Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

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

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

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

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.

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

Quote (is the 3% moisture content Rho=60 value reasonable?)

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:

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

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.

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

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.

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

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.

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

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.

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close