Electrical theory question...

When properly wired off the same source the two 120 volt legs are in different cycles. The transformer is wired in series with neutral being in the center point of the two coils.
If the two were off different sources, there would be issues on the neutral.

The current in one 'leg' flows from its hot, through the load and back to the panel in your RV via its neutral. Since it is AC, the flow direction reverses 60 times per second. Meanwhile, the other leg does the same thing but with the flow traveling in the opposite direction to the first leg. From the panel in your RV to the loads, nothing really special happens but between the supply and your panel (i.e. your shore power cord), the neutral only carries the difference in current between the two legs. This is because the neutral is all connected together in your RV panel. Current returning from one leg's neutral just flows out the other leg's neutral without traveling back to the pedestal or power company.

So to your comment about 'canceling out', they do cancel out in that any opposing currents simply don't flow.

This...



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and this...

"If" the two hot lines are perfectly balanced, (that is to say both have identical current flowing) the actual neutral current will be zero, because the first hot current will flow to the second hot and vice versa. This is how you are able to have a neutral conductor that is rated for 50 amps. and yet have two hot conductors, EACH rated for 50 amps. If both hots are running 50 amps current EACH, the total current would be 100 amps. but none of it will flow in the neutral conductor because it will flow back and forth between the two hots. The absolute worst case would be if only one hot wire is loaded (with 50 amps) and the other had no load at all. This would cause 50 amps return current to flow in the neutral wire.

Actually this is just a hypothetical situation but demonstrates the theory.

The electrons do not flow both directions in the same wire at the same time. Somewhere there will be a common connection for the neutral. At that point the electrons will decide which direction to flow based on how much current each hot leg is carrying.

BB_TX wrote:
The electrons do not flow both directions in the same wire at the same time. Somewhere there will be a common connection for the neutral. At that point the electrons will decide which direction to flow based on how much current each hot leg is carrying.

That is correct.

What happens (as described by larry and wnjj pretty well, among others) is a consequence of Kirchoff's Current Law, which is a foundational priciple in electrical circuit analysis. It states that at every point in time the sum of currents flowing into a node in a circuit is zero--or, stated another way, that the current into the node and out of the node is exactly equal. (Current flowing into the node has an opposite sign from that flowing out of the node.)

Taking the neutral to be the 0V reference, when one leg is in its positive cycle the other is in its negative cycle, so the current due to one leg is going into the node at the neutral and the current due to the other leg is going out of that node. The difference of these two currents, which is just the amount they are unbalanced with respect to each other, is what flows through the neutral connection.

It might make more sense if you think of a plumbing analogy. If you have a tee in a pipe system, with some amount of water flowing into one side of the tee and the same amount flowing out the other side, the net flow through the third opening must be zero.

If you were to perfectly balance the loads so that the neutral current is zero, you can think of it as 2 loads in series, connected to a 240V supply.

Since perfectly balances loads are not practical, the neutral's job is to keep the point between these loads at 0V by providing or taking current as needed when the system is not balanced. This is why open neutrals can lead to serious damage. If the voltage between the "series" loads is not held at 0, it will move toward + or - 120V in proportion to any mismatch in loads. This means loads on one leg will be at voltages above 120V while the other will be below 120V.

Thanks to all for the information; it's answered my question well.

Ok, Draw yourself some plumbing

Two pumps, One sucks out of a tank, the other pumps back in

The lines from these pipes flow to devices that are powered by the flow of water (Turbines) and then to a common "T" connection.. The 3rd leg of the T connects back to the tank

For ease let's have the left hand pump the pressure pump Sucking water from the tank into the pipes, and the right and the suction pump, returning water to the tank.

Fire up both pumps..No water flows due to the turbines being turned off.

now we open one turbine, Let's make it the left one.. Water flows out and back on the "T" Line

Of course there is some pressure loss both ways due to friction with the pipe.

Now let's close that turbine off and open the other one.. again water flows.. This time FROM the tank, via the "T" line,through the turbine and back to the tank.

Now... Lets fire up BOTH turbines.

Water flows out to the left turbine the to and through the T, to the other turbine, then back to the tank.. NO WATER FLOWS IN THE "T" line

That which flows out on one side returns on the other

Oh, I mentioned pressure loss due to friction.. Well it is still there but now it is effectively halved (Since half of it is on both legs.

And that not only explains what is happening... but why as well.