Rated full load current? another equation first? Not the breaker rating?
If you work in KVA or MVA, it doesn't matter which side of the transformer you are on. Yes, you can calculate current first by choosing which side of transformer you are on.
Say 500KVA @ 208V 3-ph current is 1388A, and short circuit is 1388/0.027 = 51.4KAIC. You get to the same value either way. I always work in KVA, or MVA, but that's me.
A transformer, because it is almost all reactance, basically only will allow the amount of fault current through it based on its own short circuit capability. This is not absolute, but engineering wise so darn close it is a very accepted practice.
When you divide the transformer kVA or MVA, by the transformer impedance, you are calculating the maximum possible short circuit capability, regardless of the impedance ahead (HV side) of the transformer. Meaning; that for this example you have 500KVA/0.027 = 18.52MVAsc. It would not matter if you had a system ahead of the transformer with a 9999MVAsc capability, the most you would see on the LV side is 18.52MVAsc.
You simply calculate the sc current (KAIC) by dividing by the appropriate voltage (and sqrt 3 if 3 phase). I used 208V in my example.
Yes, there would be some reduction due to reactance in the cable connecting the transformer LV side to the Main breaker, but if this distance is short, the limitations would not likely reduce the fault current enough to allow a change in rating of the breaker.
i.e. without cable 51.4kA (at 208V) and with cable maybe 49kA? Still would require 65kA equipment. Keeping in mind these are the maximum.
Typically, using the actual utility value on the HV side of transformer will not be of much benefit because the system fault current level is going to be greater than the transformer capability, and therefore the maximum is what you may see. I say typically, but not every case, there are always exceptions.
Further, the KVA value used in the calc should be based on what size you need for your service, NOT what the utility provides. There have been a lot of jobs that go south on that issue. The utility will most likely provide a transformer that is up to half what you determine. See how it changes the numbers:
Determine 500KVA is needed, utility provided a 300KVA instead. 300KVA/.027 = 11.11MVA, and at 208V that is 30.84KA. 42KAIC equipment would work.
That is a huge reduction in short circuit current. Your cable and breaker are all sized for continuous current based on 500KVA, but if you size your KAIC rating on the 300KVA, you could fall short in the future. Say the transformer burns up so utility brings out a 500KVA, still meets your continuous current rating but your KAIC rating of equipment is now obsolete. 42KAIC now needs to be 65KAIC. The owner is screwed, and worse, he doesn't even know he has a potential disaster on his hands.
