Using 12-2 for travellers between two 3-way switches

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Dennis Alwon said:
No, I don't believe it is an issue with the nec if the wiring is non ferrous-- 300.3(B)(3)
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al hildenbrand said:
Because the wiring is NM cable, 300.3(B)(3) specifically allows the installation that Howard found.This is not a NEC issue.
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Ok, the main reason I kept asking about the travelers being run by themselves is this:
I had a customer that wanted a "3rd switch" added to a lighting circuit. So I told him I could do that with a 4-way switch. This was in a partially unfinished basement/garage. The problem was the 2 existing 3-ways were in a finished wall and the wiring ran above the finished ceiling in another room. He didn't want to cut or damage the sheetrock so I had to improvise. I put the 4-way midways of the unfinished area beside a door on a finished wall. I had 14-2 on the truck so I ran 2 sets of cable to the first switch to tie in the 4-way. This left the travelers to the 4-way running by themselves. There was 3-wire ran between the 3-ways, including the neutral. I in effect just extended the travelers. I didn't think much about it until this thread came up, but I don't think it is a violation.
 
Little Bill said:
Ok, the main reason I kept asking about the travelers being run by themselves is this:
I had a customer that wanted a "3rd switch" added to a lighting circuit. So I told him I could do that with a 4-way switch. This was in a partially unfinished basement/garage. The problem was the 2 existing 3-ways were in a finished wall and the wiring ran above the finished ceiling in another room. He didn't want to cut or damage the sheetrock so I had to improvise. I put the 4-way midways of the unfinished area beside a door on a finished wall. I had 14-2 on the truck so I ran 2 sets of cable to the first switch to tie in the 4-way. This left the travelers to the 4-way running by themselves. There was 3-wire ran between the 3-ways, including the neutral. I in effect just extended the travelers. I didn't think much about it until this thread came up, but I don't think it is a violation.
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I agree with you (your opinion highlighted in red).

And, if the two 14/2 NM cables were threaded along together, any EMF of any amount would exist only in the gaps between the cables. When the switches are ON, the load current out on one traveler is close beside the same load current back on the traveler in the other cable. Their magnetic fields are equal, but exactly opposite to each other, and, therefore, cancel out in the space surrounding BOTH cables.
 
jumper said:
I just checked, the white as a traveler would violate 200.7(C)(2).
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(NEC 2005) 200.7.C.2 Says when the white is used as the supply, it has to be permanently reidentified. It does not prohibit the white wire from being a traveler.
 
Originally Posted by K8MHZ
So the center of a run of Romex (where the grounding conductor is) has the highest magnetic field?
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al hildenbrand said:
Yes. The most concentrated magnetic field.
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I would have to disagree with this statement Al.

The highest effect of cancellation is the point of equal distance from two 180? out of phase sources, place two speakers together and supply them with a mono tone signal with the polarity reversed at one speaker, and one will cancel the other out if you are exactly centered between them, lean from one side to the other and you will notice that this canceling effect will go away as you move closer to one or the other speaker, this is no different then the effect in wires and as K8MHZ already knows is used heavily in phasing antenna arrays for side and rear rejection which is why I think he was questioning this.

This is the whole reason for placing all circuit conductors as close as possable together because of this cancelation, the field is always higher closer to each conductor, it is always lower when you are between the two 180? out of phase conductors.;)
 
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hurk27 said:
The highest effect of cancellation is the point of equal distance from two 180? out of phase sources, place two speakers together and supply them with a mono tone signal with the polarity reversed at one speaker, and one will cancel the other out if you are exactly centered between them, . . .
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Hi Wayne,

The analogy doesn't work for me.

Sound is a wave that is following a path that would be at a right angle to the magnetic lines of force, if both the speaker cone and the straight current carrying wire were in the same place.

The further one moves away from the two current carrying conductors in an NM cable, the greater the effect of cancellation.

As one's point of measurement moves toward and then in between the two current carrying conductors the magnetic field increases. To be clear, most of the field is out around both conductors, however, that part of the field that is passing through an imaginary line drawn from conductor to conductor is moving in the same direction. That's the really interesting thing. The magnetic field from the two NM current carrying conductors is additive exactly between the two conductors.
 
