Travellers or switch loop in a separate raceway

I came acrioss some online articles recently that I want to understand better and may be relevant in the way I have installed my home wiring. It is about how all wires for a circuit needs to be housed in the same raceway, and that wires need to be always in pairs (hot and neutral together), and how not having the wires in pairs may lead to overheating and electromagnetic interference EMI especially if the raceway is metallic like EMT, BX, MC etc...

I don't quite understand it, or the science behind it, but I do have my own wiring done in ways that the wires are not always together in the same raceway, especially on three ways and switch loops. So I want to show two examples below and see if the pros can comment on whether there are potentially issues with NEC, or have EMI problems, or even if code compliance but still bad practice. In my house, most wiring are done in EMTs with metal boxes, there are some NMB, and a few MCs.

To keep the diagrams simple and less clutter, I am not showing any ground, but ground is there, either as a ground conductor in the NMB or EMTs acting as EGC.

First example is three way wiring. In this case I have power going into the ceiling box where the fixture is. I then have the hot going to the SW1, a pair of travelers to SW2, and a switched hot going back to the fixture. In this case there is no neutral conductor going to switch box 1, no neutral going from switch box 2 back to fixture.



(1) Is the lack of neutral from fixture to SW1 and from SW2 back to the fixture is bad thing in terms of heat inductance and EMI?
(2) I know NEC requires a neutral in switch boxes since I think 2012, so in newer wiring I did carried a neutral conductor to switch boxes, but I have no need for the neutral at all, so that extra neutral is always capped in the switch box. So that capped neutral does not make it a "pair" because there is no current flowing through it right?
(3) The pair of traveler wires in it's own raceway, is that OK? does it need to be in the same raceway as the hot and have the neutral included. In other words, is the preferred installation is to have a 12/4 from fixture to SW1 to SW2 back to the fixture?

The next example is where I may have many fixtures in a room. It could be several sets of lights, or fan/light combinations and there is a multi-gang switch box. The power goes to the first fixture, then I would feed a 12/2 to the 4 gang switch box with the constant hot and neutral. Again neutral is capped since not needed. From there I run a 12/4 back to the fixture, each conductor of the 12/4 carries one switched hot for each fixture, as shown in the diagram below.



Again, the switch legs in a 12/4 are totally separated from the 12/2 feed. There is no conductor with current flowing in an opposite direction, nothing is in pairs. Is this an issue? If so if the 12/4 is in an EMT is it an even bigger issue?

 

M -

Not a real electrical kind of guy.

But- I think the answer starts with basic electrical theory which says that moving electric charges produce an electric and magnetic field. If the hot and neutral are in the same conduit, with the electric currents therefore going in different directions, then the produced magnetic fields cancel each other out. That cancellation is good.

If the magnetic field wasn't cancelled out, then, because a changing magnetic field induces an electrical current, the changing magnetic field could case moving electric charges in the conduit, i.e., could cause current in the conduit. That could cause the conduit to overheat.

At least I think that’s the basic explanation. I think that’s why NEC requires both hot and neutral to be in the same conduit. Not absolutely sure though. Don’t the grounds have to be in the same conduit? I guess that’s for another reason though. Maybe to ensure integrity of the grounding?

Maybe someone else will opine.

 

The code requires all wiring to be contained in a common conduit or cable. (NEC 300.)
One reason is for over heating of wiring but that's more for commercial applications.

Wiring is contained within a single enclosure for troubleshooting and future modifications.
If you were to move out.... no one could tell how it was wired.
It could allow for several circuits to share the same neutral.

Your wiring will work... it's technically not dangerous... it's just not code compliant.

 

I thought the reason for requiring the neutral in switch boxes is newer smart switches need neutral. So when I take a neutral to a switch box then I just cap the neutral, but wouldn't that mean the neutral is doing nothing, and the singular hot conductor in that 12/2 is bad news?

 

Yes.... the newest code requires a neutral in the box.
It has nothing to do with current balancing or overheating.
The neutral is there for the future use of a smart switch that requires it.

 

Why not add another neutral wire from the switch box back up to fixture 1? To any extent that conduit presents a problem, wouldn’t that solve it? Seems like it would.

 

These two scenerios have been present in my house for years and it works electrically without me knowing this being code violation. But then again, I could use a green insulated conductor to pass hot and it will work electrically but incredibly dangerous for anyone to follow my work and a really bad idea. So I want to correct whatever is wrong either code wise, or if it's bad practice like EMI or heat inductance issues that were inferred when I read these articles.

