NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT

[duki]

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?

 

[dpc]

I would contact your cable supplier for this data, rather than trust any generic tables. The dielectric losses are a sensitive subject for cable manufacturers. EPR insulation has higher dielectric losses than XLPE, so the XLPE suppliers or those who supply both are more likely to be forthcoming.

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.

 

[gordonl]

IEEE S-135 provides tables which give a generic value. As DPC stated this will vary for material, but S-135 only gives "Rubber". The DELTA-TD ranges about .6 to 1 depending on the cable size for your application which is minor compared to the other factors.

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.

 

[Ares]

You may want to check software from

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.

 

[stevenal]

If the ridiculously long link below doesn't work, try searching on standard 835 (update of S-135) at the IEEE web site. The Aluminum Electrical Conductor Handbook also lists the rubber deltaTDs, but shows how to adjust them for different insulation dielectric constants and power factors. I would think the IEEE standard they referenced would include that information also.

http://shop.ieee.org/store/product....5FSTANDARD%5FNO+%2A835%2A%29+&mh=10&sh=0&ae=1