310.15(B)(2)(a) - need a little help

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RD35

RD35

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Bloomington, Indiana
Currently working on a design for a small RV park. Owner is a not-for-profit. They are trying to keep costs as low as possible. There will be a long (300ft+) feeder run that needs to be buried and comprised of parallel conductors. I could, conceivably, do this run as direct burial with URD aluminum cable runs. However, for the parallel DB cable runs to not be subject to derating, the subject code rule states that "spacing" must be maintained throughout the run. Can anyone tell me what "spacing" is required for a design of this type? Would, say, installing the runs side-by-side with a minimum of 2" of clean soil between the runs suffice in the bottom of a 24" deep trench?
Just for the sake of my own integrity, I plan to propose doing this install with conduit. But I want to be sure I give the owner all legal options.
Thoughts? OBTW this project is in Indiana and subject to the 2008 NEC.
 
Oh and to clarify, the run has a calculated service load of 720 amps. I'm considering 4 runs of 250, 250, 250, 3/0 Aluminum as the direct bury option.
 
Imo, I would not do direct bury. I know it is a hell of a lot cheaper but unless you have sandy soil - no rocks I would avoid it.
 
Rural setting, no 3-phase available so this is a single phase 120/240V system. And...this is a branch feeder, not a service feeder. I mis-typed that one. This is the first time I have encountered a situation such as this. Any suggestions for a better approach are welcome. I'm always ready to learn something new!
 
Thanks Dennis! I am leaning more and more toward not even offering it up to them. Really just want to know if it would meet NEC minimum requirements. A bad design is embarrassing.....but an illegal design is down right horrible from an engineers perspective. The soil at the site does tend to have some rock. I would insist on a sand bed and backfill if they were to choose the option. Still trying to decide!
 
I live in a neighborhood that was built circa early 1980's (my house was 1980). We have u/g service and transformer feeds. About 20 years ago, the PoCo had conduit installed (directional boring) and installed new feeders for the transformers (they were having problems with the distribution wiring). I've heard of other locations going through the same thing.

Remember that commercial, "pay me now or pay me later"? It applies to your feeder. IMO, direct burial should only be used for temporary feeds or 120V, 15/20 amp circuits.
 
Thanks ptonsparky and oldsparky52! I just finished having a new UG feeder installed at home for my new workshop 100 ft from my house. I chose to have it installed in PVC conduit. You folks are making me feel better about my decision to spend the extra $$. (y):)
 
The does not define spacing. There was a rejected PI that would have required at least one diameter between cables or conductors for spacing. The duct bank tables in Annex B specify 7.5" center to center between the ducts, but not a code rule.
 
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