In the first example, if I add a neutral from the fixture to SW1 and from SW2 back to fixture, it won't be sufficient because those two neutral legs are not doing anything being terminated in SW1 and SW2 right? I really need to carry a neutral from fixture to SW1 then along the traveler pairs to SW2 back to the fixture to complete the run I think.

Although I have seen so many implementation of three way with a lone 12/2 between the two 2W switches in books and in articles, with no neutral carried through, then they are all wrong?

In the second example again I need a neutral back probably, but the issue is the 12/2 and the 12/4 needs to be in the same raceway. There isn't a 12/6 cable so a conduit needs to be used?

 

As you've pointed out, there are lots of ways that work (as in the light goes on), but are not code compliant. Code is intended to ensure consistency and safety, and while many non-code-compliant installations work and may be reasonably safe, most codes are based on actual fires, electrocutions, or disasters that have happened in the past.

I'm sure someone with enough research could point to a handful of fires or other catastrophes that happened in the past due to separate conductors. Do I think your house is going to be one of them, most likely, no... but if you run the wiring in the way code specifies, we can all agree that there's a basically zero chance of any overheating or issues with unbalanced raceways.

Back to your first drawing, how did you even do that? Cables don't have a single wire, so it seems like a challenge to actually install that wiring in that way.

Your second drawing, conduit is one solution. But most residential electricians would run single 14/2 cables from each of the 3 switches to each light. 14/3 is used sparingly on most sites, and 14/4 is almost never used. (or 12ga of the same).


IMO, if you're going to continue doing electrical work, I would read up on standard practices. There's a lot of that isn't intuitive about the code and standard installations that are worthwhile to follow, both from a safety perspective as well as someone down the line looking at your work and not wanting redo it.

 

Well I didn't read Zorfdt's response yet - but it takes me so long to type I'll post this first and go read the good info. Here's what i wrote:

If conduit is not being used then I don’t see a problem with what you are showing in your diagrams. I don’t see why you would have to make any changes. I think the other guys will jump in if that’s wrong.

But if conduit is being used, then I think you don’t normally put cables in the conduit. You run individual conductors through the conduit. I think you are allowed run cables in certain cases through the conduit but fill capacity has to be considered and wet/dry location considerations. I think though generally you run individual conductors through the conduit. It is also easier to do it that way.

So, if conduit is being used in your diagrams, then you can always add a neutral conductor through any 2 points in the configuration, thereby paralleling hot and neutral. You can add the number of conductors needed between any 2 points, but fill capacity for the conduit has to be taken into consideration and calculated according to the NEC rules. You can increase the conduit diameter if fill capacity becomes exceeded.

In other words, if in your first diagram conduit is being used you would route the neutral as:

Panel neutral to Light Fixture Box-->Light Fixture Box to SW1 Box --> SW1 Box to SW2 Box -->SW2 Box to Light fixture Box-->Light Fixture Box to Light.

 

Forgot to say, if you do need to use conduit some time, you can get individual conductors that you can use in conduit at Home Depot (and I guess the other home centers also). They have them on spools, all different colors: red, black, white, green, etc. You just tell them the length and they’ll cut you a piece.

 

NEC 300.200 "Where conductors carrying alternating current are installed in ferrous metal enclosures or raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be grouped together."

The rule of thumb I was always told is you do not run one wire in a metal pipe. I suspect even the presence of another wire in a conduit is enough to disrupt the magnetic field induced by current carrying wire. None of this would apply to cables used these days.

 

Here is the thing. The house was built in the 1960s. Back then whoever wired it was using all EMT conduits. The house is still 60% EMTs and metal boxes today. The EMT conduits all goes from the panel up into the attic, then the conduits run to different areas of the house. Once a conduit reached that area, say a bedroom, the electrician put in a metal junction box in the attic and terminated the EMT conduit. Now let's say the room needs 1 closet light and 6 recessed lights and one fan, what they did was run an EMT conduit down next to the door for the switches. They have 1 hot and 4 switched hots (one switch for closet light one for recessed lights east and one for recessed lights west and one for the fan). They use solid THHN conductors for each switch loop. No neutral conductors were being sent to the switch box and back. Once the switched hot are back to the junction box, then they go from there to the individual fixtures with BX cables. That's how it was installed originally, It was to code I believe, except that there is no neutral conductor in the switch box.

