Ambient Temperature

[valassi]

I came to this question during a calculation for ampacity where there was a small diference between 30?C and 35?C, at the end I had more questions than answers:
a) Ambiente temperature, average or maximun?
b) Guidelines for inspectors about the way to evaluate the ambient temperature or just their judgement
c) Ambient or surrounding temperature?

there is a lot of money involve in this matter, not only how apply THHN insulation......

 

[bphgravity]

Re: Ambient Temperature

a. The code is not exactly clear on this issue, however a carefull look at the conditions involved should be evaluated. Taking the maximum temperature that could be involved would be impractical as every state in the US has reached a temperture exceeding 100 degrees at least one time. An average temperature seems more suitable as that is the average condition the conductors will be exposed to.

b. I know some jurisdictions have already made evaluatons about the ambient temperature that will be accepted for use in their community. For example, the Florida Building Code offers a regional temperature and climate map of the state that can be used for purposes for applying code requirements.

c. Ambient means surrounding.

 

[charlie b]

Re: Ambient Temperature

I?ll only talk about your question (a). As an electrical engineer with at least some understanding of the physics behind the degradation of insulation at elevated temperatures, I might answer ?year-round average.? But to calculate the correct average temperature to use in any specific geographic area is far more complicated than I can handle, so I have never used that answer in a real design project.

But as to what the code says, well it says nothing on the subject, just as Bryan has said. For practical purposes, I would guess that most of us use the 30C ampacity values, without considering any temperature corrections, for most of our work. The exceptions might be in an industrial plant or a cable tunnel in which it is known that the area will be hot almost all the time. But as an example, if I judged that a cable will be exposed to 35C ambient temperatures for less than 10% of its entire life, and would be under 25C for over 80% of its life, then I would not apply a derating for the times at the higher temperatures.

 

[sandsnow]

Re: Ambient Temperature

I think one area of the Code that has addressed elevated ambient is the requirement that MN conductors be 90 deg, but the ampacity be 60 deg.
I always thought this was due to to things. Elevated ambient in attics and elevated temperatures in lighting outlets above incandescent fixtures. I'd be interested to know the history of that change.
Another area to consider for elevated ambient wood be roofs.

 

[dana1028]

Re: Ambient Temperature

Val -

Regarding (b) - I think as bob said most folks just default to the 30 degree table. I don't know of any inspectors wanting to get into the battle of derating NM cable passing through a hot attic.

FYI - I took a Fluke digital thermometer into the attic to check temps when the outside temp was 88 degrees...by mid-afternoon the temp in the attic was over 140 degrees! It doesn't take much checking to see there are a lot of places around the country that run over 88 degrees day-in-day-out throught most of the summer (heck, there are places that exceed these temps most of the year!). When you consider solar heat gain (a building acts as a heat sink and absorbs heat all day, at night the building is still hot and continues to radiate this heat), attics never do get below 88 degrees during these continued hot days. Yet most jurisdictions don't consider derating NM under these conditons. (I said most, not all).