Grab the conductor with your right hand, with the thumb pointing in the direction of the current. The curl of your fingers around the conductor is the direction of the magnetic lines of force.
 
al hildenbrand said:
Grab the conductor with your right hand, with the thumb pointing in the direction of the current. The curl of your fingers around the conductor is the direction of the magnetic lines of force.
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Ok so the magnetic lines of force will be spinning in opposite direction on each conductor, pulsating at 1/60th of a second alternating at maximum magnitude between each conductor at opposite timing am I missing something?
 
hurk27 said:
Ok so the magnetic lines of force will be spinning in opposite direction on each conductor, pulsating at 1/60th of a second alternating at maximum magnitude between each conductor at opposite timing am I missing something?
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The little circles of lines of force, small enough to fit in between the two conductors, will be moving in the same direction where the lines of force are passing through the line between the two conductors.

The larger circles of lines of force that surround both conductors will be moving in opposite directions.
 
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Forget EMF!

Forget EMF!

Forget about EMF's for residential wiring. The EMF/health effects issue was from high voltage power lines where currents are 100's and 1000's of amps, and conductors are separated by a few to several feet. Those lines can produce magnetic fields of several milligauss at ground level. Much research (a couple $Billion worth - yes Billion with a B) was done and never did prove or show EMF's cause cancer or anything else. I assure you, a romex (or similar) carrying a few amps produces negligible magnetic field. And what field it does produce falls off quickly from the wire.

The magnetic field midway between two wires carrying equal but opposite current will be ZERO.
 
stevero said:
Forget about EMF's for residential wiring. The EMF/health effects issue was from high voltage power lines where currents are 100's and 1000's of amps, and conductors are separated by a few to several feet. Those lines can produce magnetic fields of several milligauss at ground level. Much research (a couple $Billion worth - yes Billion with a B) was done and never did prove or show EMF's cause cancer or anything else. I assure you, a romex (or similar) carrying a few amps produces negligible magnetic field. And what field it does produce falls off quickly from the wire.

The magnetic field midway between two wires carrying equal but opposite current will be ZERO.
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I have fixed emf's in homes that range from 6 mg to 13 milligauss. Now maybe that is negligible to you but there are studies that show low levels of emf cause childhood leukemia. There are probably studies being done that show the emf's are both safe and unsafe. My point is that the emf's are a bit of an unknown entity so why run wires that create the field if we can avoid it.
 
stevero said:
Forget about EMF's for residential wiring. The EMF/health effects issue was from high voltage power lines
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Actually there were many studies done at many different levels and all showed levels of varying ranges

stevero said:
Those lines can produce magnetic fields of several milligauss at ground level. Much research (a couple $Billion worth - yes Billion with a B) was done and never did prove or show EMF's cause cancer or anything else.
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It was never proven that it didn't either
stevero said:
I assure you, a romex (or similar) carrying a few amps produces negligible magnetic field.
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And there are others with respectable credentials that will debate that.

stevero said:
The magnetic field midway between two wires carrying equal but opposite current will be ZERO.
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And that is not what you have with two travelers.


Roger
 
stevero said:
The magnetic field midway between two wires carrying equal but opposite current will be ZERO.
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See my posts above. Wayne and I just discussed this.

The net magnetic field around BOTH conductors is zero.

The magnetic field midway between both conductors, as you describe is more than zero. The lines of force of the magnetic field form circles around the conductor (and the current creating the field). The circular lines or force get larger all the way to infinity.

When the radius of the lines of force is large, compared to the distance between the two conductors that have equal currents flowing in opposite directions, the circular lines of force appear to be centered upon the same point, and, since the fields are of equal strength but of opposite direction, they sum to zero.

When the radius of the lines of force is equal to half the distance between between the two current carrying conductors, the two magnetic fields are going in the same direction, and they sum to each other, they don't cancel out.
 
You know, this reminds me a bit of a conversation I had with a fella a couple months ago about this very thing. I let him know my opinion, that it was a better design to keep it simple; when wiring a threeway switch for a switched receptacle, just make it a dead end and avoid any question. You're still using the same amount of wire, I said.

He couldn't be persuaded that the method was more correct, because he had wired his neutrals in parallel on his lighting circuit around 70 times on that job alone, so he decided he was right just due to sheer volume. He was also incredulous that the same could not be done with EMT.

Someone on here has it in their signature to "Never argue with an idiot, they will drag you down to their level and beat you with experience." Truer words never spoken. :slaphead:
 
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