Trench Under Switchboard

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charlie b

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Lockport, IL
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Semi-Retired Electrical Engineer
We will have a 2500 amp, 480 volt, main service switchboard inside an electrical room. It will be about 12 feet long and 3 feet deep. The electrical contractor wants us to provide what he calls a ?concrete wire trench? under the switchboard. He intends to stub up the service conduits at one end of the switchboard, and start the trench just past that point, so the trench will be about 9-10 feet long. He wants to run feeders from the switchboard breakers down into the trench, and over to the side wall, where they will enter conduits that run under the slab to distribution panels.

This arrangement does satisfy the requirement for separation of service and feeder conductors. Also, a 10 foot wide trench would easily satisfy the minimum size requirements of 314.28. But what about derating factors of Table 310.15(B)(2)(a)? The EC claims that the trench is similar to a manhole or handhole, and does not constitute a ?raceway.? Therefore, he claims that Table 310.15(B)(2)(a) does not apply. I have never encountered such a trench under a switchboard.

Thoughts? Seen this done before?
 
I think you could make the arguement that it is an Auxilliary Gutter subject to Article 366 and the derating factors of 310.15(B)(3)(a), per 366.23.
 
It was a common practice in my area. I see it often in older buildings.

I wish they still did it.
I can't say it was common practice here but it was done and I was on one project where it was actually part of the EE's design.

I also wish it were more common practice


Roger
 
Trench Under Switchboard

We will have a 2500 amp, 480 volt, main service switchboard inside an electrical room. It will be about 12 feet long and 3 feet deep. The electrical contractor wants us to provide what he calls a ?concrete wire trench? under the switchboard. He intends to stub up the service conduits at one end of the switchboard, and start the trench just past that point, so the trench will be about 9-10 feet long. He wants to run feeders from the switchboard breakers down into the trench, and over to the side wall, where they will enter conduits that run under the slab to distribution panels.

This arrangement does satisfy the requirement for separation of service and feeder conductors. Also, a 10 foot wide trench would easily satisfy the minimum size requirements of 314.28. But what about derating factors of Table 310.15(B)(2)(a)? The EC claims that the trench is similar to a manhole or handhole, and does not constitute a ?raceway.? Therefore, he claims that Table 310.15(B)(2)(a) does not apply. I have never encountered such a trench under a switchboard.

Thoughts? Seen this done before?

When I was contracting we did this type of installation on several projects. You have the separation from the utility service conductors that is required and typically we would have the underground conduits entering the trench at different points along it's length so you are not running all the conductors for the total trench length. On build-out projects where we would install the UGP, CT Comp and Main and maybe only one distribution section; we would run the trench for the length of all the future sections and cover it with diamond plate. As the new sections were added all we had to do was remove the diamond plate for that section and install the feeders.This method works very well when you want to install all the under floor feeder conduits but not all the gear. Also this method takes away the problem of hitting each section with the correct feeder conduits, especially if a change is made to the gear layout during the construction process.
 
I don't see how a trench under the switchgear lineup is any different than the top hats you can add to the top to allow feeders to cross from section to section to get to their respective overhead conduits.
 
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