Transmission & Grid calculator
Transformer Full-Load & Fault Current Calculator
Full-load current on both windings and the maximum available fault current at the secondary terminals, from kVA and percent impedance.
GridBasis: IEEE C57.12.00 · IEEE 141 (Red Book) · IEC 60909 (note)
Results
- Enter values to calculate
- —
Formula
Full-load current: three-phase IFL = kVA × 1000 / (√3 · V); single-phase IFL = kVA × 1000 / V
Maximum secondary fault current (infinite bus): ISC = IFL,sec × 100 / %Z
Fault level: MVASC = kVA / (%Z / 100) / 1000
Worked example
1000 kVA, 13.8 kV to 480 V, 5.75 %Z, three-phase: IFL,sec = 1,000,000 / (1.732 × 480) = 1203 A; IFL,pri = 41.8 A; ISC = 1203 × 100 / 5.75 = 20.9 kA.
Assumptions and standards
- Infinite-bus (unlimited source) method: it ignores utility source impedance, so it overstates the real fault current. That is conservative for equipment ratings.
- Motor contribution is not included; add it for a full study.
- IEC 60909 studies also apply a voltage factor c (typically 1.05 or 1.10 for low voltage) and the actual source impedance.
- Transformer impedance tolerance (often ±7.5 % of nameplate %Z) is not applied.
Formula and data last verified: October 2026.
Frequently asked questions
Why is the infinite-bus fault current higher than my study shows?
A full study includes the utility source impedance and cable impedance, which both reduce fault current. The infinite-bus value is the worst case and is useful for a quick equipment-rating check.
Where do I find %Z?
On the transformer nameplate or test report. Typical values range from about 2 % for small units to 5.75 % or more for large distribution transformers.
Can I use these results for final design?
Use them for preliminary design and checking. Final designs must be verified by a qualified engineer against the code edition and standards adopted where the work is installed.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you type is uploaded or stored on our servers.