Concrete trench used for MV Power Install

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diamonddean

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I am seeking thoughts on the NEC code issues around concrete trench such as made by "Trenwa" or Utility Vault.

How do you derate cables laid on an open cable tray inside a 96" x 48"" concrete trench with a removable concrete cover set at grade in the ground ? The trench is over 2000' long and is sloped with drainage issues addressed. The covers do not seal the trench, i.e. heat can escape.

Does the requirements for cable tray in free air apply for this? How small of a trench would still be handled the same way? Is it an issue of cable versus trench cross sectional area ?


Are there any papers or anything that address this installation.
 
Still no thoughts? Looks like we are going to treat as a simple cable tray installation with an increased ambient temperature. Per NEMA WC-51, ambient temperature in upper trays of a multiple tray rack or the upper portion of a cable tunnel can be elevated and recommends using 50 degree C.
 
diamonddean said:
I am seeking thoughts on the NEC code issues around concrete trench such as made by "Trenwa" or Utility Vault.

How do you derate cables laid on an open cable tray inside a 96" x 48"" concrete trench with a removable concrete cover set at grade in the ground ? The trench is over 2000' long and is sloped with drainage issues addressed. The covers do not seal the trench, i.e. heat can escape.

Does the requirements for cable tray in free air apply for this? How small of a trench would still be handled the same way? Is it an issue of cable versus trench cross sectional area ?


Are there any papers or anything that address this installation.
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Use the covered raceway rating, such as tray.
 
Thought of that but I feel that approach is way too conservative. There is alot more air to distribute the heat in a 4' x 8' trench than would be in a 6" x 30 " covered tray, but thanks for the response
 
Why not ask them?

Trenwa, Inc.
1419 Alexandria Pike
Ft. Thomas, KY 41075
Phone: (859) 781-0831
Fax: (859) 781-1085
info@trenwa.com

Roger
 
I did, they recommended treating as A/G Tray in free air, but had no basis for the recommendation. Again, we are going to handle it this way and use a 50 degree C ambient to compensate for the tray being inside a covered trench.
Thanks
 
I don't agree that it could be treated as a free air type of installation, sounds as though they are telling people what they think they want to hear.

Roger
 
If you don't agree, do you have a recommendation? Most Engineers that have done this installation for major utilities have have either used tray in free air (If tray was used) or no derating at all if cable simply laid in trench without tray. However no one could site a code basis for their decisions.
 
If this is for a utility the installation may not even fall under the NEC. Are there any guide lines in the NESC.

Roger
 
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