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A motor has two overcurrent devices. An overload and a ground-fault short-circuit OCPD. What exactly is tripping? If it trips as soon as it starts then your ground-fault short-circuit is not set properly. Is it an AC motor?
If you know the power factor then you divide into the KW rating which in this case is 25 KW. If you don't know the power factor then assume that it is unity. Your KVA would be the same, 25 KVA. That full load current value is probably obtained from the NEC tables in article 430.
Yes I would have thought that the KVA rating of the transformer is sort of a maximum. If the secondary is shorted the voltage would be almost zero but the current would be very high. If it were possible to determine the exact voltage even at milllivolts or microvolts and then multiply it by the short circuit curent which would be on the order of thousands of amps then you would come up with the rated KVA value. But the primary voltage would not be zero because it is isolated. I would've thought that the primary current would be rated current which in this case is the maximum. I wish there was some technical material that specifically addresses the subject of short circuited secondaries with rated voltage applied to the primary.
A measuring CT would be used for a watt meter at a service. It would determine the amount of current flowing through the wire. A protecting CT would be used with ground fault protection relays. The three phase wires pass through the donut(CT) and then it detects an imbalance among the phases. For example if there's a fault the CT would sense it and then activate the relay which will then trip the circuit breaker.
Since you already know the ratings of the generator I would refer you to article 445 of the National Electrical Code. Section 445.13 contains the rules for sizing the conductors. 80A X 1.15 = 92 A. If you look at NEC Table 310.16 the conductor size would be No. 3 AWG at 75'C.
A tertiary delta winding transformer would be used for the following:
1) To limit the fault level on the LV
system by subdividing the infeed that is, double secondary transformers.2) The interconnection of several
power systems operating at different supply voltages.3) The regulation of system voltage
and of reactive power by means of a synchronous capacitor connected to the
terminals of one winding.It is desirable that a
three-phase transformer should have one set of three-phase windings connected
in delta thus providing a low-impedance path for third-harmonic currents. The
presence of a delta connected winding also allows current to circulate around
the delta in the event of unbalance in the loading between phases, so that this
unbalance is reduced and not so greatly fed back through the system.
