sparklee,
Your first diagram is a single phase 120/240V single phase system. If you had a load of 1500W on line A and 1700W on line B, then you would have 1500/120= 12.5A on line A, 1700/120 = 14.17A on line B, and (1700 - 1500)/120 = 1.67A on the neutral.
Your second diagram is a three phase delta to wye transformer. If you just use X1, X0, and X2, and ignore X3 to supply your loads then it is exactly the system that I believe that you were talking about in your first post. In this case, if you had 1500W connected from X1 to X0, and 1700W connected from X2 to X0, you would still have 12.5A flowing through X1, 14.17A flowing through X2, but now you would have 13.5A on the neutral, as calculated by Bob and JBD at the top of the thread. Note that Bob and JBD used different calculation methods, and got answers that were not exactly the same, but this was because of differences in rounding; if you follow either method and don't round off, you will get the exact same result.
-Jon
