Selecting ambient temperature correction factors from Ta

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Re: Selecting ambient temperature correction factors from Ta

Originally posted by lauraj: Sentence 1 still seems to me that it is just dealing with conductor ampacity and terminal ratings if there are no correction factors applied.
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I would prefer to say that ?sentence 1? does not address temperature correction factors at all. It doesn?t tell us they apply, and it doesn?t tell us they don?t apply. It just talks about using the (termination?s) 75C rating, when selecting the ampacity of the conductor.

Please note that ?sentence 1? also does not tell us how to select the ampacity of the conductor. That we get from Table 310.16. It is that Table that tells us to adjust for ambient temperatures in excess of 30C.

So I did not completely address your earlier question:
Where in the code does it say that I need to use the temperature adjustment factor based off the rating of the terminal and not the conductor?
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I should have said ?110.14(C), sentence 1, in conjunction with the Table 310.16 Correction Factors.? By that, I mean that you can't complete the actions described in 110.14(C), sentence 1, without also going to Table 310.16 to find the ampacity values.

Going back to Bob's initial statement, this section is in need of clarification.
 
Re: Selecting ambient temperature correction factors from Ta

Lauraj,

One of the opposing Comments to the original 1993 NEC 110.14(C) Proposal was that it was unnecessary because it was already covered by 110.3(B); i.e., since it was already required by the UL, AALZ Caregory Code, it was also required by the NEC.

It is essential to recognize that Table 310.16 is specifically referred to in both the UL Category Code and 110.14(C) and the Ambient Correction Factors are integral to the Table including the specific instruction:
For ambient temperatures other than 30?C (86?F), multiply the allowable ampacities shown above by the appropriate factor shown below.
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In reality we are NOT derating the terminal, we are applying Table 310.16 to the conductor connected to it, and saying that - at the termination - a conductor with a higher temperature rating will be treated as if it were one with a temperature rating the same as the termination's temperature rating. See the FPN to Section 310.15(A)(2) and read it in context of the Section (Selection of Ampacity)
 
Re: Selecting ambient temperature correction factors from Ta

I agree with and understand your explanation of why we should use the 75 degree column to derate.

I still don't see why then the code book tells me I can use the 90 degree column to derate for temperature.

I'm starting to feel a little dense. javascript:void(0)
 
Re: Selecting ambient temperature correction factors from Ta

Lauraj,

First, you don't need to feel dense.

Second, as I was explaining to Dave (Wirenut1980), when 110.14(C) was first introduced, it was routinely misinterpreted. The second sentence didn't exist in the 1993 edition and it was easy to conclude the termination limited the conductor's USEFUL temperature rating to that of the termination's throughout the entire circuit.

Many people were then saying the 90C column was without any useful application with the "new" requirement. (It has been a UL requirement at least since I got into the business in the late '60s) Even many who recognized that it shouldn't limit the conductor's temperature rating throughout still didn't know where to "draw the line."

It was obvious some correction needed to be made. If you can get a copy of the 1996 TCR you will find some very interesting "corrections." The current second sentence, also from the original author, was the "solution."

I think it has permitted an "over-correction" because, while I think you (and many others) have misinterpreted it, your's is still a reasonable interpretation on its face, just as the previous misinterpretation was. In fact, there are still some who hold very close to the old misinterpretation, saying effectively the new sentence only permits me to use the 90C ambient temperature correction factors to the 75C column basic ampacities. That is, the #1/0 I used as an example would have an ampacity of 150 x.82 rather than .75 (75C Ampacity column x 90C correction factor) for a 90C conductor with a 75C terminal.

I really would like to see a copy of the IAEI article. Is the author identified?

One of the reasons I said I'm not sure I'm up to a rewrite is the original 110.14(C) author (and corrector) is very influential and has great pride of authorship. He knows I'm right and thinks 110.4(C) already clearly says so :D
 
Re: Selecting ambient temperature correction factors from Ta

The IAEI article was in the "Focus on the Code" section, written by John Stacey, CMP-6.

He dealt with a temperature derate for conductors run through an attic. Now in that case, since our terminations are not in the attic, is it ok to use just the 90 degree column since only the conductors in that portion of the run are exposed to the higher termperature?

Thanks for all of the replies, it is beginning to become a little clearer.

