This is a very interesting thread. I have been out of town doing a round of inspections in one of my communities, if I had of had acces to this forum I would have jumped in.
Let me take a stab at this...
Given... a three phase 120/208 volt 4 wire system (wye) system. What we have is...
Three separate sources whose instantaneous voltage levels are all 120 degree out of phase relative to each other. One end of each source is tied together to form the star point of the wye configuration.
Given a 1000 watt resistive load connected across phase A and phase B, what is it the resistor is actually connected across?
It is connected across two voltage sources connected in series, namely... the a phase coil and the b phase coil.
But what if any phase difference exists across the resistive load? By definition "phase difference" must describe the difference in instaneous voltage levels of "two" voltage sources. As such, the term "phase difference" can not apply here as the resistive load sees only one voltage source and not two sources.
What our resistive load sees is one voltage source whose "steady state" voltage is equal to 208 volts. The 208 volts is the sum of the instaneous voltage of the two sources connected in series.
If we look further at the two seried sources that forms the voltage source our resistive load sees, what we see is a difference in the rate of change of the voltage potential in one source relative to the other source of 60 degrees for a total of 120 degrees acros the pair.
As such... the only "phase difference" that exists in this circuit is the difference between (Phase A to Neutral) relative to (Phase B to Neutral).
I hope I have explained this in way that helps the thread.
[This message has been edited by Rick Kelly (edited 03-11-2006).]