Along the way, some owners have done some improvements. They added a wall sconce, and may be the fan became a fan with light, so they needed more switches to control these devices. So whoever did these modifications, probably 20 years later in the 1980s, the decided to run their own cables and not use the conduit (and I think I know why, 5 #12 solid THHN in a 1/2" EMT can be a bit difficult to pull new wires through even with a fish tape, if those solid conductors have kind of kinked up). So they just ran another set of 12/2 NMB from the junction down to the switch box, the 12/2 carries two new switched hots, one for the fan light and one for the sconce. Then the pick up the neutral at the junction box, and heads to the individual devices with NMB cables. So at this point, the wiring consists of some EMTs, some BXs, some NMBs. Between the junction box and the switch box is a 1/2" EMT conduit with the original hot with 4 switched hot, 5 THHN conductors, PLUS a 12/2 added later containing two additional switched hots.

It is my understanding that in this case, adding the NMB carrying two switched hots is an issue. There is a current imbalance and EMI problem, and it's an NEC violation because all these wires should have been grouped together, correct?

In addition, today's code NEC requires a neutral to be added to the switch box.

I suppose one way to fix this is to combine all of them into one conduit. That will be 5 original conductors (hot feed and 4 switched hots) plus two additional switched hots plus a neutral. That's a total of 8 conductors. That will be difficult to pull. I also believe the neutral cannot be just run to the switch box and capped, it needs to run back to the junction box to balance the current in the opposite direction right TRUE or FALSE? If true, there will be 9 conductors. I think NEC limits is 9 current carrying conductors in any size conduit before derating.

 

The first diagram I did not show "dead ended neutral" to make the diagram easier just like I didn't show the green ground conductor. The actual configuration presently look like this.



I actually do have a neutral in switch box 1 and switch box 2 because code requires it. But since my understanding was it was there for no reason other then to facilitate smart switches, I have no need for them so they are capped in the switch boxes.

It's been working but now from recent reading up on this topic, which I am still really fuzzy on, that 12/2 carrying the travelers under certain switching, will be a single current carrying conductor and needs a neutral to balance it so I really need to run a neutral from SW1 to SW2, AND when the neutral get back to the first light, the light should be connected to the neutral at the end of the run so the return current can follow the entire run?

 

I’m not one of the electrical gurus here so my questions may be off target.

Are you saying that in your living areas you have EMT coming down the wall surface from the attic, connected into a surface mounted multi-gang switch box on the wall? That seems unusual. I think usually the wiring is hidden in the walls and the switch boxes are recessed into the wall. Did something prevent them from doing it the standard way?

And how was the new wiring run which was not put in the conduit? Was that placed inside the wall?

I thought EMT wasn’t needed in unfinished attics, if NM cable was used. There are rules I think about where and how to fasten the NM cables in an attic – I believe. But I guess you are saying that individual conductors were run in the attic in your case. In that case, I think conduit then has to be used.

My house also was built in the 1960’s. NM cable was used and routed inside the walls, which I think is standard. I guess my question is: why are you stuck with conduit in the living area? I think new cables can be fished through walls, although sometimes it can get a little tricky and some patchwork might have to be done – I think.

 

Standard practices are very locale specific.

I am in south Florida and I have owned various properties over the last 30 years some to live in and some as rental investments. Most of the properties I have owned that were built up to 1980 were done with EMT conduits. Residential. Earlier than 1950 I ran into clothed cables with sheathing crumbles upon touch, and after 1980s I see mostly NMBs. So majority of homes I have are EMTs and only mixed in with Romex when later they do improvements.

The EMT example I gave is not run on the wall, they are behind walls, holes were drilled in the top plate and conduit connect to the metal box with set screw connectors. In the attic it's a 90 elbow to go where it needs to go. Sometimes to pass from one stud bay to another bay 6' over they ran the EMT up across and back down.

When I opened the multigang switch box I see both THNN conductors from the conduit and NMB sheathed conductors. That's always because the later improvement needed additional switching, but additional switching by adding cables not in the same raceway of the same circuit is a problem. My post's intent is to better understand the problem and hopefully the science behind the problem.

A lot of these improvements I found, when done by DIYers, and sometimes electrians are kind of scary. For example I noticed upon opening a bathroom wall someone ran a PVC conduit between two metal boxes, the PVC went up the attic and back down. I imagine it was an EMT straight across at one time (I can see the open KO holes on the boxes not even closed off), and someone wanted a recessed medicine cabinet that the conduit was in the way. Whoever ran that PVC didn't run a ground conductor between the two boxes, and therefore EGC continuity has been lost on that run.

