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Okay so this is a two part question. First Question is probably more of an ELI5 question. But I am a fairly experienced electrician (NOT AN ENGINEER mind you.) but I have a generally good working understanding of electrical theory, ohm's law, grounding, neutrals, etc. But I got asked this question today and didn't really have a great answer. I ask for the ELI5 explanation mainly because those are the explanations I have to give to the junior electricians all day.

To elaborate/illustrate a little though, say you have a stereotypical residential shared neutral circuit, properly wired; we use a two-pole, 120/240V 20A breaker in a 120/240 volt panel. Our hots are of course on different phases. for simplicity's sake we'll say the circuit is stupidly simple, and we have each hot go out to a single identical load, we'll say load A and load B, the same distance away from the panel. My wonderment is basically, why do the loads get nearly exactly 120volts? I mean there is still somewhat of a 240 volt circuit there, starting at one pole, going to load A, out of load A along the neutral, to wherever the neutrals branch off each other to the neutral terminal of load B, out of Load B along the hot and to the other pole of the breaker. (really wish I could draw this, lol). And to my understanding, current travels not just on the path of least resistance, but on multiple paths proportionate to the resistance of the paths available. So, hypothetically, though you've provided a neutral path for each load, why is there no voltage applied across the loads via the hots?

...Actually, In writing this question I feel like I may have generated my own answer: is it the resistance of the load of the other circuit that acts as a choke and causes the current to stay within the wiring of the intended circuits? Or something else?

Second Question! Why/how do the currents cancel eachother out on the neutral. What is happening inside that wire nut, (or other splice point) Are the electrons from one neutral coming down (I speak of this only as a snapshot of the circuit during a single moment in time. I know AC alternates.) from a load smashing into the electrons trying to move up toward the other load? What's happening inside the neutral bar of a 3 phase panel? is it just subatomic chaos in there?

Any elucidation offered is much appreciated?

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  • \$\begingroup\$ The transformer outside your house is a 120v center-tapped transformer. Just think of it like a DC split power supply, like used in audio, say (-36v,0,36v). The neutral is the ground, the -36/36 poles are the hots. If you go from -36 to +36, you get 72v, just like in your house if you go from "-120" to "+120", you get "+240v" \$\endgroup\$
    – dandavis
    Dec 14, 2020 at 20:50
  • \$\begingroup\$ No, yeah, that totally makes sense to me, transformers, especially single phase and how you can get additional voltage from taps that are further away from each other thus higher voltage is pretty understandable, especially when wired up to a single load or just voltage tested between each other. That's not really my questions though. As above and in lieu of rewriting my entire question, the mystery to me is on shared neutral circuits specifically. \$\endgroup\$ Dec 14, 2020 at 20:57
  • \$\begingroup\$ In residential wiring two phases 120V is opposite, 180 degrees. It means then one phase plus, another minus. If it is two symmetrical load on each phase, current on neutral is zero, because two currents run in opposite direction. \$\endgroup\$
    – user263983
    Dec 14, 2020 at 21:10
  • \$\begingroup\$ @dandavis: No, if HotA delivers 1 Amp, and HotB (opposite "phase") delivers 1 Amp, there will be no current in the Neutral, and there will be 240 volts between the Hots. At some instant, HotA will be +120 V, and HotB will be -120 V. \$\endgroup\$ Dec 14, 2020 at 21:14
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    \$\begingroup\$ @dandavis: Yes, if the loads on each Hot are equal. \$\endgroup\$ Dec 14, 2020 at 21:16

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To your first question: the pole transformer has a split secondary. It sorta acts like two 120V output transformers connected end-to-end, in that it enforces 120V between neutral and each hot.

So if, say, I went and turned on all the circuits in my house on one leg, and left all the others off, the voltage on each leg would be pretty close to 120V*. (And in that situation, the neutral current would probably be as high as it could ever get without faults in the various 240V appliances).

To your second question: because the two legs are out of phase, if they're equally loaded then any current flowing into the nut from one leg is matched by an equal current flowing out of the nut to the other. If the loads were perfectly matched, no neutral current would flow at all.

To your unnumbered third question: I assume you're talking about a panel that has three 120V legs and 208V leg-leg. In that case, three balanced loads, one on each leg, will sum up to zero current. Any imbalance will result in current, but since the legs are 120 degrees out of phase with each other, it's kinda complicated to figure out what the neutral current may be for any given combination (it can be done -- just make friends with Mr. Trigonometry and get to work).

* Actually, the loaded leg would have slightly lower voltage -- just close your eyes to that right now...

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