Cable Installation

[timm333]

Is there a rule in NEC or NESC which allows to reduce the required spacing between underground power cables for short distances? For example, if the required spacing is 600 mm, will it be possible to reduce this spacing to 200 mm for short lengths (of about 10 m) just before the cables enter into a building or duct bank? Thanks for help.

 

[Smart $]

What underground power cables are required to be spaced 600mm? Required by? Why the need to decrease to 200mm for 10m?

 

[timm333]

The actual system is different and specific to a project. I just mentioned an example to locate a rule which allows the reduction in spacing for short distances. If such rule exists, then I will be able to tailor it to apply to the project specifications.

 

[Smart $]

Well, let us know when you decide to be less cryptic. :blink:

 

[Ponchik]

IMO, it will depend on the project spec. You need to ask the project engineer.

What voltages are you dealing with? AC or DC? Are they just insulated individual conductors in the ground without a conduit? Or they are cables?

Just curious, since your measurements are in mm, is this installation in Europe, the drawings are from Europe or you are used to mm measurements?

 

[timm333]

I often find that mm measurements are easier to use. I was talking about two 3-conductor 13.8 kV XLPE underground cables running in parallel. These are the dual feeders to the same load (one main feeder and other standby). Most of the length of these feeders is in soil (direct buried) for which the required spacing between the dual feeders is minimum 2 feet. Some part is in concrete (duct-bank) for which the spacing can be reduced to minimum 1 foot.

The main issue is the transition from direct-buried to duct-bank. As the spacing is more in direct-buried (2 feet) and less in duct-bank (1 foot), so the cables will have to bend just before entering into the duct-bank. In this way there would be some length of cables in soil (direct-buried) just before entering into the duct-bank (and immediately after leaving the duct-bank) for which the spacing would be less than 2 foot. So I thought maybe some rule of NEC/NESC is available which allows the spacing to be reduced for short lengths.

 

[Smart $]

I often find that mm measurements are easier to use. I was talking about two 3-conductor 13.8 kV XLPE underground cables running in parallel. These are the dual feeders to the same load (one main feeder and other standby). Most of the length of these feeders is in soil (direct buried) for which the required spacing between the dual feeders is minimum 2 feet. Some part is in concrete (duct-bank) for which the spacing can be reduced to minimum 1 foot.

The main issue is the transition from direct-buried to duct-bank. As the spacing is more in direct-buried (2 feet) and less in duct-bank (1 foot), so the cables will have to bend just before entering into the duct-bank. In this way there would be some length of cables in soil (direct-buried) just before entering into the duct-bank (and immediately after leaving the duct-bank) for which the spacing would be less than 2 foot. So I thought maybe some rule of NEC/NESC is available which allows the spacing to be reduced for short lengths.

Why not design the duct bank so the ducts are 2' apart on entry then reduce to 1' after entry.

PS: I don't believe 2' is the minimum for direct burial, but without checking it may be if you want to use the table ampacities. Closer would require engineering supervision and ampacity calculation using Neher-McGrath.