I was wondering if you guys could give me a hand. Before I begin let me apologize for my ignorance on this subject since the major part of my experience is in residential wiring. We are looking at trying to run our panel box drops from the second floor to the basement through schedule 40 RNC. The reason behind this is becuase of the addition to article 334.80 requiring derating of more then 2 cables passing through fraimming required to be fire/ draft stopped. The question I have is whether or not there is a derating factor on the conductors installed in the conduit. The conduit will be 8 to 9 feet in length. My opinion is that it does but my third party inspector is trying to tell me that it doesn't since the conduit is not a complete system. Any help would be greatly appreciated.
NM cable in RNC
- Thread starter kendog
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That's a fill table. You can exceed the normal conduit fill when a conduit is used for damage protection, but you'll still have thermal derating issues. Cables bundled or in a raceway need to be derated. Even if you didn't consider an incomplete conduit system a raceway, I don't know how you could consider cables in a pipe not bundled.
For most NM cables, you can fit 4 in a bundle or raceway before the ampacity has dropped enough that you need to begin to oversize. So I'd plan for a bunch of 1" holes if you have a lot of cables.
For most NM cables, you can fit 4 in a bundle or raceway before the ampacity has dropped enough that you need to begin to oversize. So I'd plan for a bunch of 1" holes if you have a lot of cables.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
For small amperage circuits the magic number before derating becomes a factor is 9 CCC's. You could put 3-two wire and 1-3 wire cable in to the conduit without having to worry about derating.
kendog said:
What does "bundled" mean in relation to NM?
I take it to mean that they are together(no spaced) for a distance of 24" or greater..as a reference, see: 310.15(B)(2).
The 24" for derating is mentioned at least 8 times in various NEC articles...although no specific mention is made of NM.
HUH?bob said:
Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable
I could easily have more than 4 conductors in a raceway or cable and not have to derate.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
kendog said:
Well this isn't the same as the 24" bundling rule but the end result is the same. The NM cables through the hole with fire or draft stopping material require derating. But unless you have more than 9 current carrying conductors the derating should have no affect on the current carrying ampacity of small NM cables.
I would go one step further to say that there is not a requirement for any derating with regard to NM through fire or draft stopped holes because 310.15(A)(2)Exception would apply.
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