Available Fault Current Calculator

It is the largest current the system can push through a short circuit at a given point, limited only by the impedance between the source and that point. Breakers, fuses and switchboards must be rated to interrupt or withstand at least that value. At a transformer secondary the main limit is the transformer impedance, which is why %Z on the nameplate drives the answer.

Find the full-load current: kVA x 1000 divided by (1.732 x secondary volts) for three-phase, or kVA x 1000 divided by volts for single-phase. Divide 100 by the percent impedance to get the multiplier. Full-load current times the multiplier is the fault current at the secondary terminals, assuming an infinite source on the primary. A 1000 kVA, 480 V, 3.5% Z transformer gives about 1,203 A x 28.57 = 34,370 A. The tool keeps full precision and shows 34,367 A.

Use the point-to-point method. Compute f = 1.732 x L x I / (C x n x E) for three-phase, where L is the one-way length in feet, I is the fault current at the start of the run, C is the conductor constant for the size and raceway, n is conductors per phase and E is line-to-line volts. The multiplier is M = 1 / (1 + f), and the current at the end of the run is I x M. Repeat for each run in series.

On the transformer nameplate, as a percentage. The Cooper Bussmann handbook this tool follows says to take %Z from the nameplate. If you only know the transformer size, ask the manufacturer or the utility, because impedance varies by design and this tool does not guess it. A wrong %Z moves the result by the same ratio: 3% instead of 6% doubles the fault current.

Yes. More conductor length adds impedance between the transformer and the fault, so the current at the far end is lower. Larger conductors, or more conductors in parallel, add less impedance for the same length. In the handbook table, steel conduit gives a lower C value, meaning more impedance, than nonmagnetic conduit such as PVC for the same conductors.

It assumes the utility supply on the primary side has no impedance of its own, so the transformer is the only limit. Real sources add impedance, so the true current is lower. The infinite bus value is the safe upper bound for choosing equipment ratings. For a closer value, ask the utility for the available short-circuit current at your service.

The handbook notes that UL 1561 listed transformers of 25 kVA and larger have a plus or minus 10% impedance tolerance, and that ANSI-built two-winding transformers have plus or minus 7.5%. To find the highest fault current from a UL-listed unit, multiply %Z by 0.9. For the lowest, multiply by 1.1. The tool applies the 0.9 and 1.1 factors from the handbook when you pick those options.

Running motors feed current back into a fault. The handbook says a practical estimate is the total motor current times 4, and that values of 4 to 6 are commonly accepted. Enter the total motor full-load current and the tool adds 4 times that value at every point. Motors are not added to the current that flows through the conductor run, matching the handbook examples.

On a center-tapped single-phase transformer, the line-to-neutral fault current at the secondary terminals can be higher than the line-to-line value. The handbook uses 1.5 times the line-to-line value at the terminals as an approximation, and uses half the line-to-line voltage in the conductor calculation. Three-phase systems use the three-phase bolted fault value.

No. The result is a bolted-fault current for choosing equipment ratings. Incident energy and PPE categories depend on clearing time, working distance, electrode configuration and more. They need a study done to the method your facility follows, usually by a qualified engineer.

Section 110.24 requires service equipment in other than dwelling units to be field marked with the available fault current and the date the calculation was made, and the calculation to be documented. Section 110.9 covers the interrupting rating of equipment, and 110.10 covers the circuit impedance and short-circuit current ratings. Read the edition your jurisdiction has adopted, and ask your inspector about local rules.

The conductor constants in the table come from the 600 V columns of the handbook, and this tool does not extrapolate to medium-voltage systems. For higher voltages use a full short-circuit study.