Originally Posted by Winepress
Interesting concept. So if I have L1 drawing 20 amps and L2 drawing 20 amps, I do not need a neutral as you state the neutral current is zero. What is the return path for L1 and L2?
Yes, if the loads on L1 and L2 are exactly
the same, you do not need a neutral conductor: the current flows in one hot lead, through the L1 load, then the L2 load, and out the other hot lead. It is essentially a series circuit at that point.
However, the loads on the two lines are never really exactly
the same, so you do need a neutral connection, and the current in it will be the difference between L1 and L2. By the time I get to the end of this rather length discourse (sorry about that!) you'll see that having a neutral connection is absolutely vital, even if it isn't used in some rare situations.
For the following discussion, I'm going to pretend that it's a DC circuit, and current is always flowing in the same direction. In reality, it's an AC circuit, and current flow is reversing 60 times a second, but it works the same way, it's just half the time the current is flowing in the opposite direction of my arrows. But the concepts still hold, and it's easier to ignore the reversing current and only look at it during the peak of one half cycle.
Consider the situation when when only L1 is loaded:
Current is flowing in on Line A, through Load 1, and out the neutral. Load 2 is off, so no current is flowing through that branch. I'm guessing you have no trouble grasping this, it's pretty straightforward.
Now consider the situation when when only L2 is loaded:
We're on the same half cycle, so the polarity of line B is the opposite of Line A, so current is flowing in on the neutral, through Load 2, and out Line B. Load 1 is off, so no current is flowing through that branch. This is really the same situation as the previous case, but because Line B has the opposite polarity, the current is actually flowing in the opposite direction than you would expect.
Finally consider the situation when when both lines are equally loaded:
Current is flowing in on Line A, through Load 1, then the same current is flowing through Load 2, and out Line B. No current is flowing through the neutral. I'll admit, it's a difficult concept to grasp, but that's the way it works.
While the pictures are drawn with 50 amp loads, the same situation applies if the loads are 20 amps each, or even 1 amp each. As long as they are the same current draw, the same electrons flow through one load first, then the other, and not through the neutral.
Now, what happens if Load 1 and Load 2 are not the same? (I drew up these pictures a while ago for a similar discussion, but I didn't make one to cover this case, and I'm too lazy to try and re-create them now.
Lets say Load 1 is 30 amps, and Load 2 is 20 amps. 30 amps flows in Line A and through Load 1. But then it hits Load 2 which is only drawing 20 amps: so 20 of those 30 amps flows through Load 2 and out Line B. Where does the other 10 amps go? It goes out the neutral line. As long as Line A and Line B have the opposite polarity, which they should in a proper service, the neutral does in fact carry the difference between the two hot lines. If Load 1 were 20 amps and Load 2 were 30 amps (swap the previous set of loads) then there would be 20 amps going in Line A, 30 amps going out Line B, and 10 amps going in the neutral (opposite direction from the previous set of loads.)
So, the neutral is still needed, even if it sometimes has no current. In fact, it's critical: one of the most disastrous wiring faults for an RV is an open neutral. Let's stick with the 30 amp Load 1 and 20 amp Load 2. At this point it starts to get complicated, using Ohms Law and Kirchoff's law, but I'll try to keep it simple. We can figure out the basic resistance of each load: 120 volts divided by 30 amps gives 4 Ohms for Load 1, while 120 volts divided by 20 amps gives 6 Ohms for Load 2. If the neutral wire is broken, we basically have a 240 volt series circuit of 4 + 6 = 10 Ohms. 240 volts divided by 10 ohms gives 24 amps of current actually flowing through the series circuit (remember, the neutral is broken.) This means 24 amps times 4 Ohms gives 96 volts across Load 1, and 24 amps times 6 Ohms gives 144 volts across Load 2. That's bad news! With an open neutral, you don't have the neutral connection to balance the loads, so if the loads on the two hot leads are not exactly the same, the voltage on one hot lead will rise, while the voltage on the other hot lead will drop by the same amount. Taken to extremes, with one line heavily loaded and the other line lightly loaded, one of the loads can see voltages approaching 240 volts.
Originally Posted by LadyFitz...
The result was 120v over each phase and 120v across the two legs instead 240v.
Very interesting story, I'm sure it was "exciting" while it was happening. However, I think you might have meant to say that there was 0v across the two legs instead of 240? I'm not sure how you would get 120v from each hot to neutral AND 120v between the two hots unless they figured out how to set up a 120v three phase delta with one of the neutrals being the third hot. (Or is this what they actually did???)
Thanks for the clarification/confirmation on the 120/208 wiring, I was wondering how well a coach designed for 120/240 would cope with it, I guess it varies.