 

added neutral wiring

I do remember now of seeing pictures of new construction where EMT was used inside the wall. Maybe they used EMT inside the walls because of a rodent problem in your area. I get mice in my walls sometimes and they like the intercom wires. Had to patch the wires several times. They haven’t gotten to my NM cables yet – I think.

The above picture is not good, just quick and dirty, but it’s what I was trying to say about adding a neutral wire if really necessary. Sounds like it is necessary by code, but it seems like some of the more knowledgeable people here wouldn’t be too concerned about it.

In your original picture above, it seems like it would have been trivial to add the neutral wire all through the conduit (like I tried to depict with the picture mod). I think that would be OK. I wonder why they didn’t do it. Maybe it’s a newer NEC requirement.

 

electromagnetic waves

M-I think you asked a good question IMHO about the science behind the NEC requirement for hot and neutral in same conduit. To me it sounds counterintuitive that more current carrying conductors would generate less heat in the conduit. But that magnetic field cancellation I mentioned, which came from some senior engineer, whose name or location I cannot remember, I think makes sense.

I have seen some good animations for electromagnetic wave propagation, but I can’t find them now (that figures). But I think the diagram above is similar to most diagrams on that subject. You can see that the blue arrows (vectors) representing the strength and direction of the magnetic field of the electromagnetic wave traveling in the direction of the black arrow, would be complemented with blue arrows in the exact opposite direction for the electromagnetic wave travelling back in the exact opposite direction of the black arrow. Thus, the magnetic field strength would sum to zero at every point. So, in that case the waves from hot and neutral together have zero magnetic field strength and thus would not induce current in the conduit, whereas the hot alone would.

I’m trying to find a really good source to confirm that explanation, but I haven’t found it so far.

But I guess if you think about it, the question really isn’t a fundamental physics question- I think. It really seems to be an engineering question. If I find a good reliable source to confirm the explanation I’ll post back.

 

If it's conduit everywhere, yes I can add the neutral without too much trouble. However, the issue is it is not conduit everywhere, what I have is a combination.

Imagine from the light to the switch SW1 is a conduit, and I have to guess at some point someone wants the three way switching so the wiring is added from SW1 to SW2 plus SW2 back to the light and those are NMB cables. For this reason I have some places I can add the neutral and some I could not do so easily.

Now, I did add a neutral from the light box to the switch SW1 because it was easy to do. But since hte neutral is not really doing anything, I capped it in the switch box. Now I think doing so does not really help anything unless I can carry the neutral from SW1 to SW2 back to the light, in other words to be along the entire run and back to the light.

 

Ah -OK, I thought that 3-way switch configuration already existed.

I don’t see why there would be a problem. I think you would just make sure a white wire for neutral is included in the added NM cable you would use for the travelers between SW1 and SW2. The same for the new cable from SW2 back to the fixture. You would just make sure a white wire for neutral is included in the cable.

In other words, if you need a white wire in the conduit, you just run a new one through the conduit. If you need a white wire where there is no conduit and you are using NM cable, you just make sure the cable includes a white wire.

 

Yes it IS pre-existing. The 3-way switch configuration exist today. I think there is a misunderstanding.

This is a house with multiple owners who have gone through multiple iterations over the years, but I am speculating here that originally, it was a single pole light like below. Power goes from the panel to the light via a conduit, a switch loop was sent to the wall switch box in conduit.

[[img]https://i.postimg.cc/nrHRnLjp/SW1992.jpg[/img]

Later, someone wanted the light to be wired as a 3-way. The three way wiring was added and it looked like this. The travelers to the second switch and from that switch back to the light were done in 12/2 NMB. Probably done a while ago, at least as old as when #12 NMB was still using white color sheathing.

[img]https://i.postimg.cc/FzDPzQ3c/SW2.jpg[/img]

Then at a later time, someone came along and tried to do something else, what I have no idea, but whatever was done, they messed up the three way wiring, so it no longer worked as a three way, but the first switch still work by itself, and the second switch was a mystery switch that no one knew what it was for.

Then I came along and bought the house. I opened up the switch boxes and figured out they incorrectly wired the switches, and I fixed it.

Then I read about the need to carry a neutral along with the travelers, about EMI and heat inductance etc...which prompted me to think that I need to add a neutral between SW1 and SW2 so that the neutral is carried all the way through the run. Like this:



Hence the reason to start this very thread because I basically have two questions.

