Effect of drying on soil TC values for duct bank design?

[mb3928]

I am designing some 480V underground duct banks for a distributed generation project, using NEC Appendix B to calculate derating. They will be designed to carry ~2,500A each. The geotech report for the site lists a number of Rho values with the soil at varying moisture contents. Down to 3% moisture content the Rho values are all below 60 °C-cm/W, which seems like a reasonable design value. However in the fully dry condition Rho inceases substantially, into the 100-120 range.

The soil is fairly free-draining and the duct banks will be above the water table. Do I need to be concerned about heat from the duct banks drying out the soil, and derate for the Rho=120 value? Or is the 3% moisture content Rho=60 value reasonable? Your thoughts would be much appreciated.

 

[cuky2000]

The initial impression is that the RHO=60 for 3% moisture content is a reasonable value considering that the encase concrete is a hygroscopic material that retains significant moisture.

However, it is advised to confirm the specific onsite moisture content. In absence of any long-term soil humidity data, it is suggested to check with the Soil Climate Analysis Network (SCAN) in the following Link

An example of soil moisture data is shown below:

 

[jghrist]

For direct buried cables, ampacity can be decreased by soil drying out around the cable. This can be accounted for by determining if the cable surface temperature exceeds a critical level. The critical temperature depends on soil type and porosity, but temperatures of 35°C and 50°C are often used. See Rating of Electric Power Cables, George J. Anders, IEEE Press

It is doubtful that the surface of a concrete encased ductbank would exceed the critical temperature, so this is not normally a consideration.

 

[dpc]

FWIW, the NEC does not require derating LOW VOLTAGE circuits to account for mutual duct bank heating. This requirement is only in Article 311 which applies to medium-voltage cable. This assumes that the conductors are sized per the NEC load calculations which are extremely conservative. You are free to derate for mutual heating if you feel it is necessary, but at 480 V it is not required by the NEC.

 

[mb3928]

jghrist, that's exactly what I was looking for, thank you.

dpc, thanks and understood. Code required or no, I just wanted to run the Appendix B checks to make sure the equipment terminal ratings/ampacities are still the governing factor, since these are (in my world at least) pretty sizeable duct banks.

 

[dpc]

Also, most cable manufacturers will do these calcs for you if you are looking for a sanity check.

Not sure of your application, but I've seen major 480 V duct banks (3000 A, 4000 A)installed with no consideration of mutual heating and AFAIK, there was never a problem. This is mostly because the NEC feeder sizing calculations are incredibly conservative and the feeders are grossly oversized in the first place.

 

[7anoter4]

From "On-line monitoring device for dry zone formation in the soil surrounding underground power cables" article

https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-gtd.2019.1118

where 6 samples were tested, minimum critical temperature was 50 oC.
A power cable outside surface may present more than 50oC, but a concrete block of a duct bank does not, in my opinion.