two-wire travellers and emf

[pg]

At the commercial job I'm presently on, there are three-way switches at opposite ends of a U-shaped corridor which operate eight or so 2x4 120v fluorescent fixtures. A two-wire MC (100') was run from switch to switch for travellers, which I realize should rather have been a three-wire, and wish I had caught the mistake before the ceiling insulation, grid, tile, etc., went in, because now it'll be an ugly job to replace.

In 300.20 it says all phase conductors and, where used, the grounded conductor and EGC shall be grouped together, but I'm unable to find the reference for travellers needing a neutral to counter EMF.

Do travellers in-fact absolutely need a 3-wire? What levels of EMF would be expected in a case like this? How much EMF is involved in loaded switch loops, and why aren't switch loops required to include neutrals if travellers are?
Thanks for any responses.

 

[luke warmwater]

Re: two-wire travellers and emf

There is not a problem in this set-up IF the grounded conductor used is of the same circut.
Your concern with EMFs should be heightened IF you are using a grounded conductor from a different circut at each end.
Did that make sense?

 

[pg]

Re: two-wire travellers and emf

l.w.,

There is only one grounded conductor (one circuit), but it only runs from fixture to fixture down thru the corridor. There is no grounded conductor in the two-wire MC (just travellers) which shortcuts through some offices, joining the two 3-way switches.

I've always thought that a grounded conductor, neutral, needed to accompany the two travellers to offset the emf of the lit traveller. Thanks

 

[ryan_618]

Re: two-wire travellers and emf

PG: I can't find a section that states "you can do this", so...maybe I'm reading too much into it, but 200.7(C)(2) discusses re-identification of the white wire when used in a cable assembly as a traveller. Considering the fact that a white conductor is allowed only to be used as a grounded conductor, other than the exceptions in the above qouted section, I think that this section is the closest thing you will find to addressing your question.

My opinion is that the grounded conductor need not be there.

 

[luke warmwater]

Re: two-wire travellers and emf

Do you have access to a gauss meter?

 

[pg]

Re: two-wire travellers and emf

Not sure if the shop has a gauss meter, but good idea. Never have used one. Thanks for your replies.

 

[pg]

Re: two-wire travellers and emf

Found this in an article written by Karl Riley from the technical section of this Holt website:

3-way switch wiring: Another common net current situation is set up by incorrect 3-way wiring circuits. When the electrician tries to use two-conductor travelers instead of three, he sets up quite a magnetic field in the room. Essentially, he is using a hot from one junction box to run to one switch and he picks up a neutral from a circuit near the second switch. The result is that the traveler carries only one current at any time, with no canceling current, so the magnetic field is at a maximum. The violation of 300-3(b) is clear when you consider the circuits supplying the 3-way section. One run contains the hot with no returning neutral. The other contains the neutral current but no balancing hot. The simple principle to follow is to make sure that in all parts of the circuit at any time there will be two equal and opposite currents flowing. You can?t achieve that with two-conductor travelers. The only cure is to replace the traveler with a 3-conductor cable and be sure you get your hot and neutral from the same point. Check out 3-way circuit diagrams in books like the NEC Handbook.


This is what I was wondering, so it looks like I (or maybe an apprentice) do need to replace the two-wire with 3-wire.

[ January 15, 2004, 12:43 AM: Message edited by: pg ]

 

[luke warmwater]

Re: two-wire travellers and emf

Originally posted by pg:
Found this in an article written by Karl Riley from the technical section of this Holt website:

and he picks up a neutral from a circuit near the second switch.

be sure you get your hot and neutral from the same point.

This is what I was wondering, so it looks like I (or maybe an apprentice) do need to replace the two-wire with 3-wire.

To me this is saying the same thing in my above post.
If your 'neutral' is of the SAME circut at both ends, then the 2-wire travellers should be OK.
If you are 'robbing a neutral' from a different circut at the other end, then you have a problem.

Did you ever 'dead-end' a 3-way switch??
One common, and two travellers.
No neutral in this set-up.

 

[pg]

Re: two-wire travellers and emf

thanks again lw,

I agree with your logic on this but Riley makes it sound like the traveller without a neutral is a problem. But then , like you say, what about dead-end 3-ways, and your basic switch loop. Maybe I'm trying too hard here. Thought I'd track down a gauss meter today and check it. I appreciate your responses.

 

[hbiss]

Re: two-wire travellers and emf

...Did you ever 'dead-end' a 3-way switch??
One common, and two travellers.
No neutral in this set-up.

