Underground trench derate of cables

[erniek]

I am installing a Trenwa trench system and am installing 4160V, 480V, 120V and 24V in the trench. This trench is going to be filled with sand once the cables are installed so the area classification will not be Div. I.
I would lke to know how to derate the cable going into the trench or where to find the information to calculate this. I am utilizing "TC" (tray cable) as opposed to CLX.
Any suggestion or information would be appericated.

Thanks

 

[dpc]

If this installation is covered by the NEC, there is no derating required, by code, for the 480V, 120V or 24V cables. If the 480 V cables are going to be heavily loaded on a continuous basis, you may choose to de-rate as an engineering judgment.

For the 4160 V, you would derate based on a Neher-McGrath calculation. If you are buying a lot of 4160 V cable, some of the cable suppliers may be willing to do this calculation for you if provided the configuration. I have been able to get Okonite to do this in the past. In addition, there are commercial software programs that can calculate this derating for you, but they are not cheap.

Special "thermal" sand can be used to slightly improve the heat transfer characteristics.

If you use TC cable for both 4160 V and LV circuits, you will need a barrier to separate the MV cable from the LV cable. You might also verify that the TC cable is listed for burial in sand.




David Castor

http://www.cvoes.com