13.2kV Duct Bank boring

[ksavoie]

I have a situation where I need to run 9 medium voltage cables in an underground duct bank. Looking at the NEC, if I run these in concrete duct bank I need to keep a 7.5” spacing center to center.

I have a client that wants to bore a 24” casing under a road and pull 9-5” pvc conduits in the 24” casing. The 5” conduits will be touching each other. Will this be to code? If the conduits are spaced 7.5” center to center with out concrete, will this be to code? I'm not very familure with boring and duct banks in bored holes any help would be appricated.

 

[dpc]

I don't think the NEC actually dictates any particular spacing. The diagrams you see in the NEC are just showing the spacings used to come up with the ampacities listed in the Tables in Article 310. For a directional boring, it's common to stuff several conduits in a larger pipe. The only issue is determining the maximum allowable current. This requires actual calculations to estimate heating effect of other conductors, using Neher-McGrath equations. Since most of the installation are done by electric utilities, they do not have to meet the NEC.

If you're buying a lot of cable, companies like Okonite or Southwire may be willing to do this calculation for you. The NEC does allow other configurations to be used under engineering supervision - meaning you need to calculate the maximum allowable current.

 

[davidbeach]

The maximum allowable current that will be calculated per the method suggested by dpc will be lower than the value in the table in the NEC. Closer spacing will mean that more heat is trapped closer to the conductor and its insulation. There is a maximum temperature and the slower the heat can migrate away (the higher the steady state temperature) the lower the allowable ampacity will be.

 

[dpc]

That's likely to be the case, I agree.

 

[Dumbo2929]

Having worked on several directional drilling projects, the thermal ampacity must usually be derated for multiple cables. The type of conduit, usally HDPE must be factored in, along with the grout used in the boring, the casing material (steel, HDPE, etc.)