Ambient Temperature for Underground Cable Ampacity Calculations 1

[Power0020]

I keep wondering that why IEC 60287 and Neher McG. equations refer to ambient temperature as soil temperature at cable laying depth though the calculations include a portion of external soil thermal resistance that is dependent on depth?.

Any clues?

 

[7anoter4]

Since the Earth is a huge mass absorbing the heat some where down and above from the ground surface-3-4 cm from surface the sun could penetrate indeed, but this distance is negligible-the soil temperature is uniform-20-30 oC.
Temperature drop from the cable or conduit-or duct-to earth according to IEC 60287 it is: T4*R*I^2 where :
T4=1/2π.ϱT*LN(2*L/De)
L is the distance from the surface of the ground to the cable axis (mm)
De is the external diameter of the cable (mm)
We don’t know where in the above space the earth temperature return to the initial temperature
indeed , so we consider the maximum distance [L]. It could be a bit conservative solution but is not far from the experimental result.