Ambient Earth Temperature for Cable Calculations
Ambient Earth Temperature for Cable Calculations
(OP)
I am working on a project that involved cable calculations for a duct bank. My modeling software defaults to 35 deg C for ambient earth temperature and typically I keep that value so that the design is overly conservative. Usually duct banks I am involved with are less than a hundred feet. And with multi-million dollar projects the cost of a short run of oversized cable is insignificant to the cost of the project. However, on this project I have two runs of about 2000 feet so cable cost is becoming significant. The duct bank is deep but not deep enough to be completely free from seasonal variations so I am not comfortable that a physical measurement would provide meaningful data. I have been unable to find a good reference of how to reasonably determine the value that should be used for ambient earth temperature. NEC uses 20 deg C for the its tables. I have found a couple of places that reference using 25 deg C. I have not found a code reference or other good reference. Any ideas or suggestions?






RE: Ambient Earth Temperature for Cable Calculations
http://www.builditsolar.com/Projects/Cooling/Earth...
So if you are in the South Florida 25oC will be good. Usually at 0.8 m depth the maximum temperature could be from 15 to 30oC.
NEC tables [310.60(C)(77) to (86)]are calculated for 20oC ambient, indeed, but one may adjust the temperature to any other according to art.310.60(B)(4) for cables above 2000 V rated. For low voltage cable the earth temperature is 30oC[Table 310.15(B)(16)].Here you could use any other temperature also according to Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F).
RE: Ambient Earth Temperature for Cable Calculations
The end customer on this project is very standards oriented and generally wants every piece of data we provide to be backed up by calculations referenced to a standards document. Any idea of an official location to get this data?
I would think NOAA would have this data and searching their site it look like the measure and track ground water temperature all over the country (USA). But I cannot find where they consolidate it in a useable format for backing up my calculations.
RE: Ambient Earth Temperature for Cable Calculations