Sparkee, yes simply use the normal three phase formulas whith one current equal to zero.
Bob,
Specifically, when using only two "hot" wires of a wye, the currents in each conductor are not 120° out of phase with each other, however, the line to neutral currents are. You can not have a phase difference if you only have two wires (what goes out must come back).
When using only two "hot" wires of a single phase 3 wire system. the currents in each conductor are not 180° out of phase with each other, however the line to neutral currents are.
Neutral currents never subtract from each other, they always add (the trick is you may be adding a negative number).
