Ambient Temperature for Underground Ductbanks

[FaradayFF]

Hi Gents,

Typically we adjust cable ampacity based on ambient air temperature, but for cables installed in underground ductbanks, I don't think we have to worry about it, because the temperature is lower than 40 degree C. Am I correct on this?

Thanks,
EE

 

[ron]

We do Neher McGrath calculations for UG ductbanks per Annex B
That mostly adjusts for the inability for the earth or surrounding material to release heat like it would if it were above ground

 

[Julius Right]

In my opinion, the captive air [ in a duct] temperature [usually 60oC] it has a small influence on ampacity. However, the main temperature drop it is from outside of the duct bank to Earth. The calculation can follow Neher &McGrath way- up-dated in IEEE 835/1994 [or IEC 60287].

 

[Julius Right]

Sorry. Wrong sentence. The temperature drop on duct air is about 45% but the air temperature does not so important[+/-3-5% from 55 to 75oC]. The approximate 40% of the total temperature drop it is the temperature drop from the outside surface of the duct [to the earth including the concrete]