NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
(OP)
We are direct buring 3/c-15KV shielded armored cable where the ambient temp. can get as high as 49 deg. C. The 15KV ampacity tables in art. 310 are for an ambient temp. of 20 deg. C. It gives a formula to adjust for the different ambients. In the equation is a factor called "deltaTD" which is defined as the temperature rise due to dielectric losses. How can this factor be calculated or are there reference tables that give these values?






RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
I haven't looked at this formula for a long time, but I suspect the dielectric losses will be a small factor in the overall derating.
Cable suppliers can also assist you with the ampacity calculation.
But if the cable is direct-buried, it is the soil temperature you are concerned with, not the air temperature. I would be surprised if the soil temperature ever reaches 49 deg C, but I'm often wrong.
RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
As DPC stated generally the ground "ambient" temperature is less than air ambient, that is why buried tables use a 20C ambient versus 40C used on air tables.
RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
http://members.rogers.com/mcgroup/download.htm
Or just download
http://members.rogers.com/mcgroup/Uground.zip
It contains tabulated ampacity values for different feeder sizes configurations, plus ambient and cable temperature rating correction factors that are posted in 2002 Canadian Electrical Code.
The CEC tabels are based on IEEE standard 835 plus some other limitations imposed by connected equipment rating.
RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT
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