winnie,
You can not have a phase angle difference on a single conductor. Yes the current flowing in the C-B winding will be at a different phase angle from the current in the C-A winding, but there is still only 1 current in the C phase conductor.
Go ahead and work out the math for the summation of these two winding currents at the C conductor and then tell me if it is substantially different from my method. I don't think it is worth the effort for sizing a conductor and overcurrent protective device. Because if we want to get down and dirty exact, we really need to look at the power factor of each load before we try to perform vector addition.
