CWilliams
New member
- Location
- Bremerton, WA, USA
View attachment Substation_Cable-Trench-Proposal1.pdf
I am providing project oversight for a government project to replace a 15kV switchgear. The existing 15kV switchgear, which was installed circa 1973, has a concrete cable trench beneath it that is 46 feet long, 17 inches deep and 14 inches wide. This trench serves as the pathway for the various 12kV feeder cables that transition down from the 11 cubicles to a manhole/duct system at the north end of the trench. The existing cables are all piled on top of one another. It does not appear as if the single phased sets or paralleled sets for each circuit are bundled or co-located. At the south end of the trench their are nine conductors (3 parallel sets for an outgoing feeder). At the north end of the trench, there are total of 45 12-kV cables piled on top of one another other. Regarding operation, none of these circuits or cables have ever been derated or adjusted by the utilities group. Given the number of cables, the heat affect and the fact that they are not properly separated, I am trying to provide some guidance to the designers of the new switchgear on how to properly separate such a large volume of MV cables. If we are unable to determine an adequate separation, what section of the NEC would cover cable derating or adjustment based on the number of cables or the potential heat impacts.
In addition to extending the width of the trench to 19 inches, the design firm has proposed installing fiberglass support bars at 4 foot intervals to create some space between cables. I still do not believe that this will completely solve the potential heating and cable weight issues and have a feeling that each circuit may still require derating where cable separation cannot be maintained. Any help you can provide would be appreciated.
I am providing project oversight for a government project to replace a 15kV switchgear. The existing 15kV switchgear, which was installed circa 1973, has a concrete cable trench beneath it that is 46 feet long, 17 inches deep and 14 inches wide. This trench serves as the pathway for the various 12kV feeder cables that transition down from the 11 cubicles to a manhole/duct system at the north end of the trench. The existing cables are all piled on top of one another. It does not appear as if the single phased sets or paralleled sets for each circuit are bundled or co-located. At the south end of the trench their are nine conductors (3 parallel sets for an outgoing feeder). At the north end of the trench, there are total of 45 12-kV cables piled on top of one another other. Regarding operation, none of these circuits or cables have ever been derated or adjusted by the utilities group. Given the number of cables, the heat affect and the fact that they are not properly separated, I am trying to provide some guidance to the designers of the new switchgear on how to properly separate such a large volume of MV cables. If we are unable to determine an adequate separation, what section of the NEC would cover cable derating or adjustment based on the number of cables or the potential heat impacts.
In addition to extending the width of the trench to 19 inches, the design firm has proposed installing fiberglass support bars at 4 foot intervals to create some space between cables. I still do not believe that this will completely solve the potential heating and cable weight issues and have a feeling that each circuit may still require derating where cable separation cannot be maintained. Any help you can provide would be appreciated.