(1) Is it necessary to add this neutral conductor because it means changing the cable between the two switch boxes from a 12/2 to a 12/3. It is NOT an easy change so I don't want to do it unless it is necessary.

(2) If it is necessary, then the neutral runs along the constant hot to SW1, along the travelers to SW2, and along the switched hot back to the light fixture box. Now, at the light, you have this neutral "twice". The neutral that comes from the panel, and the neutral from the end of the switched run. Which neutral should be connected to the light? I am thinking the one that is paired with the switched hot, or may be it doesn't matter. Does it?

I hope this makes it clear.

 

If, as you say it's not an easy change, I would not do it. You've got it working as you like for it to work. I think it's safe, though maybe it's just not as kosher a you'd like for it to be. But that's okay because you fully realize and understand that the wiring is a hodgepodge. But the bottom line is - as long as one neutral is not shared with another circuit, then I think it is perfectly okay.

However, if you do decide to change it - I would choose to connect the neutral wire that is paired with the hot to the light fixture...but it really doesn't matter because actually (in fact) neither of the neutral wires (if you did decide to change it) for this one circuit would not be shared with another circuit, therefore both those neutral wires are essentially one and-the-same return path neutral.

 

The answer is that NEC requires you to add the neutral, so yes you should, according to code, add the wire. But I think some of the knowledgeable folks here don’t think you have a dangerous situation in the least. It’s just that the preferred method would have been to include the neutral. So, I think the answer to “should you or shouldn’t you” – is just personal preference. I think I would just let it be, although I can be a fussbudget and would probably wrestle with the question myself – just as you are. But I think I would not worry about it.

However, in some instances I have pulled in a new cable by taping the new cable to the end of a current cable. I overlap them by several inches and tape them really good and as flat as possible. Then I go to the other end and pull in the old cable and the new comes along with it. But you do have to be careful that the taped connection doesn’t come apart in the wall.

I just pull very slowly and very gently and am ready to reverse the process and abort the operation if I feel a worrisome obstruction as I pull. I’ve done it several times. But whether it would be worth the trouble in your case – I don’t think it would be.

But I don’t understand the question about connecting the neutral in the fixture box. The whole point of running the neutral wire all through the circuit -if you were to do that - is because you want the neutral to be used in the conduit. If in the light fixture box, you connect the neutral from the panel directly to the light, then you would bypass all the new neutral wires that you would have added. So, you would have thrown away all your new work, although the light would work properly with the switches. But that wouldn’t make sense.

 

Duplicate of my last post #20 the image links were messed up and I couldn't edit it either. Somehow the "color" BB code got inserted everywhere and really messed things up.

Yes it IS pre-existing. The 3-way switch configuration exist today. I think there is a misunderstanding.

This is a house with multiple owners who have gone through multiple iterations over the years, but I am speculating here that originally, it was a single pole light like below. Power goes from the panel to the light via a conduit, a switch loop was sent to the wall switch box in conduit.



Later, someone wanted the light to be wired as a 3-way. The three way wiring was added and it looked like this. The travelers to the second switch and from that switch back to the light were done in 12/2 NMB. Probably done a while ago, at least as old as when #12 NMB was still using white color sheathing.



Then at a later time, someone came along and tried to do something else, what I have no idea, but whatever was done, they messed up the three way wiring, so it no longer worked as a three way, but the first switch still work by itself, and the second switch was a mystery switch that no one knew what it was for.

Then I came along and bought the house. I opened up the switch boxes and figured out they incorrectly wired the switches, and I fixed it.

Then I read about the need to carry a neutral along with the travelers, about EMI and heat inductance etc...which prompted me to think that I need to add a neutral between SW1 and SW2 so that the neutral is carried all the way through the run. Like this:



Hence the reason to start this very thread because I basically have two questions.

(1) Is it necessary to add this neutral conductor because it means changing the cable between the two switch boxes from a 12/2 to a 12/3. It is NOT an easy change so I don't want to do it unless it is necessary.

(2) If it is necessary, then the neutral runs along the constant hot to SW1, along the travelers to SW2, and along the switched hot back to the light fixture box. Now, at the light, you have this neutral "twice". The neutral that comes from the panel, and the neutral from the end of the switched run. Which neutral should be connected to the light? I am thinking the one that is paired with the switched hot, or may be it doesn't matter. Does it?

I hope this makes it clear.

 

I believe technically speaking, right now, I am incompliance with NEC in a sense that NEC requires a neutral in every switch box. I DO have a neutral in both SW1 and SW2. They are both capped.



