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I recently learned from this community about the importance of keeping currents balanced in each cable. In other words, current in one direction should be canceled by the same amount of current flowing in the opposite direction in the same cable in order to reduce inductive heating and (possibly to cut down on unwanted rf interference). I think it is a useful rule for thinking through switching-circuits, but I just wanted to run my scenario by all of you to see if you agree with my conclusions. My question is, should the neutral conductors from the same branch circuit always be tied together in each box, or is it better to do it the way I show in the following situations?

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I have three boxes: two of them in the garage, one in the laundry room. My homerun comes into the box on the right [C], where the Hot (H) pigtails to feed a single pole switch for the walk-in door light, a three-way switch for the garage lighting, and also ties into the black wire of a 2-wire cable running to the laundry room switch box [A].

The traveler terminal screws of the three-way switch at [C] are connected to the black and red wires of a 3-wire cable running to a four-way switch box [B]. The white neutral wires (N) all tie together in this box because they’ll each have to have the return path back through the Homerun neutral here. So the 3-wire cable carries two travelers and a neutral (TTN).

In box B, the black and red tie to the bottom two screws of a four-way switch. The white (N) ties together with the white from a second 3-wire cable leaving the box and running to box A. The black and red in that second cable tie to the upper two terminals of the four-way. So the second 3-wire cable also carries two travelers and a neutral (TTN) to box A.

In box A the HN 2-wire supplies a single pole switch to control the laundry room light. The black wire hits one terminal of the switch, while the other terminal is connected to the black wire of a second 2-wire (SN) running to the light. These two white (N) wires tie together in this box, but in keeping with the balanced current rule, I’ve kept them separate from others.

Also in box A the TTN 3-wire cable enters from box B, with the black and red wires tied to the traveler terminals of a three-way switch. The black common screw of the three way is connected to the black wire (S) of a third 2-wire cable running to the garage lighting. The white wire to the light ties together with the white in the TTN 3-wire cable.

Questions I have about this arrangement are:

  • Is this the standard way of wiring multi-gang switch boxes?
  • How important is it to make these distinctions since the neutrals are all on the same branch circuit?
  • Have you noticed any reduction in radio interference from fluorescent lighting by carefully balancing currents this way?
  • @ThreePhaseEel: Why did you remove the [balanced-current] tag? One of the things OP is trying to do is insure that current supplied via a hot wire returns only via a neutral wire in the same cable. – A. I. Breveleri May 8 '18 at 22:44
  • @A.I.Breveleri -- I'm not sure if that tag would provide anything useful to help folks find questions they wouldn't be able to find anyway – ThreePhaseEel May 8 '18 at 23:37
  • @ThreePhaseEel: I defer to your judgement. - That tag doesn't seem to be used enough to form a worthwhile category. – A. I. Breveleri May 9 '18 at 13:17
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    That is an excellent wiring diagram. It is perfectly clear and precisely illustrates the question. There is nothing extra and nothing missing. - You obviously have a talent for teaching. – A. I. Breveleri May 9 '18 at 13:29
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Nice drawing. Very clear and looks correct. It would be a mistake to tie the two neutral wires in box A. This would create a circuit loop and a fire risk from heating when one circuit is on and the other is off. There may also be hum if you use switches or lights that include electronic circuits such as fluorescent ballasts. It's OK to wire this up as drawn.

Green/bare grounding wires should be treated in the same way. If there are two separate grounding conductors feeding box A (e.g., NM cable) then they should be tied separately as drawn for the two neutrals. If you feed box A with two grounding wires it is best not to use a metal junction box. If junction box A is metal it needs to be bonded to one of these grounding wires and will thereby be bonded to both switches. In that case you can't avoid a ground loop. This would not be a safety hazard as these wire carry no continuous current but it may cause hum as noted above. So use a non-metal box and run NM cable or individual wires in NM conduit.

Better yet, remove the standalone A-to-C run by running HTTN(G) from box A to B as well as from box B to C. Then there are no neutral or ground loops to worry about and you are free to choose the most convenient wiring method.

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    It's not quite as critical as grounds. If the ground wires form a circle, that's only a problem if you're flying an airplane. Whereas a neutral having two paths to follow is BAD. – Harper May 8 '18 at 20:22
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    Loops in mains EGCs aren't nearly as critical -- they're unavoidable in metallic conduit or metal-sheathed cable systems anyway! – ThreePhaseEel May 9 '18 at 2:35
  • @Stanwood, Thanks for the answer and mentions about the hum created. You talk about "loops" and I hadn't thought of it that way, but rather as parallel paths which would divide the current. So, are loops what create the problem of radio interference by becoming a sort of transmitting antennae? I also am surprised about your EGC comments. The others said they are not as critical (and frankly, I've always heard it was a good idea to always tie the EGC's together) but I'm interested to hear more about this. Have you ever dealt with sound problems and EGC loops turned out to be the culprit? – Nicknamednick May 10 '18 at 9:49
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    The ground loop concern is minor. I wouldn’t have mentioned it except that the question asked about how to reduce radio interference. In general we want to eliminate loops. In the case of the neutral it’s a safety issue. For the grounding system it would have to pick up a voltage difference between the two paths inductively so the impact is more subtle. Overall, if you route the circuit with a common N and G (HTTNG passing from C to B to A) that would be ideal. – Stanwood May 10 '18 at 13:08
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    @ThreePhaseEel I agree the ground loop on a lighting circuit is not so critical. That’s what I tried to express in the answer. I don’t agree that loops are unavoidable in metal conduit. If the underlying circuits map onto a tree diagram then there are no loops. Also, code does not demand a ground loop: even with metal conduit, even if the current-carrying wire topology forms a loop. The metal parts need only a single approved bond into the EGC. We can break the loop in several places: e.g. use a NM box or a NM conduit transition or bushing. – Stanwood May 10 '18 at 13:37
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Correct. Once the neutrals go their separate ways, they must not reunite, because that would form a loop, giving neutral current two potential paths. That would imbalance currents in each cable, and cause eddy current heating, cable vibration (and soon, damage) and all sorts of mischief.

Another way to solve that is to use /4 or /2/2 cable, where the wires would be H-T-T-N(-G).

Electrical is a tree diagram.

Another way to visualize that is if you drew the diagram out, with all non-grounds, but without boxes -- and used the "paint pot" tool on the corner -- the paint should fill everywhere. Wires on a diagram should not enclose an area, except within their own cable.

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    You may want to clarify that an HTTN run would go from box A to B as well as box B to C. Saves some Cu and may prove easier to pull then a separate A to C run. – Stanwood May 8 '18 at 21:24
  • @Harper: by "/2/2 cable" do you mean two 2-wire cables bundled closely together to cancel induction or something else? I do not recognize that designation. Thanks! – Nicknamednick May 21 '18 at 20:59
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    No, I mean /2/2 cable. That is a thing. Colors are black white red white-red-stripe. /4 would also work. You can't cancel induction by bundling two cables, they will vibrate. – Harper May 21 '18 at 21:39

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