Ambient Earth Temperature for Cable Calculations

[BlownUp]

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?

 

[7anoter4]

Mean annual soil temperature in USA it is from 40 to 77oF.[4.5 to 25oC]according to:

http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm

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).

 

[BlownUp]

Thanks for the response 7anoter4. I saw that and the graphs shown "look" like they come from some standard. They also show in Figure 3 the seasonal range based on soil type and depth. This is the data I am looking for but I really need to be able to reference the standard or organization that produced it.

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.

 

[chuckd83]

Check NEC Annex B. It sites the papers used to populate the tables: AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems (Neher and McGrath) along with a couple standards. There may be information in the paper itself as to what assumptions can be made about the soil as it will not be uniform from the surface to the conductor.