- Location
- Lockport, IL
- Occupation
- 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?
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?