I really appreciate the discussion on this thread. Wow, it took a little mental leverage to force the old notions out of my mind, but I see how the 240 with center tapped transformer works now. I will tell you how my incorrect notion has been reinforced all this time. In my work, I do not deal with building power, we have electricians for that. I only troubleshoot "wall powered" equipment, 95% of which is 120 volt. I do have a handful of 240 volt devices, however, but I still do not fool with building circuits. One device in particular that is 240 powered is a high temperature distillation apparatus, the heater elements driven by 240 volts at 5 amps. Traditionally to troubleshoot the temperature control system when it fails, we check voltage from chassis to each power terminal (neutral to both hots) and then from terminal to terminal...and if the meter reads 120 from neutral to both hots and 240 across, we conclude that the element is open when it isn't heating.
In my mind, I always concluded that neutral/ground was PART of the circuit, just as it is with 120 volts. I had never been told differently, so I perpetuated this thought. And since the two 120 volt "hots" were connected opposite to the load, in order for there to be any potential between them I had to see them as out of phase with each other. But my mistake was defining the potential with respect to neutral, as I am accustomed to doing with 120 volt circuits. I never thought of the center tap of the transformer permitting this to work.
And I see now how neutral to ground permits all three breaker panel supplies to share the neutral connection. The danger that Scaedwian was referring too, and I think Mark R touched upon, is trying to supply power to a device by paralleling two 120 volt hot connections from different circuits. I can see that this would cause a dead 240 volt short circuit which would be a bad, bad thing. DO NOT TRY THIS AT HOME.
I do have ONE MORE point of confusion, and this caught my eye on the ALL ABOUT CIRCUITS single phase power tutorial:
The astute observer will note that the neutral wire only has to carry the difference of current between the two loads back to the source. In the above case, with perfectly “balanced” loads consuming equal amounts of power, the neutral wire carries zero current.
OK, this is my last question, but it is very critical in understanding this.
The all about circuits tutorial offers the explanation that the two 120 volt branches in your home are IN EFFECT a series load on the 240 transformer. The neutral connection is provided to prevent an open circuit from shutting down power to everything in case of a failure in one branch. It provides an either/or path.
So, we are defining the neutral connection as having zero current, because in effect even the two 120 volt branches work together as a series load would off the 240 secondary. But in order for the current to be zero, the load has to be balanced if I read this correctly. But that will never be!!!! considering that if I plug in even so much as a night light on one branch with all circuits open on the other, wouldn't the load become imbalanced, thus the neutral would have a detectable current flow, am I not correct???
I can visualize in my mind with a perefectly balanced load on each branch, how the current at neutral would cancel out. (Am I not correct that the direction of flow would be out of phase 180 degrees on the neutral, and this is why it becomes nil? But if one half of the transformer secondary is open circuit, with no current flow, then the other branch would have no opposing current at neutral to cancel out with, correct? So wouldn't this be a dangerous scenario considering we all consider neutral to have no potential. I might think twice in the future before touching a neutral wire!
But obviously, this is not the case, something in my understanding is amiss!!!....so I would appreciate the final piece to the puzzle that answers this and I'll be satisfied.