Ampacity in precast trench?? 1

[electengr1]

Hello everyone!

I have been reading the code, trying to figure out this problem.

We are using a precast trench (i.e. Trenwa) for 2 circuits of 4 sets of 3/c 350kcmil, copper. We are trying to figure out which table in the code would be applicable for the ampacity. It is not exactly in cable tray, it is not exactly in duct and it is not exactly direct buried. I have seen many substations utilize this system, but how is the ampacity determined?

Any help would be greatly appreciated!!

Thank you!

 

[dbaird]

Check the definition of Electrical Ducts NEC 310.60A:

http://s50.yousendit.com/d.aspx?id=1EOJS6XPAS2NU0EWDUXS4AG97X

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[dpc]

Well, utility substations are not governed by the NEC.

As far as the NEC goes, this type of installation is not actually covered in the NEC, unless it was added recently. The electrical duct definition in 310.60 doesn't seem to cover this Trenwa rectangular duct, at least I've never thought so.

In the past, I have treated it as Non-metallic Wireway (Article 378) if I needed to give the inspector a specific reference. If you treat it as wireway, Article 310.15 covers necessary derating.

For situations where I needed to put a lot of conductors in the Trenwa, I have installed metal-clad cables using conduit spacers, then backfilled with thermal sand. Okonite and other cable suppliers can calculate the resulting ampacity using the Neher-McGrath method. If you have the software, you can do the calculation yourself. This generally satisfies inspector's concerns.

 

[electengr1]

Thank you for the responses.

I have taken a look at the NEC again and tried to figure these out. I agree, dpc, the non-metallic wireway seems to be the best fit.

This installation is in an industrial setting, so the NEC applies.

One pertinent piece of information that I forgot to mention is that the cables are 13.8kV. I don't believe that section 310.15 will help us in this situation as it deals with cables under 2kV. This is in a preliminary stage, so we were trying to use the ampacity tables to get a general idea of the number and size of conductors we would need for this installation.

 

[dpc]

Are you using metal-clad cable? I don't think standard 13.8 kV cable in a Trenwa duct would meet NEC requirements for installations over 600V, but I'd have to think about it for a while. You might have to run a cable tray in the trench.

If you use MC cable, you should be OK. But you will need to derate. I would check with Okonite or other cable suppliers on options and recommendations.

 

[waross]

Hello
My code is badly out of date but your concrete trench seems to fit the description of a non-metalic tray system in Article 318.
I find a reference to solid bottom trays, and the conductor arrangements for conductors over 4/0. Article 318
I also find conductor derating factors for trays with solid covers. Article 318
Hope this helps
yours