Transformer Secondary Short-Circuit

A screening estimate of bolted fault current at the transformer secondary terminals.

1. What this check does

Estimates the symmetrical bolted three-phase fault current at the transformer secondary terminals and, if you enter one, compares it with the equipment rating.

2. Inputs used

Transformer kVA, secondary line-to-line voltage, nameplate impedance %Z.
Optional: available fault level of the source in MVA; equipment interrupting or short-circuit rating in kA.

3. Formulas and assumptions

Full-load current I_FL = kVA × 1000 / (√3 · V) Infinite bus I_SC = I_FL / (Z% / 100) Source impedance Zs% = kVA / (MVA_SC × 1000) × 100 on the transformer base Finite source I_SC = I_FL / ((Z% + Zs%) / 100)

The infinite-bus result is the upper bound for the transformer alone.
Source and transformer impedances are added as magnitudes, which assumes similar X/R ratios. This is a screening simplification.
Nameplate impedance is used as given. Actual impedance may be lower within manufacturing tolerance, which increases fault current.

4. What is NOT checked

Motor contribution, which can add significantly to fault current on buses with large motor loads.
Secondary cable and bus impedance downstream of the terminals.
Asymmetrical current, X/R effects on equipment ratings, single-phase and ground faults.
Arc-flash incident energy and protective device coordination.

5. Reference standards

NEC 2023 110.9, 110.10 — interrupting rating and circuit impedance
IEEE Std C57.12.00 — transformer impedance tolerance

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Transformer Secondary Short-Circuit.