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Derating Conductors due to bundling inside Low voltage switchgear

Former Community Member
Former Community Member
I know when it comes to conductors inside switchgear the regulations for  IEC-61439 Annex H pages 111 and 112 apply.

If cables installed in free air outside the enclosure regs of BS 7671 Appendix 4 Section 7 and IEC-60364-5-52 would apply.

If we have many cables installed in free air (let's say 15) and they are distant from each others by more than 2 cable diameter (hence no derating factor for bunching shall apply) but they would be bundled for a distance(L) inside enclosure before they directly terminate in C.B's what would be the case here?

Shall we use the rating for the whole run of cables obtained by IEC-61439 and apply high derating factors or it depends on the length of (L) of the bundling? 

Knowing that in case of thermal insulation of conductor BS 7671 Reg. 523.9 takes in consideration the Length of the part of conductor that is thermally insulated.
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    To some extent this depends on the "bunching" you wish to carry out. If all the cables are to be zip-tied together into a tight loom then perhaps some extra thought might be taken if the length is long (probably metres), although it is very unusual to find cable damage inside switchgear unless there have also been loose connections and similar problems. It is not normal practice to loom cables inside fuse boxes etc here, just loosely lay them together, and so spacing does largely meet a couple of cable diameters. The IEC spec is for long runs of loomed cables, like multicores and is very much a worst possible case scenario. You may care to check some installs with thermal imaging equipment to not see a particular problem.
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    To some extent this depends on the "bunching" you wish to carry out. If all the cables are to be zip-tied together into a tight loom then perhaps some extra thought might be taken if the length is long (probably metres), although it is very unusual to find cable damage inside switchgear unless there have also been loose connections and similar problems. It is not normal practice to loom cables inside fuse boxes etc here, just loosely lay them together, and so spacing does largely meet a couple of cable diameters. The IEC spec is for long runs of loomed cables, like multicores and is very much a worst possible case scenario. You may care to check some installs with thermal imaging equipment to not see a particular problem.
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