where to find nm kcmils

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I could see a number of cables fully loaded for a dwelling unit that has electric baseboard heat. They normally undersize the heaters and load the circuits to the max.
Don
 
don_resqcapt19 said:
I could see a number of cables fully loaded for a dwelling unit that has electric baseboard heat. They normally undersize the heaters and load the circuits to the max.
Don
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I agree, normally if electric basboard is chosen it is already a budget job.

That would be about the only scenario I can imagine.

But still....Did they mention that they found any evidence of cable damage outside the laboratory?

Like I mentioned to tallgirl I would not be surprised if we see the rules for derating expanded.

Almost anything can be shown in the lab and who stands to gain the most if the derating rules where tightened?

I think this is another case of a 'fix' trying to find a problem.:rolleyes:
 
don_resqcapt19 said:
I could see a number of cables fully loaded for a dwelling unit that has electric baseboard heat. They normally undersize the heaters and load the circuits to the max.
Don
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Well, figure out a worst-case scenario and see if it's feasible.

Someone else mentioned 7W / sqft for electric heat in a discussion about load calculations for baseboard electric heat in some arctic wasteland like Chicago. For a 1200 sqft house (what I worked on this weekend) that's 8.4kW, which is significantly below the 38.5kW it would take for 24 CCCs through a 2" hole to exceed 13.5A per circuit. (No checking on the math -- sorry for any mistakes)
 
Julie,
I think the test was done with only 2 or 3 fully loaded cables, and the temperature within the "foamed" area exceed the 90?C rating of the condutors. I can't find my information about the testing so I could be wrong.
Don
 
don_resqcapt19 said:
Trevor,
I don't agree that the exception applies for this application. It is my opinion that a specific rule (like that in 334.80) always over rules a general rule (like the exception to 310.12(A)(2)).
Don
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This exception is used to allow a limited length (10 ft. or 10%) of a circuit to have a lower ampacity (passing through a higher ambient area), and still be able to use the higher ampacity for the entire circuit. Wouldn't this eliminate the need to adjust for derating through a nipple that is between 24" and 10 ft (or 10% of circuit length)?
Edited to add: Or where cables are stacked or bundled for longer that 24"?
 
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don_resqcapt19 said:
Julie,
I think the test was done with only 2 or 3 fully loaded cables, and the temperature within the "foamed" area exceed the 90?C rating of the condutors. I can't find my information about the testing so I could be wrong.
Don
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On a similar note, I drill a bunch of 3/4' hole in the top plate of a 2x4 wall. Now to get 24 cables down that wall I need to drill 12 holes with, lets say 1/2" between holes to avoid making one big hole. I have 14.5 inches between studs- now do the math...... pretty unreasonable. Now take the same scenario and get all the wires down thru the top plate, somehow, without ruining the integrity of the top plate. Okay now the insulator comes along and does this spray insulation all over the wires. In case you have never seen it, it is a mass of foam filled solid in the cavity of the walls. They spay it on and it sticks to the outside sheathing and fills the entire void. This is being down around here alot more frequently then I want to see. What do I do now?? 24 cables totally encased in this foam- where does the heat go---i'd be better off sleeving it with 2" pipe and leaving the top open (if it were legal). Any feedback on this issue.
 
Julie, speaking to the point of having _lots_ of circuits through one hole coming out of a panel (something that is apparently a very common practise):

If you look at most electrical devices used in a home, the net result of their use is _heat_ in the home. The exceptions would be AC systems with external condenser units (heat outside of the home), well pumps ( energy stored as potential energy of the raised water), and radio transmitters. (And I am sure quite a few others...) But most of the watts used in the home will show up as heat.

Run 77KW into a 1200 square foot home, on a continuous basis, and the self heating of the NM cables will be the least of your problems. *grin* I suspect that your point is quite realistic; with all of the circuits in a house going through a single hole (or a pair of holes), the load diversity of the house would generally prevent any real problems.

When the aggregate capacity of the circuits exceeds the service capacity, then there simply is no way that all of the circuits could be extensively loaded.

The test that I linked to was in reference to far fewer circuits. They had 12 current carrying conductors in a single hole, representing 6 circuits. What they did was to take 3 cables, feed them up through the hole, loop them around and went back down through the same hole, and then out to a load bank some distance from the test (so that the heat from the load bank wasn't an issue). Looking at the pictures from the report, one could argue that there was significant bundling for more than 24 inches, but at the same time they mention that they acted to prevent such bundling.

Don, did you get any information on this test other than what was in the published report?

-Jon
 
don_resqcapt19 said:
Trevor,
I don't agree that the exception applies for this application. It is my opinion that a specific rule (like that in 334.80) always over rules a general rule (like the exception to 310.12(A)(2)).
Don
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Don,

I will have to disagree. I see nothing that says this exception wouldn't apply to NM cables in a sealed hole. If I remember correctly there is even a change coming in the 2008 NEC to eliminate the use of the exception when using NM cable under 334.80.
 
don_resqcapt19 said:
Julie,
I think the test was done with only 2 or 3 fully loaded cables, and the temperature within the "foamed" area exceed the 90?C rating of the condutors. I can't find my information about the testing so I could be wrong.
Don
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That would be really scarey if true -- it means that the derating tables are wrong since four fully loaded 12/2's should be able to being fully loaded, etc.

