The short answer is that you can't. The kVA and voltage of the transformer will give you its rated current.
If you want to know how that translates in to available (mechanical) horsepower you need to know the power factor and efficiency of the load(s).
The OP opened a new thread on the same subject, and that one has more comments than this one. So I am closing this thread, to prevent duplicate discussions. The new thread is here: http://forums.mikeholt.com/showthread.php?t=139196