Fault Current (Short-Circuit) Calculator
The available short-circuit current at a transformer secondary — the number that sets the interrupting rating (AIC) your breakers and panel must meet. From the transformer kVA, voltage and impedance.
The Current You Hope Never Flows
Normal load current is what runs your equipment. Fault current is what surges when a short circuit dead-shorts the system — often tens of thousands of amps in the first instant. Every protective device has to interrupt that surge safely. If the available fault current exceeds a breaker interrupting rating, the breaker can explode instead of trip. That is why this one number drives equipment selection.
The transformer full-load current times 100 divided by its percent impedance gives the bolted-fault current at the secondary terminals. A 500 kVA, 480 V transformer at 5.75 percent impedance delivers about 10,000 A — roughly seventeen times its full-load rating.
Why Impedance Is Your Friend
The percent impedance %Z is the brake on fault current: a higher-impedance transformer limits how much current a short circuit can draw, while a low-impedance transformer lets more through. It is a trade-off — lower impedance means better voltage regulation but higher fault duty on the downstream gear.
Interrupting Rating (AIC)
A breaker has two ratings: the continuous current it carries, and the interrupting rating (AIC, amps interrupting capacity) — the largest fault it can safely break. Pick devices whose AIC meets or beats the available fault current. The calculator rounds up to the next standard rating (10, 14, 22, 25, 35, 65 kA and so on).
Frequently Asked Questions
What is the infinite-primary assumption?
It treats the utility as an unlimited source, so all the limiting comes from the transformer. That gives the maximum possible secondary fault current – a safe, conservative basis for rating equipment.
Is the real fault current always lower?
Downstream, yes – cable and connection impedance reduce it with distance. But large motors briefly feed current into a fault, which a detailed study adds back in.
Where do I get %Z?
From the transformer nameplate or test report. Typical dry-type and liquid transformers run around 4 to 6 percent.
Related calculators
- Three-Phase Power Calculator — the load side of the same transformer.
- Power Factor Correction Calculator — trim demand and current.
- Motor FLA Calculator — motor full-load current.
- Wire Size Calculator — size the feeders.