Soil Thermal Resitivity

[JFG]

I inquiry on a situtation that I am analyzing for 15kv power cable placed in a trench system. To obtain the cables highest ampacity, it is required to surround the cables with a sand, or other soil material, that has a very good soil thermal resistivity to allow cable heat disipation, (ie. rho=60) Where does one find more information on soil thermal resistivity and what company's are able to provide such a specific soil.

 

[BJC]

Section B-310-15 of the NEC list the thermal resistivity of different soils and has the reference to ANSI/IEEE standard 442.
Sand is a very poor conductor. Some how it presist in the "standard details" of many engineering firms as backfill around cable and duct banks.

The use of sand for the direct burrial of cable is for the physical protection of the cable ( no sharp rocks )and not for heat transfer.
Wet sand has a lower value of Rho but there will always be times when the sand is not wet. Better to design around the rho you will see when the sand is dry.

 

[kraigb]

I would suggest taking a look at IEEE 442-1981 (IEEE Guide for Soil Thermal Resistivity
Measurements) if you want more information on thermal resisitivity.

Our normal design methodology is based on a study of thermal resistivities in our service region. We assume that most native soil has a rho within the range specified for that soil type. In the instances where we have a trench where the native soil is likely to have an unacceptably high rho (we don't like to go higher than 90 rho), we use a flowable fill material with a known thermal resistivity in close proximity to the cable rather than backfill with the native soil.