In reworking outlets and switches in our 1960's California home to use AlumiConn connectors + copper pigtails where needed (instead of older wire nuts + Noalox), I came across a duplex wall box in the kitchen that includes incoming and outgoing wires on a circuit that's completely separate from the other wiring in the box. That particular separate circuit only includes outlets, and the wall box is upstream of anything else on the circuit. Disconnecting the wires in that wall box should have left no power whatsoever going to the rest of the circuit … or so I thought.
After turning off the breaker for the circuit, I started disconnecting the wire nuts connecting this pair of incoming and outgoing wires, and that's when I saw unexpected sparking between the two neutral wires. Nothing was plugged in to the outlets at the time. At first I thought I made a mistake identifying the breakers, but to the best of my abilities, I am certain that the rest of what's in the box is unrelated and separate. I finally proceeded to open up every downstream outlet on that circuit, disconnect all the wires, then test and retest using both a twin-lead tester (kind of like this one) and a continuity tester. All the outlets (superficially at least) appear to be wired in an ordinary manner, with incoming and an outgoing wires and pigtails to the receptacles, much like figure B in this diagram. However, testing reveals that when a certain other breaker in the house (operating lights elsewhere) is turned on, two of the wall boxes on my problem circuit exhibit the following behavior: there is current flowing in one of the neutrals going into the box. My twin-lead tester lights up when I connect it to the neutral and the ground, but not between neutral and hot, and not between hot and ground. Turning off the "other" breaker stops this behavior.
Researching this kind of thing leads me to conclude that the most likely cause is a shared neutral between my problem circuit and another circuit in the house. My best guess is that the wires feeding the two outlet boxes are coming from a junction elsewhere (in other words, that the wiring is not strictly from outlet to outlet as I expected, but that there's unidentified joint somehere), and in that junction, someone connected a neutral from the other circuit to the neutral(s) feeding those outlet boxes. I went into the attic to investigate, but the attic uses loose-fill insulation (so I basically can't see the wiring) and is not easy to navigate safely, so I gave up for now.
Questions: are my conclusions reasonable? What are other possible causes of this behavior?
(More info: there are 2 other things in that wall box, a switch and an outlet, each on separate breakers. The circuit with the outlets is a 3rd one. I am 99.9% certain there are no connections or dependencies between these 3 circuits. Testing was done with all 3 breakers off. The circuit that unexpectedly allows current on the neutrals mentioned above uses a 4th breaker.)
Best Answer
First, there is such a thing as a multi-wire branch circuit (MWBC), in which two hots intentionally and consensually shate a neutral. They are placed on opposite poles of service, which means only differential current flows through the neutral. This works for 2 hots in single (split) phase service, and up to 3 hots in 3-phase (208V) service which you will only find commercially or in NYC.
Neutrals must be carefully wired to be monogamous to its partner hot(s), to assure that the neutral carries no more current than its partner hot(s). After all, neutrals don't have breakers. If a neutral carried current for more than its intended hots, it could overload, and nothing would detect this. Borrowing a neutral is not allowed.
Another rule is that currents must be equal in every cable or conduit. This means no loops, wiring routes must be a strict "tree" topology. It also means the neutral will always be in the same cable with its partner hot, or hots in MWBC.
And that means MWBCs will be readily apparent in the service panel, because they will enter the box in a red-black-white cable. (or in the same conduit). When you see that, place both red and black gots on a 2-pole breaker, so that when you switch off one side, the other is also switched off. That satisfies a basic rule of MWBC.
At this point, shutting off that breaker must arrest all current flow in the neutral.
If it does not, then you have a "promiscuous neutral" problem that you should aggressively eliminate.
Grounds, on the other hand, are allowed to be promiscuous, since current only flows on them during fault conditions.