Ampacity Adjustment.

[steven765]

Okay just want to check my knowledge on this as it turned into a flame thread elsewhere on the internet. *cough* reddit.

So derating/adjusting romex through holes in resi construction.

Say you have 4 12/2 romex cables, so 8 current carrying conductors

310.15(B)(16)

7 to 9 conductors = 70% adjustment

12 gauge 90 degree c wire is 30A

so 30 * .7 =21A

Great!

But what if the terminals are 75deg c? My read on 110.14(C) specifically the second sentence "
Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
Means you don't have to take the terminal into account for bundling adjustment. To me this makes sense as the threat is heat at the bundling point. You're still sizing the overall conductor for the 75deg c column for overall current capacity. Just double checking with the hive mind.

 

[Seven-Delta-FortyOne]

NM cable has to use the 60c table.


Derating for bundling should only come into play with 24“ of continuous bundling, if you have an inspector who knows the code.

 

[steven765]

NM cable has to use the 60c table.


Derating for bundling should only come into play with 24“ of continuous bundling, if you have an inspector who knows the code.

I think part of the problem is art 100 doesn't define spacing and "bundled for a continuous length" is open to lots of interpretation. I have a suspicion that many AHJ's are seeing the wires run in parallel for any length even if free floating with spacing here and there and going: AhhHA those are bundled.

 

[DrSparks]

Your derated ampacity for 4 x 12-2 cables is 21 amps. Whats the problem?

Sent from my BE2028 using Tapatalk

 

[DrSparks]

NM cable has to use the 60c table.


Derating for bundling should only come into play with 24“ of continuous bundling, if you have an inspector who knows the code.

You are allowed to use the 90° column for derating more than 3 CCC. The final value cannot exceed that of a 60° conductor.

Sent from my BE2028 using Tapatalk

 

[Little Bill]

If the holes aren't fire stopped, then no adjustment is necessary.

 

[Dennis Alwon]

@steven765, your calc is correct. Even though nm is 60C the derating starts at the 90C rating of the wire inside nm. The only issue, but not in this case, is that the overcurrent protective device cannot be larger than the 60C rating of nm.

110.114(C)
Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected and
coordinated so as not to exceed the lowest temperature rating
of any connected termination, conductor, or device. Conductors
with temperature ratings higher than specified for terminations
shall be permitted to be used for ampacity adjustment,
correction, or both.

 

[infinity]

As Bill stated a bored hole in a stud requires no derating if the hole is not filled. If the cables are bundled before and/or after the hole for more than 24" then derating would apply.