Thermal resistivity for UG cable calculations

[RedOne05]

I am currently working on calculating the ampacity of an underground cable, where the thermal resistivity of the surrounding soil is a key factor in the calculation.

The geotechnical report includes thermal dryout curves that show how soil resistivity changes with moisture content, which ranges from 41.8% to 0%. Using the highest resistivity value (2.5 m·K/W at 0% moisture) would be very conservative and may lead to oversizing the cable. To avoid unnecessary overdesign, what moisture content should be selected to ensure a practical and reliable result?

 

[waross]

To avoid unnecessary overdesign, what moisture content should be selected to ensure a practical and reliable result?

The lowest value that may be reached in service.

 

[RedOne05]

The lowest value that may be reached in service.

Thank you for your response. How can I determine the "lowest value that could be reached in service" while I'm still in the design phase?

 

[7anoter4]

First, the IEC standards use elevated earth resistivity -see IEC 60364-5-52 [2.5 K.m/W] or IEC 60502-2 [ 1.5 K.m/W].
However, directly buried power cable produces a soil heating and then the soil around stays dry. The standard IEC 60287-1-1 presents a way to overcome the actual change in earth resistivity in order to avoid using the maximum dried soil resistivity in calculation.
See chapter 1.4.2 Buried cables where partial drying-out of the soil occurs
Any way we may avoid the soil drying running the cable through a concreted duct or duct bank, since the concrete preserves the surrender soil humidity.

 

[RedOne05]

Any way we may avoid the soil drying running the cable through a concreted duct or duct bank, since the concrete preserves the surrender soil humidity.

The cable will not be directly buried. The three cables will be installed inside three conduits, which themselves will be placed inside a casing. The casing will then be filled with concrete.

 

[Sn00ze]

You could also manage it from a procedural perspective. We had a client that in order to avoid oversizing the cable, would make it so turn-around would only happen in early May which would limit the ambient temp to 25C

 

[dpc]

You can specify backfill with thermal sand that has more consistent thermal characteristics. But you've found the weak link in ampacity calculations in underground cables. You pretty much have to assume a realistic worst case.

 

[bacon4life]

The region I work is winter peaking, so it is fair for me to assume some moisture exists in typical utility duct banks serving residential load. My residential loads only peak for a few days year. For loads serving loads that are constant 24x7 or loads that peak during the dry season, I would need to assume the soil dries out.

 

[wcaseyharman]

What I saw in the last wind farm I’ve been involved with is the company took soil moisture samples over a the course of a year. Worst case measured soil moisture was used, which was at 5%.