How to use the short circuit current calculator
Choosing a breaker is not only about the load current it carries every day. It also has to interrupt the fault current that appears the instant something shorts out. A device with an interrupting rating below the available fault current can fail violently instead of clearing, which is why NEC 110.9 requires equipment to be rated for the current available at its terminals. This tool estimates secondary fault current from transformer kVA, secondary voltage, and nameplate %Z.
The arithmetic is straightforward. Find the transformer secondary full load current, then multiply by 100 divided by the percent impedance. A 5% impedance transformer therefore delivers roughly twenty times its full load current into a bolted fault. Short circuit capacity in MVA follows the same ratio applied to the transformer rating.
The method assumes an infinite utility source and ignores conductor impedance between the transformer and the fault, so the answer comes out higher than reality. That bias is safe when selecting an interrupting rating, but it is not appropriate for coordination studies or arc flash incident energy work, where an understated impedance produces a misleading result.
Because a DC offset rides on the first cycle, an asymmetrical estimate using the common 1.6 multiplier is shown alongside the symmetrical value. Final interrupting rating selection should be confirmed by a licensed electrical engineer working from a full system study.
Frequently asked questions
It assumes an infinite utility source and ignores conductor impedance, so the result runs higher than the real fault current. That is conservative for picking an interrupting rating, but a full study with source and cable impedance is required for coordination work.
A DC offset rides on the first cycle of a fault, so peak current exceeds the symmetrical value. Instantaneous interrupting performance and busbar bracing forces both have to account for that asymmetrical component.
It is printed on the nameplate and the factory test report as a percentage on the transformer kVA base. Manufacturing tolerance applies, so using the low end of the stated range yields a conservatively high fault current.