Laura
 
Re: Selecting ambient temperature correction factors from Ta

Originally posted by lauraj:
...
He dealt with a temperature derate for conductors run through an attic. Now in that case, since our terminations are not in the attic, is it ok to use just the 90 degree column since only the conductors in that portion of the run are exposed to the higher termperature?
...
Laura
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That is exactly the correct application in my opinion.
 
Re: Selecting ambient temperature correction factors from Ta

I wonder if anyone would like to comment on this: http://www.ci.mesa.az.us/building_safety/pdf/Code%20Interpretations/AmbientTemperaturesForElectricalWiring.pdf

For me it raises far more questions than it answers. For example, what is "mean peak temperature"? The highest temperature on record here is 122degF. We go above 115 frequently in July and August [but it's a dry heat]. Attics run 140degF or higher, as do rooftops.

Based on this memo a 20A residential circuit with the Romex running through the attic will require #10 copper, and then it's only good for 17.4A. In this area the typical residential service is an "all in one", so the branch circuit breakers are outdoors.

'SRP' is one of the local utilities. The cases I recall where there were cable failures were in duct banks that were too tightly spaced. In my opinion the "catch all" approach for outdoor underground is far too vague and unsubstantiated. I'd believe 109degF just below a slab, but not at 18" below grade.

Developers aren't going to be happy.

Martin
 
Re: Selecting ambient temperature correction factors from Ta

I have two issues with this paper. One, there is no 2003 NEC?

Two, where in the NEC does it say to use the "mean peak temperature"? It doesn't. I don't totally disagree with that choice but think it will most likely result in a higher than necessary correction factor.

The problem is no long term data. I own a home built in 1957. All the 15-ampere lighting branch circuits are 14 awg and run through the attic. I have recorded temperatures of over 120? F in the summer. Yet, nearly 50 years later, the cable and conductors are in really good shape. My 1930's home which blew away with the hurricane also had mid-50's rewiring that did not appear to have any degradation from the day it was installed.

So in effect, are high ambient temperatures that destructive to conductor insulation? Perhaps over lots and lots of time or extremely drastic situations.
 
Re: Selecting ambient temperature correction factors from Ta

The meaning of ?mean peak temperature? is clear enough to me, and I have no fundamental objection to its use. What you do is to record the highest temperature during the day, and write that number down. Do this every day for a century or so (you might need some help with this), and then calculate the average value. But I think you can just get the information from the local weather bureau, and I suspect that that is the basis of their choice of 115 degrees.

It is far better than using the ?highest peak temperature.?
 
Re: Selecting ambient temperature correction factors from Ta

This problem has been a confusing one since 90 degree C, THHN insulation was introduced. The temperature correction factor under the 90 degree column is applied to the ampacity of the conductor ampacity in the 90 degree column. After multiplying the ampacity by the correction factor, you must compare it to the 75 degree ampacity of the conductor in that column. The LOAD connected to the conductor cannot exceed the ampacity value under the 75 degree column. If the corrected value is less than that in the 75 degree column, IT is the determining factor for the amount of load that can be applied to it. The Code does not require you to apply the temperature correction factor to the 75 degree ampacity if you are using 90 degree conductor insulation.
 
Re: Selecting ambient temperature correction factors from Ta

The meaning of ?mean peak temperature? is clear enough to me, and I have no fundamental objection to its use. What you do is to record the highest temperature during the day, and write that number down. Do this every day for a century or so (you might need some help with this), and then calculate the average value. But I think you can just get the information from the local weather bureau, and I suspect that that?is the basis of their choice of 115 degrees.

It is far better than using the ?highest peak temperature.?
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Even if we used this method would the data be accurate for most installations? With the exception of equipment being served in an attic we always seem to have our cables running through an attic under the insulation. In many cases they're below 12" of fiberglass so the ambient temperature of the their surrounding air would be somewhere in between the attic temperature and the temperature of the living space below it. So what temperature should we really use to derate these conductors that are passing through an attic?
 
Re: Selecting ambient temperature correction factors from Ta

It seems that some calculations in these threads create as much confusion, as opposed to clearing up the issue.


I just read this thread and I am a little confused myself. So I printed out this thread and I am going to try and make heads or tails of it. Then I will post what I think is supposed to be and wait (with a suit of armour :D ) for the responses.

Math has never been my strong suit and it seems here I get even more confused when I read some of these posts.
 
Re: Selecting ambient temperature correction factors from Ta

Originally posted by pierre:
It seems that some calculations in these threads create as much confusion, as opposed to clearing up the issue.
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You must be thinking of some other Forum. :D
 
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