Ambient Temperature

[derf48]

The correction factors of table 310.16 lists factors to change the allowable ampacity of a conductor for ambient temperature other than 30 C. How do you select the proper ambient temperature? I have asked wire manufacturers as well as code panel members, and their answers are varied and inconclusive. What I want to know is how you actually do it in the field, or if you consider it at all. Thank you.

Fred

 

[bphgravity]

Re: Ambient Temperature

I only consider it for extreme situations or conditions were it is clearly obvious ambient temperature will be an issue.

The correction used is not always that easy to determine. In most cases, I will consider the temperature the conductors or cables will be exposed to from most of the time. I generally never consider the worst cases temperature and feel the average temperature is more suitable. The FPN gives some guidance for installations near or on the roof exposed to sunlight. On other occasions, the exception to 310.15(A)(2) nulifies the issue.

 

[charlie b]

Re: Ambient Temperature

I agree with Bryan. If I were smart enough to write a convincing argument, I would submit a code revision to make it clear that "year round average temperature" is the temperature that should be used for that table. But I doubt that anyone could convince the code committees to declare a clear intention on this issue. No criticism of committee members is intended, but I just don't think a consensus could be reached.

 

[raider1]

Re: Ambient Temperature

I also agree with Bryan. For example a boiler room in a hospital that has a high year round ambient temp, I would use the correction factors for the wires that originate and terminate in the boiler room.

Chris

 

[bthielen]

Re: Ambient Temperature

Charlie,

How would one justify using year-round average temperatures when the conductors may be exposed to extremes for considerable periods of time?

Example: for conductors that must endure outdoor climate, Minnesota's year-round average temperature in Grand Rapids (centrally located) is roughly 43 degrees F. however, extremes from +90 degrees F. during the summer months to -30 degrees F. during winter months for extended periods of time are not uncommon.

Bob

 

[charlie b]

Re: Ambient Temperature

I didn't say it would be an easy sell. But I believe there is a valid technical basis for it, and I've explained my views before. I would have to look for my "car using gasoline" analogy or my treatise on the Arrhenius equation.

The short answer is that any time spent above the rated temperature (e.g., 30C) degrades the insulation faster. Any time spent below that temperature degrades the insulation slower. The net effect of spending 10 months below 30C and 2 months above 30C might work out to a net decrease in the rate of degradation.

It's all in the math, and the math (i.e., the Arrhenius equation) is not simple. It's not easy to predict the results for all combinations of ambient conditions. That is why I think I could not come up with a convincing argument supporting my viewpoint about using "year round average temperature."