Cable Derating in cable tunnel

Hello Gents,

I'm working in a project where I have a tunnel of 3-meter height and 3.7-meter width. In this tunnel there are around 37(single core) medium voltage cables laid on cable trays staked on both sides as illustraed in the attached image. The separation dsitance as shown between each tray is 350mm and every cable is sperated from the adjacent other cable by more than double the diameter of the single core cable as also shown within the same tray. The question is how to figure out the derating factor for such arrangement, taking into consideration that the IEC 60502 table B.23 considers maximum of 3 horizontal cable trays above each other ?

Thanks in advance for your help..

  • IEC 60287-2-3?

    (Not used it myself so can’t advise any further)


  • Well the bundles are far enough apart, that there is almost free airflow between them, so the 'close grouping' model does not apply - they are thermally independent, except that they all contribute to the overall heating of the tunnel, and unless the air is moved in some way, it will be hotter at the top than the bottom. I presume the top of the tunnel is closed, which is not the case assumed for table B23.

    If so one approach is to deduce the overall temperature rise by looking at the surface area of tunnel in contact with the ground around, and how much that can dissipate to get a new ambeint temperature..

    ERA 69-30 suggests that vertical spacing of more than 6 times the cable diameter can safely be treated as if the cables have no neighbours, but again this assumes that the whole ambient is not allowed to rise.