Ambient Air Temperature

[andrew grant]

Is there an easy way to determine the outdoor ambient air temperature for different locations around the country (Houston would be different than Anchorage, etc)?

One source might be the ASHRAE Handbook of Fundamentals - are there others?

Stallcup refers to the 1971 NEC that used to have a table - what else?

 

[bphgravity]

Re: Ambient Air Temperature

Your local AHJ and the local building official should carry an administrative code for their jurisdiction that specifies areas in this regard such as average ambient air temperatures, rainfall amounts, wind-borne debry structure requirements, flood zoning etc.

A national almanac of sorts may not be approved by the AHJ.

 

[bennie]

Re: Ambient Air Temperature

All you need is the maximum temperature at the location in question.

The newspaper, Weather service, long time resident, Chamber of Commerce, are a few sources.

If the inspector does not believe you, ask him for a number

Averages don't work, they create smoke.

[ May 19, 2003, 10:27 AM: Message edited by: bennie ]

 

[charlie b]

Re: Ambient Air Temperature

Originally posted by bennie: All you need is the maximum temperature at the location in question. . . . Averages don't work, they create smoke.

Andrew: I do not agree with Bennie on this one, as he will no doubt recall. This topic has been the subject of a previous discussion, and you are welcome to look it up in the forum archives. For my part, I am convinced that it is appropriate to use ?year-round-average temperature,? for the purposes of de-ratings.

But I will echo Bryan?s and Bennie?s statements that your best bet is to ask the AHJ.

 

[bennie]

Re: Ambient Air Temperature

Charlie: I respect your opinion. This reminds me of the old analogy... Put one foot in a fire and the other in a bucket of ice water. On the average you are in good shape. The reality is you are in a world of hurt.

Heat flow charts, for dissipation of heat, are calculated on the maximum temperature surrounding the material.

Using the average temperature may work, there is built in tolerances in most calculations.

[ May 19, 2003, 12:43 PM: Message edited by: bennie ]

 

[charlie b]

Re: Ambient Air Temperature

Bennie: I respect your opinion also, and your experience as well. But I must point out that the ?hot-foot, cold-foot? image does not accurately represent the manner in which temperature affects cable insulation. Let me offer a different image.

Suppose you need to take a long drive (300 miles) through a rural area (no intermediate gas stations - once you run out of gas, you are stuck). Suppose you car gets 20 miles per gallon at 40 miles per hour. You figure that your 16 gallon tank should get you there with a little bit of safety margin built in.

What happens if you try to drive the whole trip at 60 mph? Your car burns fuel too fast, and you might not make it all the way. What happens if you drive the whole trip at 30 mph? Your car burns fuel at a lower rate, and you will have extra gas left over at the end of the trip.

HERE?S THE IMPORTANT PART:
Finally, what if you spent one hour at 60 mph, then spent two or (even three) hours at 30 mph, and drove 40 mph the rest of the way. Do you need to calculate your overall fuel consumption on the basis of the 60 mph burn rate? No. Will you make it all the way to your destination? We would need to perform a calculation to find out, and to do that calculation we would need to know the burn rates at 60 mph and at 30 mph.

This process is similar to the way that temperature causes cable degradation. The insulation is designed for a given useful life (e.g., 40 years). If you subject it to higher temperatures for its whole life, it will fail much sooner. But if it is 10 degrees above the NEC?s 30C for one day, and then 10 degrees below 30C for 3 days, the net result might be that it has degraded slower (and will last longer) than the manufacturer had promised. ?Time at temperature? is a far more important variable than temperature itself.

 

[bennie]

Re: Ambient Air Temperature

You are probably correct Charlie. Maybe this is another code section that should be clarified by the officials.

The ambient temperature is the surrounding temperature. If the surrounding temperature is 120?F. and the conductor exceeds the rating of the insulation this appears to be a violation of 310.10 to me.

In technical writing, when averages are intended, the wording is "mean temperature" or mean ambient temperature.

But I guess the NEC is not a technical manual

 

[dana1028]

Re: Ambient Air Temperature

www.worldclimate.com - has all the temperature info you'll ever need...average highs, lows, by month, etc.

 

[dana1028]

Re: Ambient Air Temperature

For what its worth, I agree with Bennie - I've crawled up into the attic space in the summer with a calibrated Fluke thermometer and recorded average daily temperatures (day in and day out throughout the summer) in excess of 135 degrees. I just cannot believe that the NEC does not think these temperatures matter because come winter it will cool down and average out below 86 degree F.

 

[andrew grant]

Re: Ambient Air Temperature

This is a reply from a fellow at askearth.com - thought it might be of interest. He is not an electrical person - I'm guessing a physics teacher. His suggestion is very much like the ASHRAE Handbook.

Commented: Re the temperature/conductivity thing - I knew that if you got it cold enough you could make it superconduct, but I thought that maybe at more typical temperatures, an increase in temperature might increase the electron mobility and improve conductivity. Now I think about it a bit more, I can see that since it delocalisation of electrons among atoms in the "conduction band", any increase in temperature, by virtue of increasing the movement between atoms, may interfere with the efficiency of this transfer.

As you say - the cooler the better.

I wanted to clarify those points in order to figure out what a "reasonable" value for an ambient temperature might be. The standard seems to be based on the idea that this is constant - no so in this case.

That is what led me to the idea of trying to see what your acceptable excursion from the nominal minimum capacity of the cable might be.

My tentative suggestion is this:

Simply using some sort of "average" temperature will probably take you out of spec on most warm days.
Using the average maximum temperature would be safer, but still too risky (in my view) since (almost by definition) on half the days of the year, the maximum temperature will be above this value!
Using the maximum recorded temperature should be a safe conservative temperature, since your cable would fall out of spec only if a new record was set. This seems too restrictive.

My suggestions therefore is: for the locality in question, obtain the historical data for maximum daily temperatures, and derive a frequency distribution for this. Choose an acceptable "out of spec" probability - say 1 day in 1000. From the distribution of maximum temperatures, find the temperature that is only exceeded in average 1 day out of 1000 (or whatever), and use that as your "ambient" value.

This is just my own line of thought, and like you, I would expect that there are some references to the "accepted" method. I haven't yet found any though.

xarqi