Fault Current
Calculator
Calculate available short-circuit current at the transformer secondary terminals from kVA rating, secondary voltage, and percent impedance. Toggle between single-phase and three-phase.
Calculating Fault Current at the Secondary
You need three values from the transformer nameplate: (1) kVA rating, (2) secondary voltage, and (3) percent impedance (%Z or IZ). All three are required for the calculation.
Choose 3-Phase for three-phase transformers (most commercial and industrial units) or 1-Phase for single-phase transformers. The formula differs by a factor of √3 (1.732).
Enter the three values and calculate available fault current in amps. Compare this against the interrupting rating of your downstream breakers, fuses, and switchgear.
This result is the worst-case (maximum) available fault current assuming an infinite bus on the primary. In practice, utility impedance and cable resistance reduce the actual fault current — consult a licensed engineer for detailed coordination studies.
kVA = Transformer rated capacity
V = Secondary line-to-line voltage
IZ = Transformer % impedance
| Transformer Size | Typical % Z |
|---|---|
| 25 kVA | 1.8% |
| 75 kVA | 2.0% |
| 167 kVA | 2.0% |
| 500 kVA | 2.5% |
| 1,000 kVA | 3.5% |
| 2,500 kVA | 5.0% |
Values are typical ANSI/IEEE standard impedances. Actual values vary by manufacturer and specification.
Available Fault Current (also called Available Short-Circuit Current or ASCC) is the maximum current that can flow in a circuit if a bolted three-phase or single-phase fault occurs at the transformer secondary terminals. It is used to verify that downstream protective devices and equipment have adequate interrupting ratings for the installation.
Percent impedance (% Z or IZ) is a characteristic of the transformer that limits fault current. It is found on the transformer nameplate and in the manufacturer's data sheet. Standard impedance values for distribution transformers typically range from 1% to 6%. A higher % impedance limits fault current but increases voltage regulation under load.
The calculated value is the available fault current at the transformer secondary terminals assuming infinite available fault current on the primary (worst case). In practice, the utility supply impedance, cable impedance, and fuse resistance reduce the actual fault current seen by downstream equipment. Always consult a licensed engineer for protective device coordination and arc flash analysis.
A lower impedance transformer allows more current to flow during a fault — it is 'stiffer' and has better voltage regulation but produces higher fault levels. A higher impedance transformer limits fault current but has worse voltage regulation under heavy load. The impedance choice is a design trade-off specified for each application.
Need a Transformer with a Specific % Impedance?
We specify and build transformers to custom impedance values — whether you need a lower impedance for voltage regulation or a higher impedance to limit fault current.