Short-circuit current equation
The available transformer secondary fault current is estimated from transformer full-load current and total per-unit source impedance.
where:
- IFL
- Transformer full-load current[A]
- S
- Transformer rating[kVA]
- V
- Secondary voltage[V]
- Zsource,pu
- Optional upstream utility impedance on the transformer base
- MVAutility
- Available upstream utility fault contribution[MVA]
- Ztransformer,pu
- Transformer impedance converted from percent impedance
- Isc
- Estimated available short-circuit current[A]
- Isc,kA
- Estimated available short-circuit current[kA]
This is a first-pass estimate at the transformer secondary. A final study should include cable impedance, motors, X/R ratio, utility data, and the applicable IEC or IEEE method.
How to calculate transformer fault current
A first-pass transformer secondary fault-current estimate divides transformer full-load current by total per-unit source impedance. Transformer percent impedance is always included; optional utility fault MVA adds an upstream source-impedance allowance on the transformer base.
- Enter transformer kVA, phase, and secondary voltage.
- Enter the transformer nameplate percent impedance.
- Add utility fault MVA when reliable source data is available.
- Calculate secondary full-load current and divide by total per-unit impedance.
- Select equipment with verified interrupting and withstand ratings above the complete study result.
Short-circuit current example
A 500 kVA, 400 V three-phase transformer with 5.75% impedance has a full-load current of about 722 A. With an infinite-bus assumption and no downstream cable impedance, the estimated secondary terminal fault current is about 12.6 kA.
IFL = 500,000 / (√3 × 400) = 721.7 A
Isc = 721.7 / 0.0575 = 12.55 kA
| Input or result | Value |
|---|---|
| Transformer | 500 kVA, three-phase |
| Secondary voltage | 400 V |
| Transformer impedance | 5.75% |
| Utility contribution | Ignored / infinite-bus estimate |
| Estimated fault current | 12.6 kA |
Why transformer impedance changes fault current
Fault current is inversely related to source impedance. A lower percent-impedance transformer can deliver more secondary fault current, while added utility, transformer, and conductor impedance reduces the current available at downstream points.
Transformer-only estimate vs. complete short-circuit study
This calculator is useful at the transformer terminals, but a complete study includes utility source data, cable and busbar impedance, generators, motor contribution, transformer tolerance, voltage factor, X/R ratio, fault type, and the applicable IEC or IEEE calculation method.
| Included here | Required in a detailed study |
|---|---|
| Transformer kVA and %Z | Transformer tolerance and R/X components |
| Optional utility fault MVA | Verified utility source model |
| Terminal fault estimate | Cable, busbar, motors, generators, and fault location |
Use fault current to check protective equipment
Circuit breakers, fuses, switchgear, busbars, and panels must have suitable interrupting or withstand performance at their installation point. Do not use current rating as a substitute for short-circuit rating.
Transformer Sizing CalculatorCircuit Breaker Size CalculatorBusbar Current Rating Calculator
Assumptions
- Transformer-secondary estimate only
- Cable impedance and motor contribution are not included
- Utility MVA is optional and treated as upstream source impedance
Important Warnings
- Final interrupting capacity must be based on a complete short-circuit study including upstream utility data, feeder impedance, motors, X/R ratio, and the applicable IEC or IEEE method.
- Always select protective devices with interrupting rating above the available fault current at the installation point.