Influence of burial depth on HV cable ampacity 3

[Foudrette]

Dear All,
I commonly use IEC 60287 for calculation of underground cables. This standard is suitable for most of the ways of laying. But when you have to cross a road,a railway or a river the burial depth can reach important values (40' or 75') for example. If ampacity calculations are made according IEC 60287, results obtained are very pessimistic because it is assumed there is no change in ground resistivity and ground temperature what is surely false. This leads to chose cable with very big cross-section what of course dramatically increases the cost.
I am looking for a chart or any other reference which could give the evolution of ground characteristics versus depth.

Thanks for all of your help, Marie-Hélène

 

[gordonl]

It is pretty hard to give generalizations on what soil will be 40-75' down, if you want accurate ampacities you need actual conditions. Intuitively I would say it will be cooler in the heat of summer, and damper. (I vital characteristics)

 

[jbartos]

Question: Is not there a typo by any chance, 40'=40feet versus 40"=40inches, etc.?

 

[Foudrette]

Unfortunately, there is no mistake in the text.
Depth is really 40 feet or more !

I really appreciate your help,
Best regards, Marie-Hélène

 

[advidana]

I though than temperature get pretty constand for a certain depth below the 12' level, something like 55-60 degrees F. I could be wrong. In the United States check with the U.S. Dept of geology.

 

[sdlloyd]

Foudrette,

IEC60287 considers that all the heat generated by cables below ground escapes from the earth's surface into the atmosphere and that for continuous ratings that the ground has no thermal capacity.

In the case of deep cables you must consider the thermal capacity of the soil. This is reasonable as if you consider the thermal capacity of a cylinder of earth 40 feet in diameter. It would take a very long time for the cables to warm the ground such that the boundary temperature changed significantly, 10's of years perhaps.

I am afraid you have to return to first principals to calculate this rather than rely on a specification.

http://www.cableconsulting.net