As a private practice EE I have run into the same difficulty. As a rule you are not going to be able to get the utility to tell you what the available fault current is on the primary side of the transformer for the usual commercial projects. They are too busy to take the time to give you an accurate number on an issue that has little bearing on their operation
In reality however the biggest determinant of fault current delivered to a point is the transformer kva and impedance. The run of cable also contributes to lessening of the fault current delivered to a point.
That leaves being able to determine what the KVA and impedance of the transformer that will be utilized. In the case where you may be utilizing an existing transformer
if you can determine the KVA, you can get an approximation of the impedance from the IEEE BUFF book. The issue you run into is that the actual values can bounce within a
range of values. Worst case is a minimum value impedance.
On new builds, there is a question of what size transformer the utility is going to use on the load. As you have probably noticed, the utilities do not size their units the way we do.
What I have done is call the utility and give them my NEC Code calculated demand and let them tell me based on their experience with that particular occupancy/building what size transformer that they are going to put in. Hopefully you will be able to get a more accurate impedance from them as well and not a guestimate.
Here is a quick and dirty method for determining fault current from a three phase
transformer.
xfmr fla x 100/%impedance of xfmr
Of course this neglects issues such as the impedance of your service conductors and possible motor contribution. But if your run of conductor is relatively short and your motors are small, it probably will
get you an idea of what rating gear you need to be buying.
At this time the AHJ's are sporadically enforcing this provision of the NEC in my part of the world anyway. I've had some tell me that they want to see it, others I believe don't even know what it is. As a rule I do it on all my risers, so I can size the gear right.
