Quote Originally Posted by electrofelon View Post
A typical 75 kva 480 to 120/208 has an impedance between 3.5 and 5.5 - at least every one that I have seen. I am not saying there are zero exceptions, but very likely AIC not an issue. Agree though, always check transformer.
Keep in mind that the fault current calculation tool hosted on this website, makes the assumption that the primary voltage and primary fault current are infinite. Obviously they are not, but the function takes off to a cruising altitude (i.e. asymptote) as these two parameters get large, and making that assumption is practical for a worst case scenario. If this is a medium voltage to utilization voltage transformer, it is a good assumption. But if it is a utilization voltage to utilization voltage transformer, it is probably a lot more fault current than there really is.


Here are the details of the complete formula, where you can apply primary voltage and primary side available fault current (if known):
http://forums.mikeholt.com/showthread.php?t=171709