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voltohm 11-20-2008 09:41 PM

How many wires in a conduit?
I have notes from a building class that refers to code requirements for the maximum number of wires you can have in a given size conduit. Does anyone know that info?

speedy petey 11-21-2008 04:48 AM

Yes, it is in the NEC Annex C.
There are about 12,000 entries for different combinations of conductors and raceways.

Kerrylib 11-21-2008 02:00 PM

How many of those variations do you have memorized speedy?

speedy petey 11-21-2008 04:59 PM

All I have committed to memory is that with anything over nine CCC's (current carrying conductors) we must derate. :cool:

triple D 11-21-2008 08:52 PM

In my neck of the woods....
I get called by the inspectors here in the small city of Seattle, if I run more than three current carrying cond for more than 24" in any conduit, they must all be de-rated by table 310.15(B)(2)(a) The de-rating of your conductors can range from 80% of capacity to 65% of capacity. Hope this saves you a big headache later. Good luck.....

speedy petey 11-22-2008 05:06 AM

I should have been clearer. With #14, #12 & #10, you do not have to be concerned with derating until you have ten or more conductors. This is due to the restrictions of 240.4(D).

Larger conductors you need to start at three, but this is not typically an issue in residential work as it is rare to have over four CCCs of #8 or bigger.

triple D 11-22-2008 09:03 PM

Good evening all....
240 simply deals with overcurrent protection. I learned the hard way on a service re-locate. I had a panel that some one built a bathroom around, so I had to move the panel down the wall about 4'. I needed to leave the old box in place for my junctions, as the house was finished and beautiful. I ran 2 2" pipes on exterior of house with lb's to get in, etc, etc. One pipe measured 24.5" and the other 32". Turns out the 24 had the range and a sub feeder and dryer. It was fine at 6 ccc's because the inspector was feeling nice enough to let me off. But the other pipe had a number of 14 and 12 ga thhn wires, and a pair of 10ga for h.w. All those ccc's got hit at 75% because of the fact that there was more than three ccc's and the conduit was longer than 24", with no means of separation between the wires. I had to rip them all out and re-wire a size larger on all wires. I won't be making that mistake again. Just thought I would share that with ya'll. Remember its up to the inspector to enforce this, but it is clearly the way the code reads. Good luck fellas.....

speedy petey 11-23-2008 05:43 AM

First off there is no "75%" adjustment factor. Second, my point about 240.4(D) is that it way underrates 14, 12 & 10. Don't forget, we do NOT use the bottom line circuit amperage for derating. We use the actual 310.16 amperage for the conductor being used.
#14 THHN is rated at 25 amps.
#12 THHN Is rated at 30 amps.
#10 THHN is rated at 40 amps.
We used these numbers for derating.

For instance, in your scenario, if there are nine or less CCCs in the conduit,, you would have to derate by 70%.
For #14; 70% of 25 is 17.5 amps. Still we within the "standard" of 15 amps rating for #14.
Fro #12; 70% of 30 is 21 amps
For #10; 70% of 40 is 28, which gets rounded up to 30 according to 240.4(B)

Also, let's remember that grounds are not CCC's and neutrals of multi-wire circuits are not counted as CCC's.

triple D 11-23-2008 11:37 PM

sorry my bad
I looked back on my post, and in my haste to show the truth, or code, to everyone, I did make a mistake on my exact de-rating. But a sensible experienced person could clearly see that. As I stated, my other pipe had all my general loads in it. In the average, on a 200amp service, it takes not a lot of experience to know the average service holds about 15-20 single lighting and appliance circuits, at 120v. That's a substantial amount of wires. Well, not really for a 2 inch pipe. But it wouldn't matter if it were a 4 inch pipe. If there are more than 3, you lose. In 310.15 (b)(2) it is clearly stated for all to see, 4-6 ccc's is 80% ampacity allowed, 7-9 is 70% allowed, 10-20 is 50%, that's what I got hit at, and 21-30, 45%, and 31-40, 40%. So my 14 gauge wires were only now good for 12.5 amps, and my 12 gauge for 15amps. So like I said, my bad. So how does the number 9 for ccc count get involved in this? Always a hoot you guys! Good luck!:p

speedy petey 11-24-2008 04:56 AM

Well, I guess in your infinite wisdom and experience I am not sensible or experienced enough to see the obvious. So sorry sir.
I was not being a jerk, I was simply pointing out a typing error.
You on the other hand are very good at the "polite insults", aren't you?

I am also not experienced enough to determine that "a number of 14 and 12 ga thhn wires, and a pair of 10ga" mean 20 or more, or 6 or more.
If this were a 50's home with an old 100A service this could have been the latter.


Originally Posted by triple D (Post 25471)
So how does the number 9 for ccc count get involved in this?

See post #8. It explains it quite clearly.
Ten CCCs is the difference between a 70% derating and a 50% derating.
This is also the breaking point at which a 14, 12 or 10 THHN conductor must be derated below it's "standard" rating.
So when working with #14, #12 and #10, by staying at nine CCCs or less you avoid any derating issues with regard to conduit fill.

See how much fun this can be Voltohm?

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