How many 12-2 romex cables in 3/4" and 1" emt?

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  #1  
Old 10-31-02, 10:51 AM
L
lars3159
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How many 12-2 romex cables in 3/4" and 1" emt?

I can't remember how you arrive at a number. Do you use 40% fill? I you could give me code references it would be helpful.

Thanks
 
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  #2  
Old 11-01-02, 10:41 AM
H
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NM-B (Romex) cable is not to be used in any conduit. You must use THHN or equivelent. You can use Romex for short runs if you remove the outer sheath. You can run as many conductors in the conduit as the manufacture of the conduit permits (sometimes written on the side) but you must derate the wirers. You should look it up, but I believe you can have up to 3 current carrying conductors in a conduit before you need to derate. If the conduit is filled under 40% I do not think you need to derate at all. Hope this helps. Check on a wire mfg. site like Belden. They should have a chart online.
 
  #3  
Old 11-01-02, 12:38 PM
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Wgoodrich
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NM-B Romex cable is allowed to be installed in a conduit not mattering the length of that conduit. However NM-B cable will fill that conduit rather rapidly.

Chapter 9 note 9 requires us to calculate that Romex cable as a round cable regardless that it is flat. A romex cable is approximately 1/2" wide making a round cable to be approximately .196 sqare inch in cross sectional area while using the formula pie R squared. Then if we look at Chapter 9 table 4 it tells us that more than one conductor [one romex cable considered as a round conductor] must be calculated at 53% fill with EMT 3/4" having a capacity @ 53% being .28 square inch. When dividing .196 into .28 you would be allowed to install only one 12/2wgrnd Romex cable in a 3/4" EMT conduit.

A 1" EMT conduit if we tried to put two cables in that conduit we would have to use a 40% fill equalling .21 square inch for more than 2 conductors. If you multiplied .196 x 3 you would get .588 square inch exceeding the 40% fill allowed for over 2 conductors in a raceway. Therefor we now know that you are limited to two conductors if that will compute. If we install only 2 conductors in a 1" raceway then Chapter 9 table 4 allows us to fill to 31% which would be .16. If we multiplied the .196 times 2 cables we would get .392 square inch required in that conduit. YOu are limited with two conductors to .16 not meeting the capacity required. Therefor you must not install two cables in this 1" conduit because you would be exceeding NEC rules. This puts us back to only one conductor or cable allowing a 53% fill. This would create a capacity of .28 cubic inch @ 53% fill for one conductor which is more than the .196 cubic inch required for one 12/2wGrnd cable.

After we are done calculating hte answer to your question is one 12/2wgrnd cable is allowed in an EMT conduit whether it be a 3/4" or 1" EMT conduit.

YOu do not exceed the 3 current carrying conductors in a raceway so no ampacity deration is requried per table 310.15.B.2.A of the NEC.

Hope this helps

Wg
 
  #4  
Old 11-01-02, 12:43 PM
J
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My views differ from Homer's.

There is no prohibition to NM cable in conduit. The code has many, many references to cable in conduit. Having said all that, however, it is usually better not to run cable in conduit.

Do not remove the outer sheath from NM.

Derating of NM-B usually doesn't play a factor until you get up to more than 9 current-carrying conductors. Derating is independent of conduit fill.

Tonight, I'll get out my code book and try to answer the question posed in the subject line (if somebody else doesn't do it first).
 
  #5  
Old 11-02-02, 06:55 AM
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workatit
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Romex is run through conduit into the breaker box at 90% fill rate. In basements I have seen romex run through conduit when it is hanging on the wall or attached to the underside of floor joists. What are the technical concerns with romex in conduit? Will it overheat?
 
  #6  
Old 11-02-02, 07:11 AM
J
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Code differentiates between "complete conduit systems" and "sections of conduit used to protect exposed wiring." Unfortunately, it doesn't define when one becomes the other (but everybody has an opinion on it). It also differentiates "nipples" which cannot exceed 24" in length.

Nevertheless, when properly done, cable can be used in all of these. The concerns are both heat (which ampacity derating rules take care of), and physical damage that might result from pulling (which conduit fill rules take care of).

I compute the same answer as Wg. You can put only one 12/2 in either a 3/4" or 1" EMT, if you have a "complete conduit system" rather than a "section of conduit used to protect exposed wiring."

P.S. workatit, where in the code book did your 90% number come from?
 
  #7  
Old 11-02-02, 07:37 AM
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workatit
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John,
My 90% is just an observation. I am a DIY and really appreciate input from you on my projects.

In our house there are 3 large diameter plastic conduit pieces (18" long) coming in the top of the breaker box. All the romex from the different circuits run through these and they are 90% or more full. Based on your comment about "protective" versus "enclosed systems" am I correct to assume that conduit below floor joists and the conduit into the breaker box are considered "protective" and therefore can be filled with romex?

I am running 3 new 12-2 circuits for a basement project and there is an area that is a 17' length of space that I cannot get up between floor joists. I thought I could use 1" plastic conduit with sweeps at the end to come down below the floor joists travel 17' then above the joists into the breaker box. Is this considered protective? Is this acceptable?

Thanks in advance. I always get a positive education with this forum. I appreciate the folks that know sharing advice with those of us learning. Teach a man to fish...
 
  #8  
Old 11-02-02, 07:45 AM
J
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That nipple may be limited to 60% fill (depending on factors I can't see). Did you actually compute 90%, or is it just a gut feel? Please be careful about citing a single observation as if it was a rule.

Please be generous when you size that conduit. I think you'll later be disappointed that you only used 1" conduit. Whether or not it is considered protective is up to the judgement of your inspector -- I suggest you ask for an interpretation.

Also be careful -- your project has some implications regarding the fire code. You may need firestopping material at both ends of that conduit. Ask about that too.

You are doing right to investigate these issues before charging ahead. Good luck with your project.
 
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