# Theory Question: A single conductor in metallic conduit.



## McClary’s Electrical (Feb 21, 2009)

Frasbee said:


> I'm not sure where else to post theory questions, or any questions in regards to school, but anyway:
> 
> So my teacher the other day had said that if you had a single current carrying conductor, in a metallic raceway, theoretically, the magnetic field could cause the conduit to become heated, because a parallel magnetic field is not present to cancel it out.
> 
> ...


 

True.....


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## Dennis Alwon (May 9, 2009)

Frasbee said:


> I'm not sure where else to post theory questions, or any questions in regards to school, but anyway:
> 
> So my teacher the other day had said that if you had a single current carrying conductor, in a metallic raceway, theoretically, the magnetic field could cause the conduit to become heated, because a parallel magnetic field is not present to cancel it out.
> 
> ...


This is definitely possible if a grounding electrode is installed in metal conduit and it isn't bonded at both ends- it creates a choke.

If you read 300.3 you will see there is a code prohibiting this install in most cases-- there is an exception for underground parallel conductors- 300.3(B)(1) except.


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## 76nemo (Aug 13, 2008)

Dennis Alwon said:


> This is definitely possible if a grounding electrode is installed in metal conduit and it isn't bonded at both ends- it creates a choke.
> 
> If you read 300.3 you will see there is a code prohibiting this install in most cases-- there is an exception for underground parallel conductors- 300.3(B)(1) except.


 
"Is" or "isn't" bonded at both ends?


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## McClary’s Electrical (Feb 21, 2009)

76nemo said:


> "Is" or "isn't" bonded at both ends?


 


isn't.....


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## Bob Badger (Apr 19, 2009)

Frasbee said:


> I'm not sure where else to post theory questions, or any questions in regards to school, but anyway:
> 
> So my teacher the other day had said that if you had a single current carrying conductor, in a metallic raceway, theoretically, the magnetic field could cause the conduit to become heated, because a parallel magnetic field is not present to cancel it out.
> 
> ...


True with AC, not with DC.

That is why 300.3(B) and 300.20(B) exist in the NEC.


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## Bob Badger (Apr 19, 2009)

Dennis Alwon said:


> This is definitely possible if a grounding electrode is installed in metal conduit and it isn't bonded at both ends- it creates a choke.


That does not cause heating, that raise the impedance of the GEC (if it is not bonded to the conduit.)


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## Frasbee (Apr 7, 2008)

300.3 referred me to 300.20 (B).



> Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by
> (1) cutting slots in the metal between the individual holes through which the individual conductors pass or
> (2) passing all conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit.
> 
> FPN: Because aluminum is not a magnetic metal, there will be no heating due to hysteresis; however, induced currents will be present. They will not be of sufficient magnitude to require grouping of conductors or special treatment in passing conductors through aluminum wall sections.


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## Bob Badger (Apr 19, 2009)

Frasbee said:


> 300.3 referred me to 300.20 (B).


Here is one example of 300.20(B) 










What you see are 3 - three phase, four wire feeders run in single conductor MI cable.

The brass steel of the enclosures was cut out and the brass plates installed so there would be no heating.


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## Frasbee (Apr 7, 2008)

Bob Badger said:


> Here is one example of 300.20(B)
> 
> 
> 
> ...


Wow, I have never seen that...

So are the conductors inside that brass tubing...within a conduit or flex? I just can't make out what they're coming out of above the picture.


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## Jlarson (Jun 28, 2009)

Frasbee said:


> Wow, I have never seen that...
> 
> So are the conductors inside that brass tubing...within a conduit or flex? I just can't make out what they're coming out of above the picture.


That's MI (Mineral Insulated) cable. It's a single conductor that looks like copper tube. It looks like they go into some conduit hangers right out of the frame there at the top to secure it. 

Here's some more info;
http://www.micable.com/


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## oldtimer (Jun 10, 2010)

Frasbee said:


> Wow, I have never seen that...
> 
> So are the conductors inside that brass tubing...within a conduit or flex? I just can't make out what they're coming out of above the picture.


 I M O the copper tubing was called Pyrotenax. The conductors are in it.

I remember a green compound in the connectors. I seem to recall they were called Potheads. Can anyone verify this?


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## Bob Badger (Apr 19, 2009)

Frasbee said:


> Wow, I have never seen that...
> 
> So are the conductors inside that brass tubing...within a conduit or flex? I just can't make out what they're coming out of above the picture.


That is called MI (Mineral Insulated) Cable. It will contiue working even when exposed to direct flames for I think 2 hours. 

