derating or not nm

Status
Not open for further replies.
macmikeman said:
3rd question is "who is the knucklehead who went around sticking thermometers in firestopped top plates with more than two cables in them in the first place? Didn't he have anything better to do?
Click to expand...
In the 2005 ROP said:
7-150a Log #CP700 NEC-P07
(334-80)
Final Action: Accept
Submitter: Code-Making Panel 7
Recommendation:
Add a new paragraph to 334.80 to read as follows:
"334.80 Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing which is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a)."
Substantiation:
Code-Making Panel 6 Rejected Proposal 6-31 to add the proposed text to 310.15(B)(2)(a) and provided the following Panel statement: "The Panel agrees with the intent of the Proposal, however, this material is more appropriately addressed in 334.80 since the Proposal only applies to one type of cable, and Code-Making Panel 6 covers all wiring methods. Therefore, Code-Making Panel 6 has forwarded this Proposal to Code-Making Panel 7 for action."
The substantiation provided by the submitter, Travis Lindsey, of Proposal 6-31 was:
"Recent experimentation shows the possibility of dangerous conditions when loaded circuits are brought into close proximity to each other inside a fire- or draft-stop, where the ability to dissipate heat is extremely limited. Cable temperatures well in excess of their 90?C rating were encountered, with no overcurrent protection present for these conditions. Results indicate that immediate adjustments should be made to the NEC to apply at least to the specific case represented by the experiment. Such a proposal is being made, with a supplemental report offered as technical support."
Panel Meeting Action: Accept
Click to expand...
 
dnem said:
infinity said:
He would only have a problem if the 310.15(B)(2) exception did not apply.
Click to expand...
What exception applies ?
Click to expand...
It's before the several pages of tables prior to 310.15(B)(2)(a).
310.15(B)(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adja-cent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a dis-tance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
Click to expand...
 
infinity said:
It's funny George, when I read 310.15(B)(2) I don't see that existing language in the 2005 NEC.
Click to expand...
I figure that's what my rebuttal is going to be in the comment stage. I can't comprehend how they can answer with some of the answers they do, seemingly without reading the affected section over again to be sure of their position.

They should be required to get Charlie's Rule tattoos before sitting at the ROP meetings. :lol:
 
infinity said:
dnem said:
infinity said:
He would only have a problem if the 310.15(B)(2) exception did not apply.
Click to expand...
What exception applies ?
Click to expand...

Here is another post that addresses 310.15(B)(2) Ex.:

http://www.mikeholt.com/codeForum/viewtopic.php?t=25893
Click to expand...
Infinity,

You were throwing me off because you posted 310.15(B)(2)X and I was reading thru the 5 exceptions after 310.15(B)(2)(a)
It's actually 310.15(A)(2)X

I read the other thread you posted and understand now. I never knew there was a conflict between 310.15(A)(2)X & 334.80 until I read that thread.

My question now is this:
I would think that the times that it is necessary to pass thru firewalls with groups of NM is very very limited on each job, so wouldn't it be best to just limit the number of NMs thru firestopped wood to 2 Romexs [as allowed in the second paragraph of 334.80] or run more than 2 Romexs per hole but limit the number of total current carrying conductors to 9 ? [which keeps you in 70% column of Table 310.15(B)(2)(a)]

Then you don't have to worry if you fall under 334.80 or 310.15(A)(2)X.

David
 
I believe there is a distinct chance that there is a difference in the theory behind 310.15(B)(2)x and 334.80 in this issue.

334.80 deals with the heat that develops in one specific location of the conductor, not along the length of the conductor. Since the effect has nothing to do with "10 feet or 10%", it deals strictly with the area affected by the heat developed within the plate - which is a concentrated area. I believe the CMP will reject the proposal sent to try to correct this, and may even write something in the NEC as to what I just mentioned.


Someone mentioned that they question a persons intention and free time issue about checking the temps of conductors in areas such as this. If the fact of his testing is correct, we should be grateful to the people who spend their time making these tests, as it brings to light things we may never have thought about.
It is the checks and balances that keep us all on our toes.
 
Infinity,

You were throwing me off because you posted 310.15(B)(2)X and I was reading thru the 5 exceptions after 310.15(B)(2)(a)
It's actually 310.15(A)(2)X
Click to expand...


Dave you're correct. I should have posted 310.15(A)(2)Ex. not (B).

Sorry for the confusion.
 
Pierre C Belarge said:
If the fact of his testing is correct, we should be grateful to the people who spend their time making these tests, as it brings to light things we may never have thought about.
Click to expand...

Agreed. It's a bitter and odd pill to swallow, though, that we can overload a conductor for ten feet and the NEC allows it, but 3" is going to kill someone. It doesn't make sense.

I'd like to see the evidence, all things being equal. :?
 
I'd like to see the evidence, all things being equal.
Click to expand...

