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NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT

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 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.

RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT

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.

RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT

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.

RE: NEC CABLE AMBIENT TEMPERATURE ADJUSTMENT

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