Yup. And how about a single pole switch, hot and a switchleg. Same thing. How many times is that done? Does 300.3(B)mean that this is a violation also? I don't think so.

300.3(B)- Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway...

It should be clear from "where used" that a grounded conductor is not required if none is needed.

As to magnetic fields and EMF's, the NEC doesn't address the issue other than to prevent inductive heating. While it might be a good idea to minimize fields to reduce interference to sensitive equipment, it is not our job as electricians to concern ourselves with perceived health issues. If there was a substantiated risk the NEC would address it.

Discussing health issues with customers and offering to mitigate them can backfire. I can just see an electrician being dragged into court because someone died of of a brain tumor after he "took care" of the EMF problem and said it was safe.

In this litigatious society we don't need any more exposure than we already have. EMF's are a controversal issue and despite your personal beliefs I would suggest you leave your gauss meter at home.

 

[don_resqcapt19]

Re: two-wire travellers and emf

If the MC has a steel jacket, then the two wire MC used for the travelers is a violation of 300.3(B). If the jacket is aluminum then it is permitted by 300.3(B)(3). 300.3(B) does not require the grounded conductor to be in the same raceway or cable, but does require more than one current carrying conductor. In a single pole switch loop, the hot and the switch leg are both in the same cable with the current flowing in opposite directions. This permits the magnetic fields to cancel. Same thing when you have the two travelers along with either the hot or the switch leg. When you only have the two travelers, there is no opposite current flow to permit the magnetic fields to cancel.
Don

 

[hbiss]

Re: two-wire travellers and emf

...300.3(B) does not require the grounded conductor to be in the same raceway or cable, but does require more than one current carrying conductor.

That's my take also. You can't run two cables or conduit runs and put the hot in one and the neutral in the other by themselves. Since with two travellers only one conductor is in use at a time that would be in violation of 300.3(B).

 

[brian john]

Re: two-wire travellers and emf

Luke: I am your father, opps wrong site for that post.

Luke: I think you are wrong in you assupmtion that as long as the both ends are from the same circuit.......

 

[pierre]

Re: two-wire travellers and emf

When you dead end a threeway, there are two conductors in the same cable/raceway that have current traveling in opposite directions that are cancelling each other, a neutral is not required for this situation. If you are using a two wire in one cable/raceway for travelers and using a conductor in another cable/raceway for the pivet, they will not cancel each other as they are not close enough to do so. This will create potential problems.

Pierre

 

[luke warmwater]

Re: two-wire travellers and emf

Originally posted by brian john:

Luke: I think you are wrong in you assupmtion that as long as the both ends are from the same circuit.......

You could be right.

But I don't think that you have to have a 'neutral' running with travelers. You could have a feed and two travelers, or a switch leg and two travelers.

Doh! Which would make this a 3 conductor cable.
I concur.

 

[a.wayne3@verizon.net]

Re: two-wire travellers and emf

I guess I have been wrong all these years.I never gave it a second thought.Many times when roughing in and there was a 3way ,4way ,3way and lets say there was a single pole at the 4way I would feed the 3 way and use 2, 2 wires to the 4 way and the single pole has a neutral and a hot and finish with 3 wire or continue the circuit if needed at the other end.But after this thread, I have now see I have been doing this wrong ....i never considered the need to cancel the effect .....

 

[svaurez]

Re: two-wire travellers and emf

Summary:I can use two-wire MC Cable for travelers, but only if it is aluminum.300.3(B)(3)

And if I use this exception I get stuck with having to cut slots in the steel box I am landing the travelers in.300.20(B)

OR

I can run a three wire cable with the neutral.

 

[delfa]

Re: two-wire travellers and emf

If I was losing money on the job I think I would remark the grounding conductor as a grounded conductor. Or if it was worrying me too much, I would start running the 3-wire.

 

[ryan_618]

Re: two-wire travellers and emf

Originally posted by delfa:
If I was losing money on the job I think I would remark the grounding conductor as a grounded conductor.

Why, I think thats a wonderful attitude! Tell me...what else will you do for money? :roll:

 

[jimwalker]

Re: two-wire travellers and emf

Am i understanding this right? The problem with just travelers in mc or emt i understand,but does this affect NM in any way if i ran 14-2 or 12-2 from 3 way to 4 way to 3 way ?

[ January 18, 2004, 09:24 AM: Message edited by: jimwalker ]