Electrical duct banks

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jtnsig

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The minimum depths of conduit is shown in table 300-5. This table covers the minimum depth of conduit in various areas. If the PVC conduits are concrete encased and routed under a parking lot, what would the minimum depth be? We have always placed the duct bank a minimum of 24". I do not see in the code where it discussed concrete encased conduit. The table does have a section that addresses conduit in a trench under 2" thick concrete, but that is not in a parking lot.

Does the code cover concrete encased duct banks?
 
Re: Electrical duct banks

In the 2002 NEC that I have, the table 300.5 shows the same minimum cover for underneath streets, highways, roads, alleys, driveways, and parking lots which is 24" for all conditions in the columns. :)
 
Re: Electrical duct banks

Yes, we came up with a 24" minimum but the columns on table 300-5 don't address PVC conduit concrete encased. The closest column is #2 rigid metal conduit. Column #3 is nonmetallic without concrete. The duct bank has a minimum of 3" of concrete surrounding the PVC pipes. Three rows above the "streets, highways, parking lots..etc" states that if it is in a trench with 2" thick concrete you are allowed a lesser depth.

We are sticking with our original 24", but concrete encased conduit doesn't seem to be discussed in the code. Wouldn't a concrete encased duct bank be stronger than RGS and there fore have a smaller depth requirement?

thanks
Joel
 
Re: Electrical duct banks

Table 300.5 is confusing; but it does cover your situation.

The problem is that Column 3 seems to talk only about raceways that aren't concrete encased. That is not true. It is talking about approved raceways, among which are non-metallic raceways listed for direct burial without concrete encasement. Any raceways (other than those in Column 2) suitable for underground use fall in this column. And any of them MAY still be concrete encased (or covered) whether they are required to be or not.

If there were no traffic, Line 4 permits Column 3 raceways under 4" of concrete.

But there is traffic, so it "drops" to Line 5, and 24" is minimum cover - no matter what.
 
Re: Electrical duct banks

Would it make it clearer if the text in Column 3 were something like: "Other Raceways Recognized for Underground Use"?
 
Re: Electrical duct banks

I completely missed that text of "Other Approved Raceways" at the top of column three! The way it is worded, unless you read carefully, I think one might think that column was only for direct buried cables without concrete encasement. I think I was just being dense though. :roll:
 
Re: Electrical duct banks

Well then, would the text I suggested have made it clearer. You know you'll get an assignment over on the 2008 Proposal forum if you say yes :D .
 
Re: Electrical duct banks

Hmmm...ok Bob, the text you suggested would make it clearer, because it does not really single out just one type of approved underground installation, with other approved types as an afterthought. So, how does this whole proposal thing work, anyways? :D :cool:
 
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