Derating NM cable

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ryan_618

ryan_618

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Salt Lake City, Utah
I have (5) 12-2 NM cables with a total length of 100' each. These cables are not bundled whatsoever. They do, however, all pass through the same bored hole that is required to be firestopped. Both conductors in each cable are current carrying. Considering the 2005 change to 334.80, what is the largest size breaker I can protect these conductors with? (assuming typical circuits for recpetacle outlets, luminaires, etc).

A. 30
B. 25
C. 20
D. 15
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Derating NM cable

Originally posted by jimwalker:
Do not have 05 yet if that changes anything
Click to expand...
There is a change in the 2005, that says: "Where more than two NM cables containing two or more curretn-carrying conductors are bundled together and pass through wood framing that 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)".
 
roger

roger

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Fl
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Re: Derating NM cable

Ryan, in your question you make note that these cables are not "bundled together" so using the the pure wording of 334.80, even if they do pass through a bored hole requiring fire- or draft-stopping "C" would be the answer.

Roger
 
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bphgravity

Senior Member
Location
Florida
Re: Derating NM cable

The trick is that the cables must meet two requirements. They must be bundled and they must pass through the bored hole. Since your cables are not bundled, they don't completely qualify.

The problem I have is that bundling is really the only required qualification for derating. That requirement already exists. So in my opinion, the point where all the cables come together in order to pass through the hole qualifies them as bundled at that point.

On a side note, derating of conductors and especially NM cables is almost never enforced in my area. I believe the FBC indicates an attic temperature of around 120? F in the summer here in Florida, yet cables are typically installed to full ratings in most structures.

[ June 21, 2005, 06:48 PM: Message edited by: bphgravity ]
 
don_resqcapt19

don_resqcapt19

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Re: Derating NM cable

Ryan,
I think that the 50% derating applies in this case. I don't know why they used the word "bundled" in this section as the supporting information indicated that the fire or draft stopping foam was the problem and that a fully loaded NM cable would exceed its 90 degree C rating under that condition.
Don
 
J

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: Derating NM cable

Where would we have that many NM that are fully loaded ?Seems in a case like that it be cheaper to have seperate holes than to run number 10
 
roger

roger

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Re: Derating NM cable

Jim, read Ryan's second post. It's a hypothetical situation for an article he is writing.

Roger
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Derating NM cable

Here is what I am thinking:

The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that 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).
Click to expand...
Now we go to 310.15, and this exception
310.15 Ampacities for Conductors Rated 0?2000 Volts.
(A) General.
(1) Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15(B) or under engineering supervision, as provided in 310.15(C).
(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 adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
FPN: See 110.14(C) for conductor temperature limitations due to termination provisions.
Click to expand...
I submit that because this derating rule only applies when conductors are bundled (but less than 24"), this rule would only apply when the total length of circuit is less than 25' in length, and that is assuming an absolute worst case scenario.

A more likely scenario: The conductors aren't bundled, but they pass through a 2" X 4" top plate this is fire stopped. This means that if the total circuit length exceeds 15", it can be ignored!


Agree?
Disagree?
Am I reading this right?

BTW: Thanks very much for your opinions :D

[ June 21, 2005, 07:52 PM: Message edited by: ryan_618 ]
 
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bphgravity

Senior Member
Location
Florida
Re: Derating NM cable

I was wondering why you intentionally made the seemingly non-important reference to the cable lengths. The arguement seems to hold, but does that make it right?

The code "seems" to be indicating this is a special circumstance and the normal rules don't apply. And if you read the section in order, you would first determine the ampacity of the cables and then apply this rule. So for your example, you would have 5 cables, 100' long. I would take this information first and determine amapacity. Now I continue with the section and it tells me to derate if I meet the conditions of that paragraph, which we do in your example. At this point, I don't think it would be appropriate to go back and apply a rule that effectively eliminates the entire paragraph.

It certainly makes for an interesting conversation and I definetely think you should include the idea in your article. Perhaps the section should indicate the exception does not apply.
 
J

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: Derating NM cable

You know years ago it was common to run all your home runs (NM) thru a piece of 2 inch pvc into a panel,sometimes even over the 2 foot rule a little.Never once seen the wires or insulation harmed.It made for a neat install and was fast.Now for some reason it will no longer work cause of bundling and wrong connectors.Now we are going after wires that are being fire calked.Lets take it one more step,can we legally apply fire calk to a piece of nm ?
 
don_resqcapt19

don_resqcapt19

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Location
Illinois
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retired electrician
Re: Derating NM cable

Ryan,
In my opinion a specific rule, like the one in 334.80, always overides a general rule like the one in 310.15.
 
roger

roger

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Location
Fl
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Retired Electrician
Re: Derating NM cable

Don, you may be right (I think you are) that the use of the word "bundled" in this section was not intended, but unless there is something to change it, (errata possibly) the fact is, it exists and can be used to get around the derating requirement.

Jim,
Lets take it one more step,can we legally apply fire calk to a piece of nm ?
Click to expand...
absolutely, there are a number of UL through penetration assembly' where fire caulk is applied directly to a NM cable, one example would be UL System .No W-L-3001

Roger

[ June 22, 2005, 08:15 AM: Message edited by: roger ]
 
George Stolz

George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
Re: Derating NM cable

Originally posted by macmikeman:
Stay tuned, soon enough it will be one cable per hole, and the only allowed place to locate an electrical panel will be in the house next door.
Click to expand...
I don't take any chances. I slit the sheating and run the individual conductors through their own holes.

And panels are way too dangerous to be in the neighbor's house. I bury them in the yard at least 25' from the house. :)
 
sandsnow

sandsnow

Senior Member
Location
Southern California
Re: Derating NM cable

I don't know of any fire stop systems that utilize "sealing foam", but there are hundreds if not thousands of systems so maybe there is.
By sealing foam, I'm thinking "Great Stuff" or equal; the stuff that expands after you squirt it.
The fill material for firestop systems has the consistency of putty or caulking so the section in question would not apply.
This may not be the intent of the CMP, I think it's a literal reding of the section.
Also if you are just sealing against cold or warm air migration the section would not apply.
 
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