Conductor Derating

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xformer

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Master Electrician
Good Day,

Most of my electrical work has been commercial with little residential. So here goes my question. When running non-metallic sheathed cable in a dwelling unit, must the ampacity of the conductors be derated when multiple cables are ran through the same hole in the wall stud, ceiling, or floor joist?
 
xformer said:
Good Day,

Most of my electrical work has been commercial with little residential. So here goes my question. When running non-metallic sheathed cable in a dwelling unit, must the ampacity of the conductors be derated when multiple cables are ran through the same hole in the wall stud, ceiling, or floor joist?
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Yes, if the inspector considers that 'bundling'.

Personally I do consider it bundling but many people disagree with me about it.

I think you need to ask the inspector who will be looking at the job. :)
 
If the cable insulation at these points fails significantly before the 'free air' portions of the cable then this rule has some experimental basis to back it up.
The effect should be less with metal studs because wood is a better thermal insulator.
The IEEE probably has published something on this.
 
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If the hole is to be sealed with thermal insulation, caulk, or sealing foam then you will need to adjust the ampacity. See 334.80.

Chris
 
xformer said:
Good Day,

Most of my electrical work has been commercial with little residential. So here goes my question. When running non-metallic sheathed cable in a dwelling unit, must the ampacity of the conductors be derated when multiple cables are ran through the same hole in the wall stud, ceiling, or floor joist?
Click to expand...

If there is no sealant in the hole then no since the conductors are not bundled for more than 24" in the hole.
 
must the ampacity of the conductors be derated when multiple cables are ran through the same hole in the wall stud, ceiling, or floor joist?
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I am not as up to date as you guys but this sounds like a one time occurrence less than 24 inches. Would the derating still be required? I know he may do this throughout the house.
 
bob said:
I am not as up to date as you guys but this sounds like a one time occurrence less than 24 inches. Would the derating still be required? I know he may do this throughout the house.
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I'm not sure I understand your post, but the 334.80 rule has nothing to do with 24". Unlike to 310.15 bundling, 334 references penetration.
I think it is important to remember that NM is rated 90? so you have to exceed 9 ccc before it becomes a problem.
 
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iwire said:
And almost everyone is trying to sell something.:grin:

Copper Orgs purpose is the sale of copper, not public safety.

http://www.copper.org/about/cda_mission.html
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I hear ya'.

Using the appropriate insulation R values for the insulation material and layer thickness and for wood, the Neher McGrath formula should be able to confirm/disprove their numbers or clarify the OP's situation.

I have a spreadsheet on this somewhere but it's somebody else's turn to crunch numbers.
 
If the hole in the wood is filled (draft stopped, fire stopped, etc.) then derating applies. If not the NM cables would need to be bundeld for more than 24" for derating to apply. Short sections of bundled cables more than 24" still might not require derating if they fall under 310.15(A)(2)Exception. That exception does not apply to the cables in the filled hole. NM cables in thermal insulation without spacing require derating also. Confusing? :roll:
 
Been reading some posts on this topic - does de-rating only apply to enclosed cables (within wall assemblies and/or where fire stopping is applied) or would it also apply to the installation I saw yesterday - 18 Type-NM cables (36 current carrying conductors) theaded through single holes in joists (holes are about 1 1/2" high and up to about 4" to 5" wide.

What does conductor de-rating actually entail with respect to calculations and conclusions?

http://i15.photobucket.com/albums/a352/TonyPE/IMGP4096.jpg
 
tonype said:
Been reading some posts on this topic - does de-rating only apply to enclosed cables (within wall assemblies and/or where fire stopping is applied) or would it also apply to the installation I saw yesterday - 18 Type-NM cables (36 current carrying conductors) theaded through single holes in joists (holes are about 1 1/2" high and up to about 4" to 5" wide.

What does conductor de-rating actually entail with respect to calculations and conclusions?

http://i15.photobucket.com/albums/a352/TonyPE/IMGP4096.jpg
Click to expand...

If those conductors are considered bundled then yes derating would apply. 36 CCC's would require a derated value of 40%.
 
tonype said:
Been reading some posts on this topic - does de-rating only apply to enclosed cables (within wall assemblies and/or where fire stopping is applied) or would it also apply to the installation I saw yesterday - 18 Type-NM cables (36 current carrying conductors) theaded through single holes in joists (holes are about 1 1/2" high and up to about 4" to 5" wide.

What does conductor de-rating actually entail with respect to calculations and conclusions?

http://i15.photobucket.com/albums/a352/TonyPE/IMGP4096.jpg
Click to expand...

Inspectors where I work would say that derating applies to installations shown in the photo.

FWIW my house is wired in such a manner and I have no fear of living in it.
 
infinity said:
If the hole in the wood is filled (draft stopped, fire stopped, etc.) then derating applies. If not the NM cables would need to be bundeld for more than 24" for derating to apply. Short sections of bundled cables more than 24" still might not require derating if they fall under 310.15(A)(2)Exception. That exception does not apply to the cables in the filled hole. NM cables in thermal insulation without spacing require derating also. Confusing? :roll:
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In a normal residence, when is foaming the holes required? Is that a local thing because I've read several posts saying "that all the holes were foamed." I believe they were talking mostly about the top plate above the studs.
 
Spray foam is common here for penetrations.thru the top & bottom plate.

All around the exterior fixtures & weather proof recptacles of course.

Typically between basement & 1st floor, 1st & 2nd floor, 2nd floor & attic
 
Little Bill said:
In a normal residence, when is foaming the holes required? Is that a local thing because I've read several posts saying "that all the holes were foamed." I believe they were talking mostly about the top plate above the studs.
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As others have mentioned, this 'foaming' is typical of top and bottom plates because of the energy code.

However, today I inspected a house where the electrician went crazy with the foam and foamed every single hole in every stud [top/bottom plates, studs, etc]...!!

Regarding the above photo - yes, most inspectors would consider that bundling.
 
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