Conduit Installed Underground Beneath a Stormwater Retention Basin

[CompositionB]

I am working on a project that involves equipment racks constructed on pilings in a retention basin. The civil engineers designing the basin will calculate the elevation of the 100 year flood line.

Concrete pilings will extend above grade to 1' above the 100 year flood line. The metal equipment racks will be mounted on the pilings and all electrical equipment will be mounted >2' above that flood line per 2005 NEC 682.10.

Others involved in the project are insisting that electrical conduits may not be installed underground beneath the retention basin since the basin will hold water for probably a few days per year. They want to run all the conduits as RMC mounted above grade.

That seems like a poor approach since it will require installation of additional pilings at <10' intervals to support the conduits per code. The change from PVC to RMC will add substantial cost to the project on top of the additional pilings. Furthermore, running these conduits above grade will obstruct access to the equipment.

I can't find anything in the code that prohibits RNC from being installed underground below a retention basin, however I also can?t find anything that specifically allows it. Any suggestions?

 

[walkerj]

There is no problem with the conduit being installeded under the basin as long as the conductors are rated for a wet location.

I would try to select a wire rated for submersion.

 

[CompositionB]

There is no problem with the conduit being installeded under the basin as long as the conductors are rated for a wet location.

I would try to select a wire rated for submersion.

I typically spec XHHW for outdoor or wet location applications.

 

[Pierre C Belarge]

Others involved in the project are insisting that electrical conduits may not be installed underground beneath the retention basin since the basin will hold water for probably a few days per year. They want to run all the conduits as RMC mounted above grade.


I can't find anything in the code that prohibits RNC from being installed underground below a retention basin, however I also can?t find anything that specifically allows it. Any suggestions?

There is a train of thought, that buried conduits are not prone to be immersed in water once installed underground.
Earth can become saturated during the right conditions, which is very similar to installing the conduit underwater.

If they have such concerns and they are not code concerns, then it sounds like a design issue.
If they want to design as such, so be it as long as you are getting paid to do so.

 

[wbalsam1]

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Others involved in the project are insisting that electrical conduits may not be installed underground beneath the retention basin since the basin will hold water for probably a few days per year. They want to run all the conduits as RMC mounted above grade.

That seems like a poor approach since it will require installation of additional pilings at <10' intervals to support the conduits per code. The change from PVC to RMC will add substantial cost to the project on top of the additional pilings. ................

Not to mention all the thermal expansion fittings necessary. See 300.7(B) and refer to paragraph #1 of the FPN for coefficient.