34.5kV to 15kV Duct Bank Separation?

[bxny]

Can anyone tell me if there is a 'separation distance' requirement for separating underground ductbanks - for example, one bank with 34.5kV circuits and the other with 13.8kV circuits? How far apart do they have to be (minimum edge to edge dimension)?

 

[alehman]

Requirement by who? Who are you regulating authorities? It may be important to separate them for reduction to mutual heating, but that depends on the loads and soil conditions.

 

[bxny]

Actually, that is part of what I am asking. Does some code or standard exist which contains a recommendation for separating duct banks at voltage levels such as these.

 

[Zogzog]

I think what alehman meant is where do you live and what type of facillity is this? This forum has members from all over the world, big difference between a gov facillity in Bagdad or a industrial plant in Kansas.

 

[bxny]

The project is located at a government facility overseas, not a US territory and we are not aware of any local code that exists governing such an installation.
I am asking if any recommendations exist in some industry standard. I am interested in any engineering/scientific basis for such a requirement, in lieu of a local code requirement. The only thing I have been able to find is in the NEC (Annex B, B.310.15(B)(2))which concerns 5kV duct banks, and it recommends keeping them 4ft. apart (edge to edge)to avoid any effects from mutual heating. However, since we have a 15kV and a 34.5kv bank, side by side, the question has arisen.

 

[alehman]

I don't know of any standards. The only technical concern for spacing that I'm aware of is mutual heating. There are computer programs that can help you model that e.g.

http://www.calcware.com/

Other reasons:
maintainability - If you have any concern that you might later need to dig up one circuit without disturbing the other, more space makes that easier.
reliability - If the circuits are in any way redundant for one another, separation reduces chances that one accident will disrupt both.

 

[dpc]

I am not aware of any codes or standards that would require minimum separation between these ductbanks.

 

[7anoter4]

I agree with alehman.
The minimum separation is required only by duct bank construction system: in order to pour the concrete between ducts [or pipes] a minimum clearance of 3" is required.
The 4 ft distance from one duct bank to another required by NEC B-310-15(b) which one has to keep it is for use with the table attached to the figure B-310-3,4,5,11 for cables without shield[0-5 KV].
If you could calculate the ampacity based on J.H.Neher and M.H.McGrath calculation mode [or IEC-287]
you could combine any kind of cables [shielded or not] keeping any distance, depth, soil thermal resistivity and pipe size and material.
Of course, the bigger separation, the higher ampacity you'll get.
What you need it is the cable cross section that means conductor dia, insulation core dia, assamble dia, jacket thickness, shield dimensions.
Regards