Ambient temperature definition by Neher-McGrath

[Power0020]

I noticed that Neher-McGrath equations for underground installations consider the ambient temperature to be soil temperature. I wonder if this will omit the depth factor?

My understanding that the heat generated will be dissipated about the soil surface and a temperature gradient will be obtained with hottest point about the cable/duct surface and ambient air temperature will be the lowest at the soil surface.

Nevertheless, the equations consider the ambient soil temperature at the depth of cable installation. This may mean that the soil temperature will not change due to the cable heat flow....I am a bit confused since the soil thermal resistant is composed of both resisitivity and depth, this is for the basic of heat flow.

Any clue?

 

[7anoter4]

I think the "ambient temperature" it is the earth surface temperature.
See [for instance] Calculation of Underground Cable Ampacity by Francisco de Leon [CYME International]

http://eeweb.poly.edu/faculty/fdeleon/publications/PDFs/Calculation.pdf

ch.IV Heat Flow in Underground Cable Installations

 

[LDKGR]

Hi Power0020,
You can read the following :
"Understanding the Neher-McGrath Calculation and the Ampacity of Conductors"

http://www.electrician2.com/articles/ampacity.htm