# When to Derate NM Cable



## jar546 (Nov 25, 2018)

This is a great example where 334.80 comes into play, specifically the last paragraph.  In this case, the affected circuit conductors must be derated and adjusted in accordance with Table 310.15(B)(3)(a)


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## Builder Bob (Nov 26, 2018)

more than a derating problems --- I don't see any protection for the conductors from the sharp edges of the track and the great stuff is being used outside its listing capacity for fire blocking - Great stuff is evaluated for annular spaces only
https://icc-es.org/report-listing/esr-1961/


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## ICE (Nov 26, 2018)

_334.80 Ampacity. 
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.80(A). 


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 sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply. 


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)(3)(a). _


The second paragraph has to do with three cables through holes in wood framing that are sealed.  That is not the case here.

The third paragraph has to do with three cables in contact with thermal insulation.  That is not to be taken to mean incidental contact as is shown in the picture.

This is a case of bundled cables.  The code says spacing without any indication of what distance shall be maintained.  I use 1/2".  The correction should state "Eliminated bundled cable".  Now I suppose the correction could state, "Apply deration factor for bundled cable".   Then we get into trying to figure which are bundled with which....is it six wires or only four.....In all the confusion I lost track...but if it’s longer than 24”......

Beyond that, if 334.80 is the violation, how many current carrying conductors should be used for the calculation? 6 or 44?

_310.15(B)(3) Adjustment Factors. 
(a) More Than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). _


Regarding the foam: If there were to be a skin on the metal studs and a covered lid, then that's probably the wrong foam as the location requires a fireblock.  If the lid is not to be covered there is no requirement for a fireblock and therefor any foam is okay and in fact not necessary.


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## RJJ (Nov 26, 2018)

It is a good example for ampacity adjustment. What about the wires in contact with the block?


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## steveray (Nov 27, 2018)

Doesn't the block act as a heat sink therefore offsetting the bundling?......


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