P.S. When you work through the above link, you will be able to answer your own question (A=10A,B=5A,C=15A, what does N=?) After you do this, try answering the following:
Given a three wire feed from a 208V/120 wye connected supply, with phase A, phase B, and neutral (no phase C). 15A is flowing on phase A. What current flow on phase B will minimize the neutral current? What will this minimum neutral current be? Assume all resistive line to neutral loads (this lets you off the hook for phase angles, and lets you assume 0 and 120 degrees)
-Jon