HOWEVER, there is no neutral from SW1 to SW2. When the light is on, one of the two travelers between SW1 and SW2 is energized, the other is not. This is a single conductor in that cable and there is no return current running in opposite direction typically served by a hot/neutral pair. This is what I gathered from the articles I read about extra heat and EMI when the hot/neutral are not paired up. Which was the reason for my original post, is it necessary to run a neutral between SW1 and SW1, to provide a neutral path from the fixture box to SW1 to SW2 back to the fixture box, which will be the best practice way to do it? But I know I have seen x/2 run between three way switches all the time in older installations, so this current imbalance and EMI is probably not a significant issue.

Of course, if I do carry the neutral wiring all the way through as described above, it follows that the light fixture *SHOULD* be connected to this neutral conductor that went around SW1 and SW2 because this is what would provide the return current to run through that path to cancel out the switched hot legs. Connecting directly to the neutral that came from the panel by adding a splice here will not be the same, it will work electrically for sure, but it won't be the same in other aspects, is my thinking.

For me to connect between the two switch boxes, I have to re-pull a new /3 cable in the attic and down to each switch box because the switch boxes are on opposite ends of an open area. I have previously checked in the very low head room attic in this area that the hole drilled for the cable has to be enlarged to pass a clamp connector through, since I cannot free the connector at the switch box unless I break the plaster at the box to disengage the clamp connectors. Breaking plaster at the box and enlarging these two holes are challenges, There is also a 9" AC duct that obstructs my path to the area above SW2 where the head room is about 10" only.

See my earlier paragraph, if I have a neutral from SW1 to SW2, I will have a neutral from the light fixture box to SW1 to SW2 to the fixture box. The only way to have the return current run parallel to the hot side to cancel the EMI is to use that end of the neutral and not the one that came from the panel, Adding a splice in the fixture box to use the neutral closer to the panel will work electrically, but will render the effort of adding the SW1 to SW2 neutral segment useless because the purpose of adding it is to have that parallel return current.

 

I think that’s what I said. So, I’m a little confused. Are you confirming that is correct, or are you asking me to re-confirm that is correct, or are you informing me that is the correct way to do it? As far as I can tell that is the correct way to do it

It would not be an issue because those considerations only apply to metallic conduit – EMT. Unless you mean that in your area you see the traveler wires running in EMT without the neutral.

Yes, but as someone mentioned earlier, that requirement has nothing to do with the requirement to have the neutral running through metallic conduit – EMT. The neutral requirement for the switch box is to have the capability for smart switches and sensors and other devices which require a neutral.

You can remove the box from the wall without enlarging the hole. I have plaster walls and I do it. You would just replace the box with a new work box. Those are cheap. But I’m not saying you should run a new cable with a neutral from SW1 to SW2. But you would have to do that to be technically – or I guess actually – code compliant. But again – I’m not suggesting you do it.

I have the NEC 2002 and section 300.3 and it basically says all conductors of the same circuit should be in the same raceway, etc. You only have one section of EMT, if I understand, it’s from the fixture box (or junction box) to SW1. But that is enough to make the requirement relevant.







​​​​​​​

 

What I am saying is, I believe the picture as depicted below is the correct and best practice way to wire this. That is:

(1) There is a neutral in each switch box SW1 and SW2.
(2) The neutral is paired with it's respective hot along the entire path so the inductive heat and EMI is cancelled.
(3) That the light fixture must take the neutral from the end of the run even though the neutral is already there from the panel once in that box, in order to make #2 happen that panel neutral must just pass through the box.



The present condition I have I am not sure if I violate NEC. I do have a neutral in each switch box, but they are capped dead. Actually I believe SW1 does not even need a neutral as NEC says if one can be pulled in the future it doesn't need one. I most likely will not try to run a new 12/3 between SW1 and SW2 as that's just too much work.

I didn't realize a regular switch loop (with no neutral) has no EMI because that loop has current in opposite directions and there is no EMI even without the neutral. But a traveler pair does.

 

.That seems right to me also. It looks like everything was OK with just SW1. But when someone added SW2 and replaced SW1 to create (or attempt to create as you say) a 3-way switch setup, it looks like they created a problem because now just a single hot wire is used in the EMT connecting the light to SW1. So now, the inductance problem appears. But how bad is it really. We are talking about a single light on that loop.

​​​​​​​Seems like it would be very low amperage and not much of a problem. Maybe someone somewhere would know exactly how bad the problem really would be. My bet would be not much of a problem.

(I think others here have thought that also)