How the calculation works
This page reports only an idealized transformer-secondary estimate using Isc ≈ kVA·1000 / (√3·V·%Z/100) for three-phase (or kVA·1000 / (V·%Z/100) for single-phase), assuming an effectively infinite upstream source. It does not calculate downstream panel fault current or determine equipment AIC/interrupting ratings. A documented short-circuit study must consider utility/source fault level, impedance magnitude and angle/X/R, conductor impedance and temperature, motor contribution and other relevant system details. Scalar addition of resistance and impedance is not a reliable general system model.
Worked example
500 kVA, 5.75%Z, 480 V three-phase gives approximately 10.46 kA at the transformer secondary under the stated idealized assumption. Downstream current and equipment ratings are not determined.
Common mistakes to avoid
- Using the transformer terminal value for equipment far from the transformer.
- Entering line-to-neutral instead of line-to-line voltage for three-phase.
- Treating this estimate as a replacement for a study.
Frequently asked questions
Is this a substitute for a study?
No. Use an engineering short-circuit study for service equipment and arc-flash labeling (NEC 110.24).
What is an AIC rating?
Interrupting ratings must be coordinated with the documented available fault current and applicable code. This calculator does not establish that value at a panel or select a device.
Why does wire length reduce fault current?
Conductor impedance adds in series with the source, limiting current.
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For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.