Estimate transformer secondary prospective short-circuit current from transformer rating, voltage, and impedance.
This calculator estimates transformer secondary fault current at the transformer terminals and assumes a stiff upstream source. Cable impedance, source limitations, and downstream location are not included.
Short-circuit current is one of the key values behind breaker interrupting rating, busbar duty, and protective coordination. In many practical situations, the first fault-current estimate starts at the transformer secondary because the transformer rating and impedance provide a quick way to estimate how much current the transformer can deliver into a solid fault.
This short-circuit current calculator uses transformer kVA, secondary voltage, and transformer impedance to estimate the prospective fault current right at the secondary terminals. It also shows the full-load current first so the multiplication effect of transformer impedance is easier to understand. That makes the page useful for panel planners, maintenance engineers, and electricians performing first-pass equipment checks.
The result should be read as a terminal estimate. Real fault current farther downstream is often lower because cables, busway, and upstream source limits add impedance that this simplified tool does not include.
| Variable | Meaning | Unit |
|---|---|---|
| kVA | Transformer apparent power rating | kVA |
| V | Transformer secondary voltage | V |
| Z% | Nameplate transformer impedance | % |
| Ifl | Transformer full-load current | A |
| Isc | Prospective terminal short-circuit current | A |
The result is shown in amperes and kiloamperes because equipment interrupting ratings are commonly discussed that way. The page also reports a short-circuit MVA value to help users compare fault duty on a system basis. Since this is a transformer-terminal estimate, it is best used for first-pass checking rather than final protection coordination.
Transformer = 500 kVA, voltage = 415 V, impedance = 5.75%, three phase
Full-load current = 500,000 / (1.732 x 415) = 695.59 A
Prospective short-circuit current = 695.59 x 100 / 5.75 = 12,097 A
The result is about 12.10 kA at the transformer secondary terminals.
Transformer = 75 kVA, voltage = 240 V, impedance = 4%
Full-load current = 75,000 / 240 = 312.50 A
Prospective short-circuit current = 312.50 x 100 / 4 = 7,812.50 A
This gives a useful first indication of equipment duty near the transformer.
This page is accurate for the classic transformer-terminal approximation, but it is not a full short-circuit study. If the fault is some distance away from the transformer, feeder impedance reduces the current. Likewise, the upstream source may not be infinitely stiff, so the actual fault level at the transformer may also be lower than the simple calculation suggests.
Even with those limits, the calculator is valuable because it gives a fast reference point for checking whether a distribution panel, switchboard, or breaker may need a higher interrupting rating. It is especially helpful when the transformer nameplate data is known but a full system model is not yet available.