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How this is calculated#

The tool works from the kVA rating on the generator nameplate. It gives the full load current, the real power and the fault current at the terminals. The steps are plain circuit arithmetic. The tool reads no Standard table.

1. Apparent power. You enter the rating in kVA. The tool converts it to VA.
2. Full load current. On three phases the tool divides the VA by √3 times the line voltage. On one phase it divides by the voltage.
3. Real power. It multiplies the kVA by the power factor you enter. The result is in kW.
4. Fault current. It divides the full load current by the subtransient reactance, as a fraction. This is the first-cycle symmetrical value.

Worked example#

A 200 kVA set at 400 V on three phases has a power factor of 0.8. Its subtransient reactance is 15 %. The full load current is 288.68 A. The real power is 160 kW. The fault current is 1,924.5 A.

Limits#

• The fault current is the initial symmetrical value only. It ignores the decay to the transient and synchronous values.
• It ignores any excitation limit. A few cycles in, the real current is much lower.
• The tool does not size the generator for motor starting or step loads.
• It does not size the cable or the protective device. Use the cable size calculator for the cable.
• The subtransient reactance comes from the maker data sheet. No default fits every set.

See also: Generator fault current from the subtransient reactance, Generator full load current from kVA and the fault level calculator.