Cables within the ground or not.

Hi,

Is there any guidance or recommendations for where cables are installed within the ground for a small length of their run and appropriate grouping factors that can be applied?

I have an existing site with single core cables installed from a transformer to a main panel 14m away. 2m is within the ground, whilst the rest is on ladder on the out side of the building and out of the ground directly into the transformer.

The only way I can get the calculations to pass is to put them through the software as been on ladder through out their entire run which as I mentioned above, is not the case. Even when using and the ERA values for in the ground, I still cant get it to pass.

I appreciate that when installed in the ground, the issue is its harder to dissipate the heat so I would normally use this as worse case and use the ratings for buried in the ground in my calculations or as mentioned, is there any other guidance on this which the previous designers/installers have factored in that I am missing?

Thank you in advance.

Mark 

  • In my view, the "in ground" ratings are applicable if any part of the cable run, even just 2 meters is in the ground. If this is not done, then that part of the cable that is buried will run hotter than intended and have a shorter or less reliable life.

  • well, it presumably has not actually caught fire, so either the load is less than you think, (that may be either lower peak current or a bursty one with enough cool down in between) or the cable is better cooled than you think, or the cable is indeed running hot but you have just been very lucky so far... Or maybe maximum load only occurs on cold days.

    However, generally the 'correct' cable rating should always be set by the region that runs hottest, even if it is a small percentage of the run. The only exception may be sections that are short compared to the cables ability to conduct heat out longways ( so lengths under clamps, passing through a brick wall etc can be ignored.)

    It may well have been undersized from day 1. I have found the stickers that change colour to indicate maximum temperature to be useful in cases of possible overload, as you can put them on the cable and come back in a week, a month, even next year if the site is not too exposed, and know what the 'high tide' temperature was since you last looked. Then you will know if the risk of cable failure is real or purely theoretical.

    No chance to fit the next size down fuse I suppose?
    Mike