Full Per-Unit Calculation — Auditable Working
Build a simple radial network — utility source plus one or more transformers or impedance elements — and calculate three-phase fault current using the per-unit system on a common MVA and kV base. Unlike a black-box fault calculator, every conversion step is shown: each element's impedance converted to per-unit, the running cumulative total, and the final fault current and MVA derivation.
Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.
The per-unit system expresses all impedances as a fraction of a chosen base impedance, allowing elements at different voltage levels to be combined directly.
All series impedances are summed on the common base, then fault current is derived:
For a cable or line element, the ohmic impedance is converted to per-unit on the base impedance at that element's own voltage level:
The voltage base steps at each transformer, so a cable downstream of a 33/11 kV transformer is converted on 11 kV, not on the source-side 33 kV. Using the wrong base here would misstate the cable's per-unit impedance by the square of the turns ratio.
Cables offer two impedance input modes. Detailed mode uses genuine complex addition — resistance and reactance are combined with the rest of the network as perpendicular vector components, correct when real R and X are separately known (e.g. from a cable datasheet or AS/NZS 3008). Direct Ω is for a single flat "line impedance" figure with no resistance/reactance breakdown, common in simplified per-unit exercises. In that case the value is placed on the reactance axis alongside the source and transformer impedances, which are themselves dominated by reactance — this matches the standard simplified convention of summing all given impedance magnitudes directly, rather than silently treating an unknown-phase figure as pure resistance, which would understate the network total once combined with reactive elements. The two modes can give a genuinely different answer for the same numeric "0.25 Ω" — this is a real modelling choice, not a rounding difference, and the one that matches a specific worked reference has been verified to reproduce it to 4 significant figures.
Base 10 MVA, 0.415 kV. Source fault level 250 MVA → Z_source = 10/250 = 0.04 pu. Transformer 0.5 MVA, 5%Z → Z_tx = 0.05 × (10/0.5) = 1.0 pu. Total Z = 1.04 pu. . Fault current = 13,912/1.04 ≈ 13,377A ≈ 13.4kA.