Ambient air temperature for underground cable sizing

[Power0020]

Tables show derating factors for soil temperature, and ambient temperature, should I consider two derating factors in the case of underground cables? one for soil temperature and the other for ambient temperature? or one only for this type of installations (soil temperature).

 

[MikiBg]

One only per IEC, determined by surrounding medium.

 

[7anoter4]

The temperature of the captured air in the duct [it is not so important-usually is taken as 60oC] and the earth temperature [it is the most important] in both calculation methods [IEC 60287-2 and Neher and Mc Grath methods].The air temperature is not involved in underground power cable temperature calculation.

 

[Power0020]

though the equivalent circuit shows the ambient as heat sink with fixed temperature (the soil around the cable heats up due to cable current!), I checked Neher and Mc Grath paper and it shows Ta as ambient temperature or earth temperature.

 

[7anoter4]

First of all it is not from the actual Neher and Mc Grath paper, but from "Calculation of Underground Cable Ampacity” by Francisco de Leon -by-the way it is a very good article, in my opinion. The figure 3 and 4 explain the Neher and Mc Grath general technique by subdivided studied area in layers. So tambiant could be air, ground, water [?] and other temperature.
This paper contains the instructions how to use CYMCAP computer program to calculate ampacity and temperature rise of underground power cable-directly buried or in ducts.