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Bored Holes #88459 06/06/04 06:06 PM
Joined: Feb 2004
Posts: 494
M
mustangelectric Offline OP
Member
Hi,
Does derrating apply to bored holes in studs? Can you run more than one cable through a bored hole? What about fill capacity? Does this apply to bored holes?

I have seen a lot of 1-1/2" holes bored above panels that are filled with NM cables.

Thanks for any replies.

Mustang

2017 / 2014 NEC & Related Books and Study Guides
Re: Bored Holes #88460 06/06/04 06:43 PM
Joined: Mar 2001
Posts: 333
S
stamcon Offline
Member
The cables need to be stacked or bundled for more than 24" before derating is required.
I don't recall seeing a conductor fill requirement for a bored hole.
I guess it would depend on the length of the bored hole, as to it being considered a "raceway"?

steve

[This message has been edited by stamcon (edited 06-06-2004).]


Steve
Re: Bored Holes #88461 06/06/04 07:09 PM
Joined: Apr 2002
Posts: 558
C
caselec Offline
Member
Joe posted this proposed change a while back:

Proposal 6-31
Revise Section 310.15(B)(2)(a)

The proposed revision to this section adds a more restrictive requirement to apply ampacity adjustment factors where more than two NM cables containing two or more current-carrying conductors each are bundled together and pass through wood framing members where fire-stopping or draft-stopping material is installed at the penetration. The ampacity adjustment factors specified in Table 310.15(B)(2)(a) would be required to be applied to the current-carrying conductors installed in this manner.


Does this mean that the ampacity of NM conductors need to be adjusted anytime more than 2 cables penetrate the top plate even though its only 3"?

I have never been very concerned about derating multiple NM cables passing through single holes since they are usually not bundled tightly between the holes. The product Dave Nix was showing us was able to get a UL listing for quite a few cables without having to adjust the ampacity. I think there would be more heat in that enclosed raceway than in a stud or joist bay.

Curt


Curt Swartz
Re: Bored Holes #88462 06/06/04 08:22 PM
Joined: Jun 2003
Posts: 681
P
PCBelarge Offline
Member
When conductors pass through a short space, such as a bored hole, can enough heat develop to create a problem? I do not believe so, but that is just my humble opinion.

Pierre


Pierre Belarge
Re: Bored Holes #88463 06/07/04 06:38 AM
Joined: Dec 2003
Posts: 751
E
earlydean Offline
Member
I and many other building inspectors have always considered cables in bored holes to be bundling. Article 310.15(B)(2)(a) covers bundled cables. Derating is required for more than 3 current carrying conductors. But, if we do the math, and take into account the "small conductor" rules, we find that 4 cables are allowed to be installed in every bored hole. This works for #14, #12 and #10 NM and AC cables, 2 or 3 conductor.
What is maybe more important, is the size and placement of bored holes.
Building code requires that bored holes be a minimum of 2 inches back from the edge of joists and rafters, and 5/8 inch away from the edge of studs. NEC requires nailer plates if the hole is closer than 1-1/4 inch.
Building code also limits the size of the holes in rafters and joists to 1/3 the actual measured size of the joist or rafter. For a 2 x 8 that is a 2-1/2 inch hole, for a 2 x 10 that is a three inch hole. Again the holes have to be kept at least 2 inches away from the edges of the joist or rafter. Also, never bore a hole in a truss or engineered lumber.

Earl


Earl
Re: Bored Holes #88464 06/07/04 09:21 PM
Joined: Mar 2001
Posts: 333
S
stamcon Offline
Member
Earl, what about the part of 315(b)(2)(a) that states that the cables must be stacked or bundled for more than 24" before derating shall be enforced? Bored holes through wood framing are usually less than 24".

steve


Steve
Re: Bored Holes #88465 06/08/04 12:08 PM
Joined: Jul 2002
Posts: 717
M
macmikeman Offline
Member
I would like to ask Earl a question. If you had one 12-2 romex cable loaded to 18 amps going thru drilled holes by itself, or 5 12-2 romex cables going thru the same set of drilled holes where the total load of the 5 cables combined was 18 amps, which situation do you consider more likely to become overheated? I am not trying to be funny with this, I just do not believe bundling of romex in at least residential situations is the problem. The actual problem starts when conductors get overloaded.

Re: Bored Holes #88466 06/08/04 05:03 PM
Joined: Feb 2004
Posts: 494
M
mustangelectric Offline OP
Member
Hi,
Thanks for all the replies. Here is a situation that you may want to consider;

subfloor installation in 10" X 12' X 2" wood, where two 1-1/2" holes are drilled in the center running for 12 feet parallel to each other. Consider a 1-6/3, 1-10/3, 4-12/2, 3-12/3 running through each hole..I would say that this needs to be spread out a little..

I can see the temptation to drill a couple holes instead of 5 or 6 1" holes, but a better installation would be to restrict the number of current carrying conductors to three per hole. I wonder if there is a difference for large holes vs small holes. You can easily get 10 cables through a 1-1/2" bored hole and if it runs for more than 24" parallel bundled together then I would derate the cables.

regards

mustang

[This message has been edited by mustangelectric (edited 06-08-2004).]

[This message has been edited by mustangelectric (edited 06-08-2004).]

Re: Bored Holes #88467 06/08/04 05:34 PM
Joined: Dec 2003
Posts: 751
E
earlydean Offline
Member
It is MHO and that of many other inspectors I know, that running multiple cables through bored holes more than three consecutive 16" centers rafters, joists or studs constitute bundling. I know, they may flair out just a little between the bored holes, but they are still in close association to one another, and the heat may build-up between them. I am also aware that most cables are not loaded anywhere near to full load, and the likehood of the cables breaking down due to heat build-up is next to nothing. But, we inspectors have to enforce the code as written, not as we think it should be. Just as electricians have to install according to code, not as we think it should be.

Earl


Earl
Re: Bored Holes #88468 06/08/04 06:06 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Earl that is how the inspectors I have had look at it too.

They will ask for me to break up bundles (MC) in bridal rings spaced 4', 5', 6' apart.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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