Clarification on 334.80

[vhemp30]

Hey guys I have a few questions,

#1 - The way I'm interpreting the bolded section of 334.80 doesn't line up with i.e. derating more than 3 current carrying conductors.

When I derate four (4) 12/2 NM cables, 70% of 30A = 21A, that final calculated ampacity exceeds that of a 60 degree C rated conductor (20A for 12 CU). But what the derated ampacity tells me is that I can "bundle" four (4) 12/2 NM cables because they are rated for 21A max.

So I feel like the wording in 334.80 isn't worded correctly OR when it says exceeds, it's saying the 60 degree C rating can't exceed the final calculated ampacity. But again, I'm not reading it that way.

#2 - For "without maintaining spacing between cables", what would count as the cables maintaining spacing? I feel like any "bundling" in a typical wood framed opening WOULD NOT maintain spacing and therefore would need to be derated.


"334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15 Tu allowable ampacity shall not exceed that of a 60°C (140 rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations provided the final calculated 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 trays shall be deter mined 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 ther mal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted n accordance with Table 310.15(B)(3)(a)."

 

[don_resqcapt19]

The wording could be better, but all it is saying is you cant use 12 AWG NM at an ampacity greater than 20 amps, where the results of the ampacity adjustment calculation exceeds 20 amps. If the results of the ampacity adjustment calculation gives you a number less than 20, than that number is the ampacity of the 12 NM under those conditions.
Yes the net result is that you can bundle 4 NM cables on a single phase system and still use the 12 NM at an ampacity of 20 amps.

 

[infinity]

As Don stated it could be worded better but you have the right idea 4-2 wire cables bundled toghether would have a derated ampacity of 21 amps so you're good with a 20 amp OCPD. Bundle 5-2 wire cables and you're now only good for a 15 amp OCPD

 

[Dennis Alwon]

So many inspectors interpret this rule to mean you cannot have more than 2- cables in a hole. This is not the case at all

 

[infinity]

So many inspectors interpret this rule to mean you cannot have more than 2- cables in a hole. This is not the case at all

I have that argument with a few people, and it always comes down not reading it correctly because for one thing it states "Where more than two NM cables" so you could have 8 CCC's in one hole (2- 4 wire cables) and derating does not apply.

 

[Dennis Alwon]

Actually I can have 10 cables go thru a hole. If there is no insulation or caulking then there is no de-rating.

 

[vhemp30]

So many inspectors interpret this rule to mean you cannot have more than 2- cables in a hole. This is not the case at all

I have heard of some jurisdictions limiting number of cable assemblies/NM Romex thru a hole in their state codes??? Is that actually the case or just the way people learned to rope houses?

 

[infinity]

I have heard of some jurisdictions limiting number of cable assemblies/NM Romex thru a hole in their state codes??? Is that actually the case or just the way people learned to rope houses?

My guess is that as Dennis said many people just can't seem to read this section correctly and they enforce it incorrectly.