite source 4. Downstream fault level 5. Assumptions and limits

1. Transformer-terminal equations

Transformer rated current on the LV side:

I_n = S × 1000 / (√3 × U_LL)

With an infinite/strong upstream source:

I_sc ≈ I_n × 100 / u_k%

If the three-phase upstream short-circuit power is known, a simple per-unit source impedance can be added:

z_source,pu = S_transformer / S_sc,source

I_sc = I_n / (z_{transformer,pu} + z_source,pu)

2. Worked example: 1000 kVA, 400 V, uk = 6%

I_n = 1000 × 1000 / (√3 × 400) = 1443.38 A

I_sc = 1443.38 / 0.06 = 24.06 kA

The strong-source terminal estimate is therefore approximately 24.06 kA symmetrical RMS.

Run the Transformer Short-Circuit Calculator

3. What if the upstream network is only 250 MVA?

On a 1 MVA transformer base:

z_source,pu = 1 / 250 = 0.004

z_total,pu = 0.060 + 0.004 = 0.064

I_sc = 1443.38 / 0.064 = 22.55 kA

The finite source lowers the preliminary terminal current from 24.06 kA to about 22.55 kA. The exact study may need source R/X, transformer tolerances, voltage factors and other contributions.

4. Fault current away from the transformer

For a downstream fault, use the total upstream impedance at the fault location:

I_sc = U / (√3 × |Z_T|)

|Z_T| = √(R_T² + X_T²)

Cable, busbar and source impedance therefore matter. The transformer-terminal value should not simply be copied to every downstream panel.

5. Assumptions and limits

This is a preliminary symmetrical RMS screening method, not a complete IEC 60909 study.
Transformer impedance tolerance and network voltage factor can change the maximum/minimum fault level.
Peak making current depends on X/R and is a different quantity from symmetrical RMS current.
Motor or generator contribution may need to be included.
Protection studies need both maximum fault current for equipment duty and minimum fault current for reliable protective operation.

Adjacent checks

Read Short-Circuit Current vs SCCR vs Interrupting Rating, then use FaultChain to carry the prospective current through a radial protection path. For selectivity, continue with Protection Coordination and TCC Explained.

Engineering references

Schneider Electric Electrical Installation Guide — transformer terminal short-circuit current and 3-phase short-circuit current at any point in an LV installation.