3/4" Conduit - 16 or 17 Wires?


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Old 05-28-07, 06:05 PM
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3/4" Conduit - 16 or 17 Wires?

For 3/4" conduit, I remember seeing somewhere that the maximum number of 12 gauge THHN/THWN wires that can be run is 17. (The run is short enough so I wouldn't have to be concerned about derating.) But then I did some web searches and found some online calculators that show this number to be 16. So why the discrepancy, is it perhaps round off error in the calculation?
 
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Old 05-28-07, 06:11 PM
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what kind of conduit and how long is the run? Although you say it is not long enough to consider derating, if it is over 24 inches, you are past a nipple and need to derate.

if you are speaking of EMT, then code states 16. (that includes all of the neutrals and the ground)

there is no rounding involved.
 
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Old 05-28-07, 06:26 PM
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Sorry I forgot to mention the type of conduit, 3/4" schedule 40 PVC RNC. The run is less than 24" (outdoor service panel to junction box in crawl space). According to this webpage, for 12 gauge I could go up to 18 wires. Now I'm getting more confused.........

http://www.carlon.com/Master%20Catalog/Conduit_Schedule_40_and_80.pdf
 
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Old 05-28-07, 06:53 PM
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Have you already combined all the grounds? How many wires do you actually NEED? Are you limited to 3/4" conduit, or to RNC?
 
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Old 05-28-07, 07:08 PM
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you can put 15 #12 thhn/thwn conductors in 3/4 in sched 40 pvc.

it don't make no diff what Carlon states. NEC rules.

Now, since the nipple is less than 24 inches, you can fill to 60% which should allow you 22 (maybe even 23, I didn't calculate it other than 1.5 times the 15 count) conductors and you still do not need to derate.
 
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Old 05-28-07, 07:14 PM
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Schedule 40 PVC conduit can contain up to 15 12-gauge THHN or THWN wires. From table C.10 of the 2005 NEC.
 
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Old 05-28-07, 07:24 PM
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2005 NEC note #4 to chapter 9 Tables allows what I stated in my previous post (60% fill and no derating)
 
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Old 05-28-07, 08:43 PM
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Sorry nap, I did not mean to imply that I disagreed with you. I was posting at the same time as you, not reacting to your post. I agree with your analysis.
 
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Old 05-28-07, 08:52 PM
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that's cool. I was just reinforcing my point. I did not know if you (or anybody else posting) was aware and was giving the specific location of the info.

no apologies required. no offense taken, or even considered.
 
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Old 05-28-07, 09:49 PM
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Reading Nap's response in post #5, I assume the 15 conductors is based on 40% fill. But I just want to make sure that I'm correctly reading that I'm allowed to go to 60% full (i.e., 22 conductors) if my conduit is less than 24 inches?

FYI, I'm down to my last (easily accessible) knockout on the service panel and want to make the most of it. So I want to pull more conductors than I immediately need in order to leave room for future expansion - but on the other hand I don't want to stuff the conduit too full either. BTW, I've already planned on combining all of the grounds.

Also, are the NEC conduit fill tables available online? If not, I suppose I could find them from someone at work, but us chemical engineers don't talk to the electrical engineers!

BTW, if I haven't already, I wanted to thank everyone on this forum for your advice - your help has been appreciated.
 
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Old 05-28-07, 09:57 PM
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yes, if the nipple is less than 24 inches, you can go to 60% fill. The 15 count is 40% fill.

as far as online; not sure. I think it may be but since I have mine laying around I have never needed it online.

It seems there have been some threads here that gave a locate for it. You might try searching the threads here.

btw; you need to size the ground for the largest circuit or conductor (somewhere in life, the code changed to require the ground be upsized to match the current carrying conductors if you upsize them for voltage drop) but you need only one.
 
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Old 05-29-07, 08:14 AM
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You are not limited to the knock out size. If you have punches you can punch a bigger hole for a 1" conduit.
 
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Old 05-29-07, 02:05 PM
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The 2005 NEC is available on-line at:

http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7005SB&cookie%5Ftest=1

I think jwhite originally posted this site. I remember that there might be some trouble after you click on the agreement but I don't remember just exactly what that problem was.
 
 

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