See if you can find their test methodology. I'd like to see what they did that produced such an unexpected (to me) result.
 
rlMutch said:
Me too. 334.80 says that ampacity of NM shall be determined in accordance with 310.15. The exception is part of 310.15. (IMHO)
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I hope I am understanding this correctly because I am a little confused( ok a lot confused). It seems to me that the last paragraph in article 334.80 have absolutely no meaning at all if article 310.15(A)(2) exception overrides article 334.80. Am I misreading that?
 
infinity said:
Don,

I will have to disagree. I see nothing that says this exception wouldn't apply to NM cables in a sealed hole. If I remember correctly there is even a change coming in the 2008 NEC to eliminate the use of the exception when using NM cable under 334.80.
Click to expand...

I disagree too. We had a similar discussion regarding AC cable maybe 6 months ago. I have a ton of respect for Don and he's way more knowledgeable than I am, but I keep reading the applicable articles and don't agree with this.
 
Julie,

See the link that I posted. The building structure is described on page 2, and includes: 5/8" interior sheetrock with R-13 fiberglass insulation in the walls, and R-19 fiberglass in the ceiling. The 'cable bundle' is described on page 3.

They give no information as to how the fiberglass batt insulation was routed around the cable bundle; for example was the cable bundle pressed against the outer wall, the inner wall, or was the fiberglass split with the cables buried in the insulation? If the cables were buried in fiberglass, then a strong argument can be made that they were 'bundled' for much more than the few inches in the wood itself.

On page 3 they also describe the electrical loading, not in terms of actual amps, but instead in terms of percentages...without saying what the baseline is. Comments in the 'results' section show that the baseline is set by the common OCPD for these wire sizes, eg when they say 80% capacity they mean 16A on a 12 ga circuit protected by a 20A breaker.

-Jon
 
I see that experiment and report was issued by the Copper Development Association. I'm sorry, but it's hard to trust this organization particularly with all the economic interests they have.

Like this statement from their "Do I need an electrician?":

"Will you use all-copper wiring for any new installation?

Solid copper wiring is the material of choice for new homes or renovations. Although 14-gage wire is allowed for many circuits, it's smart to install heavier 12-gage wiring, which costs a little more but can handle more electrical current, making it safer and more energy-efficient."

It's not about safety at all, it's about selling wire with more copper content. That's how I see it.
 
I know that a lot of people do not agree with my position on the application of the exception in 310.15 to the rule in 334.80. My position is strictly based on my idea that the more specific rule always prevails over a more general rule.
Don
 
Look at this proposal.
7-74 Log #3152 NEC-P07 Final Action: Accept
(334.80)
____________________________________________________________
Submitter: Travis Lindsey, Travis Lindsey Consulting Services
Recommendation: Add an additional third paragraph to 334.80 to read:
Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining
spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).
Substantiation: Recent field experimentation shows that NM cables which are grouped or bundled together for varying lengths and installed in contact with
thermal insulation in walls and ceilings can exceed the maximum allowable design temperatures of the insulation. Even when the circuit currents were limited to eight percent or less, temperature exceeded 90?C (194?F). Full details are contained in the test report entitled NM Cable Bundles
Installed on or In Thermal Insulation, November 2005.
Note: Supporting material is available for review at NFPA Headquarters.
Panel Meeting Action: Accept
Number Eligible to Vote: 14
Ballot Results: Affirmative: 13 Negative: 1
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Don
 
iwire said:
IMO your on to it.

I read some of the test criteria, IMO the conditions they created are not what we see in real life.

A number of NMs that all happen to be in the same hole and all happen to be fully loaded (or close to it).

It seems unlikely many homes have more than one NM fully loaded at a time.
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Regardless of whether or a house would have more than one NM fully loaded at a time (I get to 2 or 3, or even sometimes 4 at night, in the winter, when it's cold, and I have friends over ...), the test methodology looks extremely suspect. Remember what I wrote a few days ago about attics here in Texas? Well, they made an attic in the Nevada desert that was going to be hotter than mine. In short, the test was rigged, in my non-professional opinion. And because they are advocating "derating" (let's don't even go there), it's clear that they are pushing the sale of copper.

Urph. I knew I forgot something.

When they talk about "80%", they really are talking about taking the cables to the continuous load limits. For, like, 8 hours a day. So that would be ((12A + 16A + 24A) * 2) that they are simulating as a continuous load. That's 104A CONTINUOUS load through that hole. In a little shed. In the Nevada desert. In the summer. With the test on the wall which receives the most sunlight.
 
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don_resqcapt19 said:
I know that a lot of people do not agree with my position on the application of the exception in 310.15 to the rule in 334.80. My position is strictly based on my idea that the more specific rule always prevails over a more general rule.
Don
Click to expand...

I agree with the notion that the more specific trumps the general rule. In this situation, I think the 2 rules can coincide (sp?) and one doesn't contradict the other. We use 334.80 to determine ampacity of cables in insulation, and can use the exception if we have differing ampacities for portions of the run.
 
John,
In this situation, I think the 2 rules can coincide (sp?) and one doesn't contradict the other. We use 334.80 to determine ampacity of cables in insulation, and can use the exception if we have differing ampacities for portions of the run.
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If the CMP intened that, they would not have even put the rule in the code. There would never be an application where you have to derate.
Don
 
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