Max conduit fill.....

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c2500

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I was talking with a Master Electrician yesterday, (about ten years younger than me but his license level is higher than mine) and in the course of conversation he said you do not have to derate wire is the conduit is 25% empty. The example he gave would be to run 100 #12 THHN through a 4" or bigger conduit. The extra space would prevent heat build up.

Have I missed something the code book? I thought 310.15 pretty much summed it up that over 3 current-carrying conductors you start to derate no matter how big the conduit.

I look forward to your response cause this is bugging me.

c2500
 
c2500 said:
. . . 310.15 pretty much summed it up that over 3 current-carrying conductors you start to derate no matter how big the conduit. . .
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You are correct. 310.15(B)(2) shall be complied with and you can't do it the way you stated and using the NEC. However, if there are any local or state amendments that changes any part of the NEC, that has to be complied with as well.

In my opinion, that sounds like a logical amendment to have but I know of no state that has adopted the idea. :)
 
I agree with Charlie, if you had just four 14 AWGs in a 6" rigid conduit you would still have to apply 310.15(B)(2).

In this case it not likely change anything till you get past 9 current carrying conductors but the point is 310.15(B)(2) has to be considered every time you pass 3 current carrying conducts in any conduit, tubing or cable.
 
That is what the rule says.

But there is another question of "is that what the rule should say"

Derating for a dozen #14s in a 4" conduit seems a bit wacky to me.
 
crossman said:
That is what the rule says.

But there is another question of "is that what the rule should say"

Derating for a dozen #14s in a 4" conduit seems a bit wacky to me.
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Running a 4" for a dozen 14s is also wacky enough that I doubt it comes up often.

I also feel there is a lot to be said for 'KISS'.

Adding exceptions or deviations for each and every particular peculiarity would swell the NEC into multiple volumes.

You could always exercise 'engineering supervision as allowed in 310.15(C). (At least I think that is the section, going from memory here)
 
crossman said:
I'll shut up now.:smile:
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No need to do that, how would we have any fun? :cool:

IMO a realistic example of 310.15(B)(2) being in the way is when you have a bank of switches, lets say 10 of them all supplied by one 20 amp circuit, you have one CCC from the panel running to the switches and 10 CCCs returning in the same conduit up to the lights.

That would count as 11 CCCs and take a large unneeded derating hit.
 
iwire said:
Adding exceptions or deviations for each and every particular peculiarity would swell the NEC into multiple volumes.
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Now there's an idea! Separate NEC versions for, say, residential, commercial/industrial, low-voltage, emergency/fire, etc.



No? :roll:
 
LarryFine said:
Now there's an idea! Separate NEC versions for, say, residential, commercial/industrial, low-voltage, emergency/fire, etc.
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Would we have even another version for when low voltage and line voltage get together? :grin:

Maybe the NFPA could just sell us all the Articles ? la carte ? :wink:

Article 250 and 310 would be top dollar, 215 would be less and Articles like 626, 630 would be at the low priced end, Chapter 3 could be purchased as a bundle. :grin:
 
LarryFine said:
Now there's an idea! Separate NEC versions for, say, residential, commercial/industrial, low-voltage, emergency/fire, etc.



No? :roll:
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No, I spend enough money on books now.:mad:
 
LarryFine said:
Now there's an idea! Separate NEC versions for, say, residential, commercial/industrial, low-voltage, emergency/fire, etc.
Click to expand...
iwire said:
Would we have even another version for when low voltage and line voltage get together? :grin:

Maybe the NFPA could just sell us all the Articles ? la carte ? :wink:

Article 250 and 310 would be top dollar, 215 would be less and Articles like 626, 630 would be at the low priced end, Chapter 3 could be purchased as a bundle. :grin:
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chris kennedy said:
No, I spend enough money on books now.:mad:
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Okay, that's two "no's." Anyone else? :cool: :smile:
 
frizbeedog said:
Installing a dozen #14's in a 4" conduit would be the wacky part.

...but I'm sure its been done.
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How about possibly a remodel where a feeder was removed, but hte conduit was left in place, and used for some branch circuits?
 
crossman said:
How about possibly a remodel where a feeder was removed, but hte conduit was left in place, and used for some branch circuits?
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Right. Now derate per code.

However, is it the size of the conduit or the proximity of the conductors to one another that is the issue?

Does size matter, or do the conductors need separation?
 
frizbeedog said:
Right. Now derate per code.

However, is it the size of the conduit or the proximity of the conductors to one another that is the issue?

Does size matter, or do the conductors need separation?
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Well, it just seems ludicrous that the 3 phase feeder with 3 -500 MCMs and carrying 300 amps in a 4" conduit is going to be less of a heating issue than 12 - #14s = 3 MWBCs, so 9 current carrying conductors in a 4" conduit.

But I like Bob's answer about it many posts ago.
 
This is an issue/topic i brung up before. It sounds like it is worth looking into.


http://forums.mikeholt.com/showthread.php?t=100951

brother said:
Ive been on many jobs and i have seen some guys who never derate, they just cram as many wires as they can in some pipe and call it a day. I try to follow as much code as possible and i even do the derating of wires even though i have to admit i have NEVER seen a fire or melt down cause people over stuffed conduits and did not derate.

But just to get your opinion, do you think there should be some leeway in the different sizes of conduit when it comes to derating?? for example if you have 12 wires of size #12 thhn in 1 inch pipe should you get a break from having to do the 50% derating as to if they were 12 wires in 3/4 inch pipe?

Seems like even though it may be a small difference in the heat its still a little different in the in heat in my opinion. what do you think? Reason i asked is because im doing a job that it would be lot easier than having to derate those wires cause the conduit runs to my area i need to go, but cant do it without upsizing everything to #10. Even though the most amps ive seen on any of them is only 6 amps. and this is at full load.
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Brother said:
But just to get your opinion, do you think there should be some leeway in the different sizes of conduit when it comes to derating?? for example if you have 12 wires of size #12 thhn in 1 inch pipe should you get a break from having to do the 50% derating as to if they were 12 wires in 3/4 inch pipe?
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My opinion is no, the NEC should not try to make allowances like this.

The book is fat enough and very confusing to many as it it.

Wording a code section to address these specific circumstances that was concise, clear, understandable and enforceable would be IMO almost imposable.

If an engineer was involved in the job the EE could design it without derating using 310.15(C).
 
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