Transformer Current & Fault Calculator

Transformer Fault

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Methodology

Transformer full-load current is calculated from kVA rating and voltage: $I = \frac{\mathrm{kVA} \times 1000}{\sqrt{3}\,V}$. The simplified "infinite bus" fault current assumes an infinitely stiff upstream source and calculates fault current purely from transformer impedance: Isc = I_rated / (%Z/100). Including source impedance uses a per-unit calculation combining transformer and source impedance on a common base, giving a more realistic (lower) fault current.

Worked Example

A 500kVA, 11000/415V transformer with 5% impedance: secondary FLC = $\frac{500 \times 1000}{\sqrt{3}\times415} = 696\ \mathrm{A}$. Infinite-bus fault current = 696 / 0.05 = 13,919A ≈ 13.9kA, or 10.0 MVA fault level.

Assumptions & Limitations

• Infinite-bus assumption significantly overstates fault current versus real network conditions — use source impedance for realistic figures
• Does not account for X/R ratio — asymmetrical (first-cycle) fault current will be higher than the symmetrical RMS value shown
• Assumes balanced three-phase fault — line-to-ground and line-to-line faults will differ