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sixspeed 11-06-2012 10:52 PM

Wiring methods: help understanding inductive heating problems
I've come across a wiring diagram that is described to be an INCORRECT way of wiring a 3-way lighting circuit:

(Diagram 1)

Regarding the above diagram, the book cites violations of the NEC: 300.3(B), 300.20(A), and 404.2(A). Basically the problems in the example are: all conductors of the same circuit are NOT being run in the same cable or raceway; and if within metal enclosures (including armored cable), the phase (hot) conductor is NOT being grouped with the grounded (neutral) conductor.

So if I understand the phenomenon of inductive heating correctly, the problem could occur between points A-B, B-C from the hot conductor, and from C-E, and E-A from the neutral conductor, since between all those points the hot and neutral of the lighting load circuit are not together in the same cable assembly to "cancel" out the electromagnetic effects of alternating current that would cause inductive heating. (Assuming no load on receptacle E.)

Now, refer to the following diagram of a switch loop:

(Diagram 2)

How is it that the switch loop is given an exemption from rule 404.2(A)?
What is electrically unique in the switch loop circuit that would prevent inductive heating on the switch loop portion, since the neutral current of the circuit is not "grouped" with PART OF (the switch loop part of) the hot conductor to cancel electromagnetic effects in that section. It seems the switch loop is like the section A-B-C in the first diagram, where the hot conductor is physically run separately from the neutral conductor of the circuit. Does alternating current electromagnetically "cancel" itself out if the single phase conductor is "folded 180 degrees upon itself" (as in the switch loop)?

mpoulton 11-06-2012 11:52 PM


Originally Posted by sixspeed (Post 1046398)
Does alternating current electromagnetically "cancel" itself out if the single phase conductor is "folded 180 degrees upon itself" (as in the switch loop)?

Yes, it sure does. That is electrically identical to the hot and neutral that supply a circuit, in this context. The circuit is just one big loop. You're focusing too much on the labels "hot" and "neutral". That's not what's important. What matters is that the net current through any single bundle of conductors must be zero. Every current carrying conductor must have a counterpart (or, in the case of 3-phase loads, two counterparts) that carries the return current in the opposite direction in the same raceway or cable.

sixspeed 11-07-2012 11:25 PM

Ok. I have an existing traditional 3-way lighting circuit, as shown in this diagram below:

The line power comes to the switch on the right, and the lighting load is located on the circuit between the 2 switches. The common of the left-side switch feeds the switched hot to the lamp; however the lamp neutral goes back to the right-side switch box, where the line originates. I understand that this set-up does not cause inductance because one of the traveler conductors carries current.

There is a type of dimmer switch that is essentially a single-pole switch, but "communicates" via powerline or RF with other such type dimmers to effectively create 3-way or multi-way switching of a common load. These dimmers require line hot and neutral to function. Would it be acceptable to use the existing 3-way circuit wiring, then repurpose the traveler wires to extend hot (H) and neutral (N) to the left-side switch box? See diagram:

The left-hand dimmer switch is wired to the load (L) and acts as the master switch. The right-hand switch becomes an "accessory" dimmer switch with no load attached, but is "linked to" and communicates digitally with the master switch to turn on and off the load. In this scenario, the traveler conductor repurposed as neutral between the two switches also carries current, albeit just the relatively minuscule amount that the left-hand switch's internal electronics use. Is this any cause for concern with inductance since now there is an odd number of current carrying conductors in the cables between the switches?

mpoulton 11-08-2012 12:00 AM

Look at it this way: Is there any net current flow through any bundle of conductors? If the answer is no, then you're fine. If the answer is yes, then you're not. Kirchhoff's Current Law (and the physics behind it) makes it impossible for there to be any net current flow through the collective set of all conductors that join to a single node. So if all the wires that go to a single device are in the same cable or conduit, then it's impossible for there to be any net current, and you are guaranteed not to violate this code rule. In order to violate the rule, there must be a "loop" in the circuit somewhere.

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