How to calculate the power requirements of a fan motor?
Impossible to say. The spec mentions a 15 A circuit, but that's probably startup current; a 120 V/15 A fan would consume 1800 W, and would probably need a fan itself. The 15 A circuit refers to the fuses. Lower rated fuses might trip when switching the fan on.  

If the fan has a 15 A startup current the 50 VA transformer, that's about 0.4 A won't probably be sufficient.