Dead-end threeways and EMF

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George Stolz

George Stolz

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I was looking at this thread and got lost in the math. It's a huge thread, so I didn't want to post to it.

My question is, if you feed and switch a threeway from the same box, leaving a three-wire NM running between threeways, will this three-wire NM have a bad EMF with it? How large of an area is effected? I tried to discern it from this thread, but it was over my head. :)

And if so, why is it explicitly permitted to run a switch loop without a neutral? 404.2(A), Exception.
 
Re: Dead-end threeways and EMF

It is not the presence or absence of a neutral conductor that matters. Many circuits have no neutral, for example, three phase motor circuits.

As long as the current flowing in both directions in the same cable or conduit is equal, the magnetic fields cancel.

Ed
 
Re: Dead-end threeways and EMF

So what is the concensus on what the Code will allow? Me, I think that the code says the conductors should be run in a manner so that any current going one way in a cable is complimented by current going in the other direction in the same cable. But I am not certain.
 
Re: Dead-end threeways and EMF

George, you've been in the "sacred scrolls".

I'm not sure I'd understand that one if I read through it again. ;)
 
Re: Dead-end threeways and EMF

If you have a pair of travellers by themselves you'll have an EMF source. If you have the same pair of travellers but with an accompanying grounded conductor you wont have an EMF source.
 
Re: Dead-end threeways and EMF

Originally posted by physis:
If you have a pair of travelers by themselves you'll have an EMF source. If you have the same pair of travelers but with an accompanying grounded conductor you wont have an EMF source.
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Sam I have to disagree here, a grounded conductor has nothing to do with it.

See Eds post.

A dead end 3 way at least the way I see it done has two travelers and one 'point' or common.

When the load is on current will always be flowing through the common and one of the travelers. This current flow in both directions cancels out the magnetic field.

This is no different then if you bring the power to a ceiling light and then run just a two wire down to a single pole switch.

Another way to look at it is this, if you put an amp clamp around all 3 wires of a dead end three way you will not get a reading as the current flow is in both directions.
 
Re: Dead-end threeways and EMF

crossman,
Me, I think that the code says the conductors should be run in a manner so that any current going one way in a cable is complimented by current going in the other direction in the same cable.
Click to expand...
It sort of says that, but look at 300.3(B)(3).
Don
 
Re: Dead-end threeways and EMF

You're right Bob, I was't even addressing the question was I? :(
 
Re: Dead-end threeways and EMF

As an aside, the use of 2-conductor cable for travellers and "borrowing" a neutral (oooops...grounded conductor) produces large amount EMF.

There is an expert here, Karl Riley, who has exact numbers to verify this.

This practice is not forbidden by the NEC if all non-metallic wiring methods and boxes are used, and the borrowed neutral is from the same circuit.

The NEC does not directly address EMF issues, so in a sense there is a loophole to allow this even though it is bad practice, IMO.
 
Re: Dead-end threeways and EMF

I guess the way I'm seeing it, the neutral has been affected by a load.

The traveller and the common are the same conductor in parallel, in a way. Does that make sense?
Another way to look at it is this, if you put an amp clamp around all 3 wires of a dead end three way you will not get a reading as the current flow is in both directions.
Click to expand...
Bob, this is definutely the best way I can imagine to answer this question. Have you physically done this? I haven't bought an ampclamp yet. :)
 
Re: Dead-end threeways and EMF

produces large amount EMF
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I think I would say produces some EMF.

Edit: darn errors

[ January 06, 2005, 09:12 PM: Message edited by: physis ]
 
Re: Dead-end threeways and EMF

George, draw the circuit out with current flowing thru the feed and returning on the switch leg running side by side and you will see the canceling of the field.

There are many cheap clamp meters on the market that are sufficient for residential use, you really need to invest in one.

Roger
 
Re: Dead-end threeways and EMF

I did, that's why I don't have one. :D

I still have the $30 Sperry meter I bought at Home Depot two years ago, without the amp clamp. Bought a cheap ampclamp from Harbor Freight, it lasted about 12.5 seconds.

Someday I'll invest in an Ideal Ampclamp. Someday.
 
Re: Dead-end threeways and EMF

This is no different then if you bring the power to a ceiling light and then run just a two wire down to a single pole switch.
Click to expand...
 
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