Ambient Temperature

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tkb

tkb

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Does anyone know where to find the average ambient temperatures for an area so I could do a calculation for 310.16
 
Best bet that I know of is to do a local search of average temps in your area. Roger would be a good one to ask. There is a lot of question as to how much should we (ECs) apply ambient derating to attic areas.
 
I haven't had a chance to stir the pot lately, so I'll do it now.

Why would you want to use the average ambient temperature, and not the maximum ambient temperature? ;) :roll:
 
Actually, I think it should be annual average. But I've made that case before, and won't push it again here.
 
SEO said:
Why would it be annual average?:smile:
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This has been the subject of much debate on this forum, and I have already made my opinion known. Others have disagreed, and have put forth very reasonable arguments against my point of view. It is generally agreed that the NEC is not clear on the meaning of ambient temperature, as used for the purpose derating of conductors. It is also generally agreed that the local AHJ will have something to say on this subject.

Here is what I posted on this forum over four years ago:
charlie b said:
I submit that the intended meaning is ?year-round average.? I concede that the NEC does not define or explain the intended meaning of ?ambient temperature,? in the sense used by Table 310.16. I would like to see an NEC revision that makes this point clear. However, I take some comfort in the fact that the laws of Physics are on my side.
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charlie b said:
Here is a brief summary of the basis for my interpretation:
? The hazard that higher temperatures impose on a conductor is an increase in the rate of deterioration in the insulation system
? For every 10 degree F increase in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will decrease by about 50%.
? For every 10 degree F drop in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will double.
? The relationship between temperature and the rate of degradation is not linear. If a cable spends one hour at a temperature of 90F (i.e., 4 degrees above the de-rating threshold), it may have to spend 3 hours, or even 5, at a temperature of 82F (i.e., 4 degrees below the de-rating threshold), in order to break even (i.e., on the overall average rate of degradation)

The concept is similar to buying a car that is intended to last you 10 years, but that has a gas tank that cannot be refilled. You start with 10 years worth of gas in the tank, with the actual volume of the tank based on your burning gas at the rate of a car moving at 30 miles per hour. Once you run out of gas, you must buy a new car (similar to replacing a cable, once its insulation system has degraded too far). If you drive faster than 30 mph for a short while (say for 2 hours), you burn gas faster than the design rate, and you might run out of gas before the car?s 10 year expected life. If you drive slower than 30 mph, you save gas. But you might have to drive at 20 mph for 6 hours, before you can save enough gas to restore the car?s 10 year expected life. Finally, if you constantly drive below 30 mph, you could, in fact, extend the useful life of the car beyond its expected 10 years.

The fact that you drove over 30 mph for a single hour would not, by itself, drain the gas tank, and render the car useless. Similarly, if you allow a cable to experience more than an 86F ambient for a limited time, without reducing the current to the de-rated value shown in 310.16, that will not instantly destroy the cable. What it will do is to reduce the cable?s useful life by some small amount. You can make this up by keeping it below 86F for an extended period.

Thus, you might be able to justify the use of higher ampacities, if the ambient temperature will be below 30C for the life of the cable. Please note, however, that you would need the services of an EE (and a PE, at that), if you want to take advantage of the lower ambient temperatures. The EE would have to be familiar with the Arrhenius equation and the methodology for applying it. The Arrhenius equation is k=A*exp(-Ea/R*T), where k is the rate coefficient, A is a constant, Ea is a property called the ?activation energy,? R is the universal gas constant, and T is the temperature (in degrees Kelvin). It would be no easy task to apply this to a cable?s insulation system. You would also need an AHJ willing to go along with the EE?s calculated results. You are not likely to get both.
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You could not resist could you Charlie? :grin:

Well then I can't either.

Based on your thoughts it would only make sense that I could size my conductors based on the average ampacity expected as well. :cool:
 
Nice try, Bob. ;) The code does not define, or even explain "average temperature." But it does, at least, use that phrase. It does not, I believe, use the phrase "average ampacity."

I will, of course, concede that yours were among the "very reasonable arguments against my point of view" that I mentioned a moment ago. And yes, the code's present wording does not support my point of view. And finally, I very much doubt that a proposal to revise the code to incorporate my point of view would likely be rejected by the Code Making Panel.

Alas! I am left to content myself with truth, and must needs give way to reality. :roll:
 
In my opinion, 310.10 FPN No.1 (1) NEC 2005

The principal determinants of operating temperature are as follows:

(1) Ambient temperature ---- ambient temperature may vary along the conductor

length as well as from time to time.

If from time to time the ambient temperature is exceeded, then correction

factors for ampacity must be applied.
 
benaround said:
In my opinion, 310.10 FPN No.1 (1) NEC 2005

The principal determinants of operating temperature are as follows:

(1) Ambient temperature ---- ambient temperature may vary along the conductor

length as well as from time to time.

If from time to time the ambient temperature is exceeded, then correction

factors for ampacity must be applied.
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You lost me at "FPN" :grin: . Which is not enforceable :wink: .
 
But it is the correct way in looking at it. Someone posted the question sometime ago about the most ignored NEC requirements and I would not be surprised if this is not in the top 10. Myself included at times.
 
charlie b said:
This has been the subject of much debate on this forum, and I have already made my opinion known. Others have disagreed, and have put forth very reasonable arguments against my point of view. It is generally agreed that the NEC is not clear on the meaning of ambient temperature, as used for the purpose derating of conductors. It is also generally agreed that the local AHJ will have something to say on this subject.

Here is what I posted on this forum over four years ago:
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The 10Degree rule is only applicable to high temperature magnet wire insulation used in motors and transformers. Normal household wiring material is different.

If we were to use the same thermal rules that derived the 10Degree rule then the ambient temperature to be taken into account should be highest mean value that is experienced in an 8 hour period for the geographical area in consideration.
 
If a conduit and wire were installed in an area that was cld, 70?F or 21?C, is there additional derating above the 105% that is allowed?
 
tkb said:
If a conduit and wire were installed in an area that was cold, 70?F or 21?C, is there additional derating above the 105% that is allowed?
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I would call that uprating, not derating. But the only way to do what you suggest is to obtain a calculation performed under engineering supervision. Can you afford my rates? ;) :grin:
 
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