Short-circuit current available in bolted-fault conditions. The system fault current is the sum of all connected source fault currents.

For utility transformers, a worst-case estimate of available short-circuit current in a three-phase fault can be determined by dividing the transformer ampere rating by the impedance of the transformer.¹⁾ This assumes an infinite utility bus with unlimited current; as this is rarely if ever the case, actual fault current will be limited by available utility bus current.

For example, a 630kVA/400VAC, 5.76% impedance transformer would be capable of delivering approximately 15,800 amperes of fault current into a three-phase fault circuit.

The three-phase bolted fault current from a generator is roughly the rated ampacity of the generator, divided by the subtransient reactance of the machine at that rating. Subtransient reactance is noted on the manufacturer's performance data sheet for that generator, using the “submittal data” on the data sheet, not the implemented or de-rated ampacity of the installation. The Cummins Power Application Manual on Gensets contains calculations for converting that submittal data to another rating in the section on calculating fault currents.³⁾

For example, with a 200kW/480vAC machine with 12% subtransient reactance (X“d), the available fault current at the generator terminals is approximately 2,500 Amps. If this generator is paralleled with two other identical machines, the total available from all generator sets is the sum of the available current from each machine, or 7,500 Amps.

A single phase-neutral bolted fault may have higher short-circuit current than a Line-Line fault on wye-connected generators with 2/3 pitch windings. The product of the three phase currents divided by the sum of the Subtransient Reactance (X”d), Negative Sequence Reactance (X2) and the Zero Sequence Reactance (X₀) gives the available phase-neutral fault current.⁴⁾

For a generator with full load current at 200 kW, 480 volts, 60 Hz of 300 amps:

• Cummins Power, Application Engineering Manuals
  • Liquid Cooled Generator Application Manual [PDF - 10Mb]
  • Paralleling Application Manual [PDF - 10Mb]