After numerous go arounds with a utility reluctant to provide available fault current data, and given our reluctance to storm the place with pitchforks and tourches, we were able to extract the following information:
The facility in question is fed from a substation with 3- 40 MVA transformers, "477 conductor" and nominal voltages of 139kV/12.4kV.
I am assuming that "477 conductor" means 477MCM and the conductor is almost certainly aluminum. We measured the transmission distance to the substation at 4.5 miles.
Now, a quick back of the envelop calculation using a table impedance value for 500MCM AL (wich is what I have handy) and guessing about 8% impedance for the transformer and 1% for the source, I am comming up with about 3800A which seems a fairly reasonable start.
My question is, Does anyone know what a reasonable impedance range for a 40MVA transformer might be.
Any input would be appreciated.

Bob Ragsdale, PE

Assuming the primary BIL is 550, ANSI C57.12.10 says 9.0% without load tapchanging and 9.5% with.

Note that with three transformers, there are three possible system configurations (source impedances) right there (neglecting all the other possible system changes the utility may make.)

I believe you mean C57.12.10

Fixed on edit. Thanks.