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Power-User
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Current-Carrying Capacity of Underground Cables

12/13/2009 6:20 AM

is there any general rule that underground cables current carrying capacity is always lower than overhead one? or vice versa? as far as i know ccc for underground ones is lower since heat dissipation is harder. But some charts says it is opposite and i am confused.. How might a undeground cable have higher ccc ??

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Guru
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#1

Re: Current-Carrying Capacity of Underground Cables

12/13/2009 10:28 AM

Conductors in conduit or direct buried?

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Power-User
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#2

Re: Current-Carrying Capacity of Underground Cables

12/13/2009 10:40 AM

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

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Power-User
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#3

Re: Current-Carrying Capacity of Underground Cables

12/13/2009 10:55 PM

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!

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Guru
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#4

Re: Current-Carrying Capacity of Underground Cables

12/14/2009 3:21 AM

British Standard 7671.

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Power-User

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#5

Re: Current-Carrying Capacity of Underground Cables

12/14/2009 7:34 AM

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?

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Power-User

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#6

Re: Current-Carrying Capacity of Underground Cables

12/14/2009 2:54 PM

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

Patrick Whowha

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