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

[al hildenbrand]

:thumbsup:

Well, for some reason my last pics didn't come up full size.

I will try again.

View attachment 5718

By bundling the cables of the loop into a C, you cause the conductors current's fields to be next to each other in a way that they cancel each other out.

The people in the room don't experience any EMF from this C shape of bundled cables.

Now, imagine the traveler cable running under the floor, from switch to switch. Now the people are in the middle of the EMF at the center of an " O " shaped loop.

 

[K8MHZ]

To imagine the answer, consider the loop in my diagram to be a circle of radius R.

On the out side of the circle, at a distance of R, there is a circle that will have some of the magnetic field. The circle has a length of Pi x 4R = C. C is a much larger region than the point in the center of the current loop.

Now to be clear, all the magnetic field on the outside of the current loop stretches, theoretically, all the way to infinity, while all the magnetic field inside the current loop goes through the cross sectional area of only the circle's area A = Pi x R squared.

The amount inside area A is equal to the amount of magnetic field outside the circle, an area that is inifinite.

The magnetic field is denser inside the loop than outside.

So the center of a run of Romex (where the grounding conductor is) has the highest magnetic field?

 

[al hildenbrand]

So the center of a run of Romex (where the grounding conductor is) has the highest magnetic field?

Yes. The most concentrated magnetic field.

 

[K8MHZ]

Yes. The most concentrated magnetic field.

Is it moving? Like at 60 Hz?

 

[al hildenbrand]

Is it moving? Like at 60 Hz?

It reverses 120 times a second.

 

[K8MHZ]

It reverses 120 times a second.

Wouldn't that make the grounding conductor produce current?

 

[al hildenbrand]

Wouldn't that make the grounding conductor produce current?

Eddy current is induced.

 

[K8MHZ]

Eddy current is induced.

In measurable amounts?

 

[al hildenbrand]

In measurable amounts?

That is an interesting question.

Depends upon the quality of the instrumentation and the nature of the measurement.

Practical observation shows that the induced energy is not meaningful.

 

[Little Bill]

All the EMF stuff aside. Are any of you saying it is a violation to run the travelers in this way? If yes, code reference please. That was the OP's question.

 

[Dennis Alwon]

All the EMF stuff aside. Are any of you saying it is a violation to run the travelers in this way? If yes, code reference please. That was the OP's question.

The op's question was based on the feed from one circuit and the neutral from a second circuit. This is not compliant based on 300.3(B). Add that to the fact that the second circuit was on the same phase.

The 2011 spells it out clearly in 200.4

 

[al hildenbrand]

Nice summary of the OP setup. I agree, Dennis.

 

[Little Bill]

Is running three way switch travellers on 12-2 legal?

Ran across this in a resi. troubleshooting call: power came into first three way at bottom of stairs, travelled via 12-2 to second three way at top of stairs.

Then, in the second box was another circuit which provided the neutral back to the panel. Ignoring the potential for overloading the neutral with two same-phase circuits, is running the travelers by themselves a violation? Or does 300.3.B.2 allow it?

No, these (in red) are his questions.

The op's question was based on the feed from one circuit and the neutral from a second circuit. This is not compliant based on 300.3(B). Add that to the fact that the second circuit was on the same phase.

The 2011 spells it out clearly in 200.4

I understand the neutral issues and the same circuit issues. I'm asking if you (or anyone) thinks it is a code violation to run the travelers by themselves, the other issues aside.

 

[Howard Burger]

epilogue

epilogue

Thank you, Al, for the refresher on California 3-ways. Fortunately, this did not turn out to be wired that way.

The additional circuit to provide the return neutral came up to the 3rd floor via a deck gfci I hadn't been told about. It fed the gfi, a hall recep. a third floor deck gfci, two lights and a small exhaust fan, then ended up in the 3rd floor switch box to provide the return neutral for that plus 8 lights on the lighting circuit.

The gfci had been replaced by the employees of the GC the homeowner had hired to correct problems turned up in a pre-sale inspection. The non-electrician employees had mis-wired the gfci and apparently not tested it, so no flow in the wires. After I re-wired it correctly the circuits worked just as designed.

This house was built in the early 70's. Would/could have this wiring method I found have been kosher back then?

And thank all of you who are participating in the (rabbit trail) discussion of EMF. These 'side' conversations by you guys who have been around the block really add bonus value to this forum.

 

[al hildenbrand]

This house was built in the early 70's. Would/could have this wiring method I found have been kosher back then?

If you are asking "was wiring a 3-way switching of a luminaire with only two conductor (with ground) NM NEC approved?" the answer was, and is, yes, IF the hot and neutral are part of the same branch circuit or are part of a multiwire branch circuit, and, IF the conductors are arranged in a manner that doesn't include the parallel conductor problem of the california 3-way.

 

[kwired]

No, these (in red) are his questions.



I understand the neutral issues and the same circuit issues. I'm asking if you (or anyone) thinks it is a code violation to run the travelers by themselves, the other issues aside.

I mentioned it earlier in the thread, I believe it is a violation of 300.3(B) All conductors of a circuit are not contained in same raceway or cable. By running conductors this way not only leads to EMF problems but increases impedance of conductors. This impedance will be magnified during short circuits resulting in less fault current which will make overcurrent device operation take longer.

 

[Dennis Alwon]

I understand the neutral issues and the same circuit issues. I'm asking if you (or anyone) thinks it is a code violation to run the travelers by themselves, the other issues aside.

No, I don't believe it is an issue with the nec if the wiring is non ferrous-- 300.3(B)(3)

 

[al hildenbrand]

I mentioned it earlier in the thread, I believe it is a violation of 300.3(B) All conductors of a circuit are not contained in same raceway or cable.

Because the wiring is NM cable, 300.3(B)(3) specifically allows the installation that Howard found.

By running conductors this way not only leads to EMF problems but increases impedance of conductors. This impedance will be magnified during short circuits resulting in less fault current which will make overcurrent device operation take longer.

This is not a NEC issue.

 

[jumper]

Jumper, you can use the white as a traveler and re-identify it as long as it isn't the return to the light.

Yeah, that is what happens when I am posting at 1:00 AM, foot goes into mouth.

 

[jumper]

No, I don't believe it is an issue with the nec if the wiring is non ferrous-- 300.3(B)(3)

Because the wiring is NM cable, 300.3(B)(3) specifically allows the installation that Howard found.This is not a NEC issue.

So, as I understand it, this would not be legal or common with MC or AC.