# derating N.M.



## Dennis Alwon (May 9, 2009)

kielarsp said:


> Hi Guys,
> Im gonna do a lecture on N.M. wiring. I need to address Art. 334.80 on ampacity and derating of conductors. Could someone explain this to me in laymans terms. I know this subject is one of the most neglected practices in the installation of N.M. wiring. Thanks in advanced.





> 334.80 Ampacity.
> The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.


This simply states that we must use the weakest link in out wiring system. Giving the fact that most terminals etc are rated 75 C that is usually our weakest link, however with NM cable we are limited to 60C. The rating inside nm cable is 90C wiring and that may be used to derate but it shall never be protected by anything higher than the 60C rating except in a few circumstances in the code.



> The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.


 For cable trays not often used.



> Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.


 Here we usually refer to as stacking or bundling. We never want to bundle to much so that our wires cannot breathe. You must derate for more than 2 NM cables bundles together.



> Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).


Here is similar to above but usually relates where you penetrate a floor of ceiling plate. If fire caulking, etc is used you must derate over 2 NM cables.

Now for an example. Suppose we have 6 -- 12/2 nm cables going thru a hole that is fire caulked. Art. 334.80 tells us to use T. 310.15(B)(2)(a). So here we have 12 CCC in the 6 cables and we must derate to 50% of the rating of #12 from T. 310.16. We may use 90C for this. 

#12 is rated 30 amps at 90c but 50% of that leaves us with a max. 15 amps on the circuit. All those wires must be derated to 15 amps. 

Same problem with 4 nm cables. Now we have 8 CCC and must derate 70%. Well 30 * 70% = 21 amps so we can still use a 20 amp OCP on this circuit. 

Hoe this helps.


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## BryanMD (Dec 31, 2007)

kielarsp said:


> I know this subject is one of the most neglected practices in the installation of N.M. wiring.


Probably because there are so few NM installs (typically resi) with (voltage drop) conditions that warrant it even being contemplated; and most of those being about plain old poor design.

Design deficiencies that can be avoided by simpler means like a better placement of the panel, additional/sub panels or just common sense install standards (as Dennis described so well).

Do you see many NM applications with de-rating issues related to the cable itself? I can only recall one.


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## kielarsp (Apr 9, 2009)

what I see is a E.C. will drill 1 2" hole and fill it with as many Homeruns that will fit. I believe this is not a good practice.


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## BryanMD (Dec 31, 2007)

kielarsp said:


> what I see is a E.C. will drill 1 2" hole and fill it with as many Homeruns that will fit. I believe this is not a good practice.


Oh. Well that's why they make large diameter drills (and nail plates). 

Your lecture subtitle:
The lesser of several evils.


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## jwjrw (Jan 14, 2010)

BryanMD said:


> Oh. Well that's why they make large diameter drills (and nail plates).
> 
> Your lecture subtitle:
> The lesser of several evils.


 
I use a 7/8 nailbiter and stick to no more than 3 so they pull easier.


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## kielarsp (Apr 9, 2009)

I did find an example of this in art.334.80 in the 08 NEC handbook. The author explains this nicely . Thanks for all your help


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