3 conductors in conduit

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Jennifer

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Hello,
My question. You have 2- 1/2in greenfields
coming out a panel. One has 3- 14ga THHN each going to a 15a breaker and a 12ga gnd. The other has 3 -14ga neutral and a 12ga gnd. The 2 greenfields go to a 4 11/16 box where they become 3 circuits going through 3 conduits/emt or greenfields. Is there anything wrong with this picture ?. In theory it sounds ok, but I have run it by several people and
they said the same but not heard of it being done.

I have looked in the NEC book, but can't decide on this one. If this is allowed, could 12ga conductors be run, or would I have to go to 3/4in
greenfield/emt ?.

Jennifer
 
Re: 3 conductors in conduit

As I understand your situation, this is not a permitted wiring method, and can be potentially dangerous - most likely dangerous.

300.3(B), 300.20

P.S. welcome to the forum.

[ November 29, 2005, 12:44 AM: Message edited by: pierre ]
 
Re: 3 conductors in conduit

Bob, "greenfeild" is a trade name for flexible metal conduit, which does not have preinstalled conductors.

BTW: I agree with Pierre, this is not permitted and is dangerous. You will experience a lot of hysteresis heating of the conductors.

[ November 29, 2005, 09:00 AM: Message edited by: ryan_618 ]
 
Re: 3 conductors in conduit

The specific problem is that you have the phase ("hot") conductor in a different conduit than the grounded ("neutral") conductors. That allows the hot conductors to generate a large magnetic field. If the hot and neutral were in the same conduit, they would be carrying current in opposite directions. That would cause their separate magnetic fields to cancel each other. But the large magnetic field that results from all the hot conductors being in one conduit will induce current into the conduit itself (and perhaps in other nearby stuff). That may overheat the conduit and start a fire.
 
Re: 3 conductors in conduit

Originally posted by ryan_618:
Bob, "greenfeild" is a trade name for flexible metal conduit, which does not have preinstalled conductors.

BTW: I agree with Pierre, this is not permitted and is dangerous. You will experience a lot of hysteresis heating of the conductors.
I thought greenfield was the same thing as BX?
 
Re: 3 conductors in conduit

As ryan says, Greenfield is Flexible Metal Conduit (Type FMC).
BX is Armored Cable (Type AC).
 
Re: 3 conductors in conduit

Jennifer

One has 3- 14ga THHN each going to a 15a breaker and a 12ga gnd. The other has 3 -14ga neutral and a 12ga gnd.
Why would you want to put only the neutrals in a coundit? Is this a question about "what if"?

[ November 29, 2005, 05:32 PM: Message edited by: bob ]
 
Re: 3 conductors in conduit

question guys (and gals)... I understand the violation and I do and will continue to reject such installations,,,however, has anyone actually experienced any overheating in these situations on 2 or 3 15/20 amp circuits ?
 
Re: 3 conductors in conduit

I was called to an old (built in the mid 1800s) church where the electrician had separated the conductors for all of the small circuits. Don't ask why, I have no idea why. The old "BX" was glowing cherry red. I was tripping over myself to turn the main off.

There have been several times when I have seen this, it is not fiction - just fact.
 
Re: 3 conductors in conduit

I haven't seen or heard news of a fire or other damage resulting from an improperly installed 120 volt 20 amp circuit. But I have seen the resultant damage from a similarly improper installation. This was related to a high voltage (365kV) isolation bus duct serving the output of a utility generator.

The duct was properly designed to have each phase in its own bus duct. However, there was an improper connection of metallic conduit between one duct and a support structure. This created a complete path for current to flow along the conduit, the outside of the bus duct, and the support structure. The current had been induced because there was a complete circular path in the presence of the large magnetic field of the generator output. The conduit became essentially the secondary of an unintended transformer. The conduit was completely burned through.
 
Re: 3 conductors in conduit

Originally posted by blasko:
I thought greenfield was the same thing as BX?
Is it scary that an engineer doesn't know the difference?
No. I think it would be scarier if one of us did. We might get it into our heads to try to install this stuff ourselves. :eek:
 
Re: 3 conductors in conduit

No iwire, I don't. Just giving him a taste of his own medicine. Those were his words he originally used w/me. I didn't start the pissing contest but I'll definitely play! One useful tool I would recommend though-THINK BEFORE YOU ACT!

[ November 29, 2005, 05:56 PM: Message edited by: blasko ]
 
Re: 3 conductors in conduit

Hi,
Wow, a lot of comments !. First thank's for the understandable how and why this is a bad thing.

This wiring idea, was only in my mind, so rest easy. The reason was to get 3 circuits from 2- 1/2in KO's. When you only have so many ko's where
"I need" them, you start to come up with "bad ideas" !.

I do hvac, mostly just the Heating, so i'm only 1/2 a electrican. Multi circuits in a conduit are never found in homes related to heating. I'm learning, so I guess theres hope !.

Much Thanks,

Jennifer
 
Re: 3 conductors in conduit

I worked with a guy (i know I didn't see it for myself) that worked on a new Red Lobster years ago.
If I remember right, it had a 800 amp service with 4 risers, A phase in one conduit, B phase in the second conduit and so on. Everything was fine until opening night when all the loads were energized. I was told they spent all night rebuilding the service.
 
Re: 3 conductors in conduit

I was thinking of a little thought experiment about this. Suppose you had two conduits. In one, you have a red and a black wire. In the other you have the two neutrals [white wires -- I like colors]. They are both running idedtical loads -- say 100 watt light bulbs.
Since the AC current travels in little 60 cycle waves, wouldn't the red and black forces cancel out? And, similarly, wouldn't the neutrals cancel out also since they are the same thing as the black and red?
~Peter
 
Re: 3 conductors in conduit

I believe it depends on if the red and black hots were fed from different phases (or buss bars of a single phase supply.

If from different phases they should cancel out (if both identical loads were "on"). same phase, they would be additive in both hot and neutral conduit.
 
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