MV Cable in Flex Conduit

[Pdan]

I’m hoping someone can offer a solution to a MV cable routing problem.

We have two structures that will experience a maximum differential seismic movement of 15” (9” north-south, 12” east-west). No vertical movement. 4” conduits pass from one structure to the other. We were considering a loop of flex conduit, but using the minimum bend radius of 24” (NEC Chapter 9, Table 2), the length of flexible conduit required is around 6.5 to 7 feet to create the loop, and 348.30 (A) Exception No. 2 (3) allows only up to 5 ft of unsupported/securely fastened flexible conduit.

This solution will not work if the flexible conduit has to be securely fastened anywhere along its length.

We don’t want to transition to jacketed MV cable, and placing the flexible conduits on a flat tray (loop would lay horizontally) would support the flex, but it would not be fastened.

Any help on this matter would be greatly appreciated.

Patrick

 

[synchro]

If you can offset the conduits vertically and then connect them with a vertical section of flex conduit, then the length of the flex could be reduced because you have no vertical movement. You would just need a shallow loop to accommodate the extra length from the "S" shape formed on the flex when it's shifted from its installed vertical position. For example, I calculated that 3.5 feet of flex installed between two fittings that are 3.1 feet apart would allow 17" of differential horizontal movement. This assumes that each half section of the "S" consists of 50 degrees of a 24" radius curve when there's a 17" shift. A longer length of flex would provide even more margin.
I'm not sure if this is practical in your situtation but I figured I'd put it out there as an option.

 

[Pdan]

Yes, this works!!! Thanks very much for the reply. -P