Current-Carrying Capacity of Underground Cables

Conductors in conduit or direct buried?

ccc of "in conduit underground > overhead> direct buried ?"

Cables are rated according to the Regulations pertinent for the country. The Laws of Physics being universal, however, in general you will find that cables are rated for a given ambient air and soil temperature. Here in NZ, for example, the ambient air is 30°C (yeah right ) while soil is 15. Thus a cable in ambient air is rated lower than one buried in soil. Soil has a better thermal conductivity than air so the cable rating is higher.

Burying the cable in a duct diminishes the thermal connection to the soil so the current rating goes down.

Thus in NZ buried direct > buried in duct > overhead

In Australia the ambient air is rated at 40. However there are other ground conditions (like sandy areas) and these must be checked wrt cable rating.

Obviously the ratings for your relevant country and the pertinent Regulations must be applied. While ground temperature remains fairly constant, air temp varies considerably. Thus in Alaska I assume

overhead > buried in duct > buried direct.

Alaskans please comment!

British Standard 7671.

You have answered your own question. Given that the comparison is between two matching conductors. Underground conductors will always be rated for lower current carrying capacity.

Which charts say otherwise?

If u are not satisfied with the answers given so far,pls refer to various regulations and also yr local regulation.

Patrick Whowha