What about 3?

[lmchenry]

334.80 states that where more than two NM cables...the allowable ampacity of each conductor shall be adjusted in accordance with 310.15B2a. 310.15B2a is for more than three current carrying conductors. So what about exactly three conductors? How are they rated?

 

[ptonsparky]

Table 310.16

 

[don_resqcapt19]

Three conductors do not require derating, but how do you have only 3 current carrying conductors with more than on NM cable?

 

[lmchenry]

Are you refering to the neutral conductor as a current carrying conductor?

 

[raider1]

Are you refering to the neutral conductor as a current carrying conductor?

The neutral of a 2 wire circuit is a current carrying conductor. Take a look at 310.15(B)(4) for the conditions when a neutral conductor need not be counted as a current carrying conductor.

Chris

 

[lmchenry]

Yes, of course. But why does 334.80 say two NM containing two or more if it will always have 4 or more?

 

[lmchenry]

Thanks for getting my mind set correct. Also, wouldn't 2 NMs bundled be 4 current carrying conductors and by 310.15b2a need to be derated? Or does 334.80 override this? Straighten me out again.

 

[infinity]

Here's what it says:

334.80 Ampacity.
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.

Under normal circumstances if the bundling is 24" or less than derating will not apply. In this instance, even though the conductors are only bundled for about 3", derating does apply.

 

[lmchenry]

So how many contractors or inspectors derate 2 NMs that are under the same staples that run the length of a stud according to 310.15b2a?

 

[LarryFine]

So how many contractors or inspectors derate 2 NMs that are under the same staples that run the length of a stud according to 310.15b2a?

I don't. Three or more cables, though, and I use stackers.

 

[LarryFine]

... wouldn't 2 NMs bundled be 4 current carrying conductors ...

Not in every case case, such as using two 2-cond NM's between a pair of 3-ways as hot and neutral as well as a pair of travelers. Besides, not all conductors carry their ampacities.

For example, there could be a 14-2 cable feed down to a 3-gang box and three 14-2's coming back up to hit three fixtures. The most the entire bundle would ever carry is 15a.

Then, you could have four 14-2's as home runs supplying four different circuits. Obviously, each cable could conceivably be loaded to its ampacity, making 60a for the same bundle.

 

[infinity]

For example, there could be a 14-2 cable feed down to a 3-gang box and three 14-2's coming back up to hit three fixtures. The most the entire bundle would ever carry is 15a.

Are you saying this configuration would not require derating if bundled more then 24"?

 

[LarryFine]

Are you saying this configuration would not require derating if bundled more then 24"?

No, just that it ought not. Or rather, need not.

 

[infinity]

No, just that it ought not. Or rather, need not.

I've argued that too because of the amount of heat shared amongst all the conductors on one circuit would be minimal. NEC doesn't agree however.

 

[kbsparky]

Why is everybody afraid to derate some romex? Derate it already!!

Take (4) 14/2 NM cables, stuff `em into a hole, foam it up, and derate:

(8) conductors total
Table 310.15(B)(2) = 70%

Table 310.16 #14 rated at 25 Amps 90 degree column

Derated value = 17.5 Amps.

Limit circuit to 15 Amps per 240.4(D).

End result? NO CHANGE IN VALUE

So what's the big deal here?

Do the same with some #12's and you end up with 21 Amps, again limited to 20 Amp breaker. Still no change!

Edit to add: Just don't try this with your A/C circuit