(More info http://en.wikipedia.org/wiki/Mineral-insulated_copper-clad_cable )

It is a copper jacket that also serves as the grounding conductor. Inside the jacket is a single solid strand of copper as the circuit conductor. Insulating the conductor from the jacket is what looks like the stuff drywall is made from. It sucks to run, it sucks to terminate. But it is sometimes required for emergency circuits.

I will post some more photos in a few minutes.


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## Bob Badger (Apr 19, 2009)




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## Dennis Alwon (May 9, 2009)

Bob Badger said:


> That does not cause heating, that raise the impedance of the GEC (if it is not bonded to the conduit.)


Okay here is what Don Resqcapt at MH wrote.



> When a single conductor that carries AC current is run through ferrous metal that completely encircles the conductor the impedance is increased and the current flow is limited...choked. In extreme cases, there will be enough heat to melt the ferrous metal and or the conductor.


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## jwjrw (Jan 14, 2010)

Bob,

Do they run MI cable for emergency lights for the fire rating of the MI cable?
Also take a peak at my thread in the nec code section. Let me know what you think please.


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## Bob Badger (Apr 19, 2009)

By the way, none of the work in those photos is mine, that job was done by IBEW 103 guys. I have to say I think they did a great job. Many of these feeders run 100s of feet through exsiting supsended ceilings. It had to be a tough job.

I was in the building doing other work and thought it was cool so I took some pictures.


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## Trimix-leccy (Dec 4, 2007)

thought MICC was only a UK thing

BICC Pyrotenax [from latin or greek maybe....pyro [fire].....tenax [hold]

copper outer sheath
magnesium oxide insulation over inner bare conductors
terminated with a 'pot' filled with 'compound'. sealed with a 'stub' enclosed in a 'gland'...that's the bit you can see.

installed some yesterday!...not done any for years before. It's a bit of an art:thumbsup:


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## Bob Badger (Apr 19, 2009)

Dennis Alwon said:


> Okay here is what Don Resqcapt at MH wrote.


And he is right, yet still I stand by what I said.

You will never find a hot GEC conduit unless it's in a fire or there is something _seriously wrong_ with the wiring system.

You will find conduits containing circuit conductors getting hot if 300.3(B) is not complied with. 

But I did not get the impression that Frasbee's teacher was talking about GECs.




> So my teacher the other day had said that if you had a single *current carrying conductor*,


I would not call a GEC a current carrying conductor.


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## Dennis Alwon (May 9, 2009)

Bob Badger said:


> And he is right, yet still I stand by what I said.


Yes but things do go seriously wrong sometimes so I'll stand by what I said also.


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## voltz (Jun 2, 2010)

they often run it for the fire pumps


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## brian john (Mar 11, 2007)

Very nice install Bob

I think MI is made in 3 conductor up to #8 or ^?


Also in 500kcmil in single conductor, NOT SURE has the ass to install that.


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## Bob Badger (Apr 19, 2009)

brian john said:


> Very nice install Bob


I wish I could say it was mine, but it is not.


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## brian john (Mar 11, 2007)

This was posted the other day and these are only nipples.


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## Bob Badger (Apr 19, 2009)

jwjrw said:


> Bob,
> 
> Do they run MI cable for emergency lights for the fire rating of the MI cable?


This building was a very large hospital and these were emergency feeders to different electrical rooms.




> Also take a peak at my thread in the nec code section. Let me know what you think please.


Ah ... thanks for the link. :laughing:


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## Bob Badger (Apr 19, 2009)

brian john said:


> Thios was posted the other day and these are only nipples.


Great example. :thumbsup:


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## 76nemo (Aug 13, 2008)

Does bend radius differ any differently to other raceways in MI installation?


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## Bob Badger (Apr 19, 2009)

brian john said:


> This was posted the other day and these are only nipples.


Looking at this I think the nipples are the least of it, (they may be non ferrous die cast) I think the steel enclosure is the real problem.

Supposedly if you cut slots between the openings it would eliminate the problem.

Much like how an amprobe clamp only works when it is closed completely.


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## brian john (Mar 11, 2007)

Bob Badger said:


> Looking at this I think the nipples are the least of it, (they may be non ferrous die cast) I think the steel enclosure is the real problem.
> 
> Supposedly if you cut slots between the openings it would eliminate the problem.
> 
> Much like how an amprobe clamp only works when it is closed completely.


 
You are right! Heck I would have cut the a 6"x26" opening and used quick edge, simple solution.


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## Jlarson (Jun 28, 2009)

brian john said:


> You are right! Heck I would have cut the a 6"x26" opening and used quiock edge, simple solution.


So I'm not the only that does that. I love the quickedge stuff.


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## Bob Badger (Apr 19, 2009)

Jlarson said:


> So I'm not the only that does that. I love the quickedge stuff.


