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Circuit Protection Tools

Short Circuit Current Calculator

Estimate available transformer secondary fault current from transformer kVA, impedance, voltage, phase, and optional utility fault contribution.

Inputs
Result
12.6kA

Available fault current is estimated from transformer full-load current divided by total per-unit source impedance.

Full-load current722 A
Transformer Z5.75%
Source Z allowanceIgnored
Fault MVA8.70 MVA
Next checks
  • Select protective equipment with interrupting capacity above the calculated value; next common reference is 18 kA.
  • Final short-circuit study should include upstream utility data, cable impedance, motors, X/R ratio, and applicable IEC 60909 or IEEE method.
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Short-circuit current equation

The available transformer secondary fault current is estimated from transformer full-load current and total per-unit source impedance.

Three-phase full-load current
IFL = S × 1000 / (√3 × V)
Single-phase full-load current
IFL = S × 1000 / V
Optional utility source impedance
Zsource,pu = S / (MVAutility × 1000)
Available fault current
Isc = IFL / (Ztransformer,pu + Zsource,pu)
Fault level
Isc,kA = Isc / 1000

where:

IFL
Transformer full-load currentA
S
Transformer ratingkVA
V
Secondary voltageV
Zsource,pu
Optional upstream utility impedance on the transformer base
MVAutility
Available upstream utility fault contributionMVA
Ztransformer,pu
Transformer impedance converted from percent impedance
Isc
Estimated available short-circuit currentA
Isc,kA
Estimated available short-circuit currentkA

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.

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.

FAQ

Why does lower transformer impedance increase fault current?

Short-circuit current is inversely proportional to source impedance. A lower %Z transformer can deliver more fault current.

Can this choose a breaker model?

No. It gives a first-pass kA level. Final breaker selection also needs voltage, poles, trip unit, coordination, standard, and enclosure conditions.

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