That is my point. I could be far off base here, but is there any real world evidence of real hazard here, or theoretical could be, maybe, the numbers on the sketch pad say so type of possible hazzard. I keep on finding myself in the position of having to argue the point that I feel it is best to write codes based on actual events, or at least substantial evidence that you could repeat in the lab, such as actual top plate combustion, prior to a code getting written. As it is currently written, it is just one more area for misinterpretation by local inspection. I am all for safety. But honestly, this is just like bundleing. Anybody have an actual case study to report?. I could tell about several hundred that are more than 25 years old now that have not caught fire where multiple cables were pretty much stuffed thru top plate holes. It is written in the code so therefore I will follow it with fevor. I just think it is dumb. One man's opinion. Fire away matey's.
 
William Laidler said:
This proposal, along with the supporting reports, demonstrates that when type NM-B cable is surrounded by fire- or draft-stop material, such as thermal insulation or sealing foam, the ability of the conductors within the cable to dissipate heat is extremely limited. This is obviously a safety concern that needs to be reviewed. The reason for my rejection of the proposal is that I believe derating the cable may not solve the problem. In my opinion, the safety concern would be better addressed in 334.80 ampacity of NM-B by restricting the ampacity rating of the conductors of NM-B cable to that of 60?C (140?F) conductors whenever it is surrounded or imbedded in thermal insulation.
Click to expand...
I wonder if they are unaware 2002-334.80 called for conductors to be loaded according to the 60? column already?

I wonder if the submitter loaded the conductors to 90?C, and then found that (surprise surprise) that spot on the conductors was higher than 90??

I wonder where to find the supporting reports?
 
This came up in a post a while ago. Supposedly someone tested multiple NM cables through a firestopped hole and found that when they were heavily loaded the temperature within the hole exceeded the temperature rating of the cable. This does make a few assumptions however, one being that the cables will be heavily loaded at the same time.


I could tell about several hundred that are more than 25 years old now that have not caught fire where multiple cables were pretty much stuffed thru top plate holes. It is written in the code so therefore I will follow it with fevor. I just think it is dumb. One man's opinion. Fire away matey's.
Click to expand...


This reminds me of years ago when I did some work in NYC. Their electrical code had a maximum derating for any number of conductors in a raceway set at 70%. This meant that most office building buildouts had many conduit homeruns run out with the maximum number of conductors installed. It was common to see an 1 1/4" conduit that may have had 30 or 40 #12 THHN conductors stuffed in it. As you've said, I don't recall ever hearing about these office spaces going up in flames.
 
Quote:
I could tell about several hundred that are more than 25 years old now that have not caught fire where multiple cables were pretty much stuffed thru top plate holes. It is written in the code so therefore I will follow it with fevor. I just think it is dumb. One man's opinion. Fire away matey's.

So you think there is no reason to make things a little safer. :p :p
While I agree with you, for the difference in cost of labor wouldn't it be just as simple to drill an additional hole or 2 or 3.

In the vast majority of cases most of these wires will not be carrying anywhere near their maximum current at any given time. But there probably will be a few exceptions.

IMHO we should be primarily interested in the safety of people! :D
 
dlhoule said:
While I agree with you, for the difference in cost of labor wouldn't it be just as simple to drill an additional hole or 2 or 3.
...

IMHO we should be primarily interested in the safety of people! :D
Click to expand...
I think the issue becomes, why is it safer?

Do we normally imperil safety by using the exception to 310.15(A)(2)?

If the exception is safe, then why is there an exception to the exception?

It's a logical paradox. Without the data, then it becomes difficult to accept the idea that in this one circumstance, the 10%/10' exception becomes unsafe. It immediately brings either the exception, or 334.80 into doubt.
 
georgestolz
I think the issue becomes, why is it safer?
Click to expand...

I am not sure that it is, but I do know that if you take copper pipe for plumbing; you can heat one end enough to solder a joint without any noticeable difference on the other end. Much less than 10% of the length, which leads me to believe that under an extreme or unusual condition it just might cause a fire.
 
De-rating for 1 & 2 family dwellings should not apply.
I have seen a test performed by UL where they have taken
38 - 14 -2 with ground NM cables installed in a 2" x 3" area
put 12 amps on every conductor (total of 76 conductors)
for 4 1/2 hours and the conductors never over heated.
Thats a total of 912 amps, Can't imagine that will ever happen in a 1 or 2 family dwelling.
 
tom25 said:
De-rating for 1 & 2 family dwellings should not apply.
I have seen a test performed by UL where they have taken
38 - 14 -2 with ground NM cables installed in a 2" x 3" area
put 12 amps on every conductor (total of 76 conductors)
for 4 1/2 hours and the conductors never over heated.
Thats a total of 912 amps, Can't imagine that will ever happen in a 1 or 2 family dwelling.
Click to expand...

Okay, Okay, I am convinced, but would it be okay for me to drill 1 or 2 extra holes anyway? :D :D
 
Status
Not open for further replies.
Top