A hire generator arrives with a rating in kilovolt amperes and a test sheet. The price of the connection depends on three numbers that the rating does not state directly. Those are the full load current, the real power output and the fault current. All three come from the nameplate.
The question
A 200 kVA standby set runs at 400 V three phase. The rated power factor is 0.8 and the subtransient reactance is 12 %. The calculation gives the full load current, the real power and the fault current.
The inputs
- Rating: 200 kVA
- Phases: 3
- Voltage: 400 V
- Power factor: 0.8
- Subtransient reactance: 12 %
The power factor is the rated figure for the alternator, not the power factor of the site load. The subtransient reactance comes from the machine test sheet. Where no test sheet exists, the tool defaults to 15 %.
The clause
Cl 7.3 of AS/NZS 3000:2018 covers generating sets as an installation supply. The clause drives the changeover arrangement, the earthing and the protection. The full load current sets the rating of the changeover switch, the generator cable and the protective device.
The steps
The tool takes three steps from the five inputs.
First it divides the rating by the voltage and the square root of three. 200 kVA at 400 V gives a full load current of 288.68 A.
Second it multiplies the rating by the power factor. 200 kVA at 0.8 gives 160 kW of real power. That is the load the engine carries. Compare the site load against that figure, not against the kVA rating.
Third it divides the full load current by the per unit subtransient reactance. At 12 % the per unit figure is 0.12, so the initial fault current is 2,405.63 A.
The result
- Full load current: 288.68 A
- Real power: 160 kW
- Fault current: 2,405.63 A
The fault current carries a condition that the tool states with every result. 2,405.63 A is the initial symmetrical value taken from the subtransient reactance. It ignores the decay to the transient and synchronous values. It also ignores any excitation limit in the control system. The current a few cycles into the fault is much lower.
That decay matters for the protection. A generator settles to a small multiple of its full load current. A device graded for a network fault level may then fail to trip. Grade the protection for the generator source as well as the transformer source. Take the changeover position into account.
What the numbers price
288.68 A sets the line items. The changeover switch takes a rating at or above it. The generator cable takes 288.68 A as the design current in the cable size tool. Add the route length and the installation method there. The next standard device rating above 288.68 A sets the protection, subject to the coordination check with the cable.
160 kW is the check against the site load. A demand of 180 kW will not run on this set. The 200 kVA rating looks larger than the load. The power factor is the reason.
2,405.63 A is the figure for the fault withstand check on the generator cable. It also starts any arc flash assessment at the board with the set as the source.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Generator sizing for a 150 kVA holiday park, Three‑phase power: kVA to amps at 400 V and Maximum demand and diversity.