Geotec report - thermal resistivity with dry out curve

[kaspor]

Hello

I am trying to calculate ampacity rating of direct buried cables. The thermal resistivity used significantly impacts the rating of these cables.

I have a report where they have performed a thermal resistivity test of the soil. It appears as if they have put the soil in an oven (or similar) and measured both the thermal conductivity (W/mK) and resistivity (mK/W) over time. The results are tabulated and continue until the moisture content of the soil is 0% (4 points are measured with moisture contents of 30.24%, 25.13%, 8.89% and 0%). The conductivity results vary from 1 to 0.2 and resistivity from 0.7 to 2.3 (in that order with relation to moisture content).

The cables will have bedding sand around them to protect them mechanically, but above this the intent will be to backfill with natural spoil.

What thermal resistivity do you use with the ampacity calculations?

IEC-60287-11 talks about this, but the rules only apply to isolated cables. There are multiple cables in this trench.

 

[7anoter4]

The earth conductivity is not employed usually, since all ampacity calculation uses only thermal resistivity of the soil.
For instance:
IEEE 835/1994 Chapter 3.1.4.1 Earth thermal resistivity
Thermal resistivities of 60 °C, 90 °C, and 120 °C centimeters per watt (oC cm/W) are shown as 60 RHO, 90 RHO, and 120 RHO in the tables. In the past, when the thermal resistivity of the earth was not known a rho of 90 was recommended for rating the cable. However, the ratings for buried cables are significantly affected by the earth's portion of the thermal circuit and therefore correct knowledge of the effective soil thermal resistivity and soil thermal stability is paramount in establishing the correct rating for a buried cable system.
IEC 60502-2/2005
Thermal resistivity of soil 1.5 K.m/W
IEC 60364-5-52/2001
A.52.3 (523.3) Soil thermal resistivity
The current-carrying capacities tabulated in this annex for cables in the ground relate to a soil thermal resistivity of 2,5 K.m/W. This value is considered necessary as a precaution for worldwide use when the soil type and geographical location are not specified (see IEC 60287-3-1/1995)

 

[kaspor]

Thanks for the response, my question was around which thermal resistivity to choose for the report. I understand how to derate the cables based on soil thermal resistivity.