Is that a fancy name for 1/2" PVC with a slot cut in it? :jester:


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## Jlarson (Jun 28, 2009)

Bob Badger said:


> Is that a fancy name for 1/2" PVC with a slot cut in it? :jester:


:shifty: If I didn't have any real edging stuff I would not hesitate to do that.


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## jwjrw (Jan 14, 2010)

Bob Badger said:


> Ah ... thanks for the link. :laughing:



Yea I probably should of provided that for ya.....


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## s.kelly (Mar 20, 2009)

Bob,nice pictures!

JLarson,nice link

Brian John great pictures, love seeing your infrareds,teaches me something every time.

thanks to all


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## Magnettica (Jan 23, 2007)

Bob Badger said:


> By the way, none of the work in those photos is mine, that job was done by IBEW 103 guys. I have to say I think they did a great job. Many of these feeders run 100s of feet through exsiting supsended ceilings. It had to be a tough job.
> 
> I was in the building doing other work and thought it was cool so I took some pictures.




Other work as in fixing what they screwed up? :laughing:


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## jwjrw (Jan 14, 2010)

Bob Badger said:


> This building was a very large hospital and these were emergency feeders to different electrical room.




But is the reasoning for running them for emergency power the fire rating of them?


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## jwjrw (Jan 14, 2010)

Magnettica said:


> Other work as in fixing what they screwed up? :laughing:


All us Rats know that union guys don't screw up.....
They are too well trained for that...:laughing:


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## Jlarson (Jun 28, 2009)

jwjrw said:


> All us Rats know that union guys don't screw up.....
> _They are too well trained for that..._:laughing:


Yeah, no one ever taught me how to walk around a burning barrel holding a sign on a stick. We rats are at such a disadvantage. 

:laughing::laughing:


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## Bob Badger (Apr 19, 2009)

jwjrw said:


> But is the reasoning for running them for emergency power the fire rating of them?


Yes, here it is in the NEC ....



> *II. Circuit Wiring
> 
> 700.9 Wiring, Emergency System.
> 
> ...



..... I think Hospitals have even stricter requirements found in NFPA 99 but I am not sure of that.


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## jwjrw (Jan 14, 2010)

Jlarson said:


> Yeah, no one ever taught me how to walk around a burning barrel holding a sign on a stick. We rats are at such a disadvantage.
> 
> :laughing::laughing:




Man thats the old school way....the "new" union pays non union people to pickett for them..:whistling2::laughing:


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## Jlarson (Jun 28, 2009)

jwjrw said:


> Man thats the old school way....the "new" union pays non union people to pickett for them..:whistling2::laughing:


How can I get in on this deal? :laughing:


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## jwjrw (Jan 14, 2010)

Jlarson said:


> How can I get in on this deal? :laughing:




I read an article and posted a link to it here a while back.. Had Badger rolling on the floor laughing. The union rep said some members couldn't find parking or something like that. So they hired joe blow to pickett. The pickett guys had no clue why they were on the line.:laughing:


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## Jlarson (Jun 28, 2009)

jwjrw said:


> I read an article and posted a link to it here a while back.. Had Badger rolling on the floor laughing. The union rep said some members couldn't find parking or something like that. So they hired joe blow to pickett. The pickett guys had no clue why they were on the line.:laughing:


Ya I know I read that one too. That was back before I was such an involved tro... er I mean member of this forum. :laughing::laughing:


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## jwjrw (Jan 14, 2010)

Jlarson said:


> Ya I know I read that one too. That was back before I was such an involved tro... er I mean member of this forum. :laughing::laughing:



It amazes me how much a tro....member can learn here.:laughing:


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## miller_elex (Jan 25, 2008)

Frasbee,

Whenever pulling wiring, just remember that current has to flow back against itself in the same raceway. Excluding stuff like that MI cable and cutting slots, which is rare.


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## TxElectrician (May 19, 2008)

Over 30 years as an electrician, and have seen and done quite a few different type of projects, but I have never used or seen MI cable until these pics.

Always something to learn in this trade.


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## Frasbee (Apr 7, 2008)

TxElectrician said:


> Over 30 years as an electrician, and have seen and done quite a few different type of projects, but I have never used or seen MI cable until these pics.
> 
> Always something to learn in this trade.


Tell me about it!

I have just 3 years, this stuff can be overwhelming!


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## RIVETER (Sep 26, 2009)

Bob Badger said:


> That does not cause heating, that raise the impedance of the GEC (if it is not bonded to the conduit.)


That is true and it could possibly increase the impedance to the strike current. 75% comes to mind. IF...it's not bonded at both ends.


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## mattsilkwood (Sep 21, 2008)

I've never seen the single conductor MI before. What little I have worked with was 3 conductor #10. I don't care if I ever work with that stuff again. What a pain.


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