Derating Conductors due to bundling inside Low voltage switchgear

I think here you need to not look at the prescriptive requirements themselves but rather what the requirements are trying to achieve, and therefore reach a sound engineering justification for how you deal with it.

If cables are bunched the air circulation around each cable is restricted and therefore the heat dissipation is markedly different. In the case of thermal insulation along part of a conductor, the part with the thermal insulation will have a reduced heat dissipation through convection but copper, in addition to being a good electrical conductor, is also a good thermal conductor and so heat will be conducted along the conductor to parts of the cable that are not thermally insulated.

This gives a certain justification for applying a derating which depends on the length of the bunching. How you determine the derating to be applied, and how you justify it, I will leave to you.

Alasdair

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.

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.

 

    - Andy.

" I suspect the 'usual practice' in the UK is not to specifically calculate for inside the DB - just work with the ratings for the rest of the circuit - but at the same time lacing/bunching/cable-tying conductors together inside a DB is considered poor practice." In many case bundling is not evitable but also I think in lighting panel boards where there are many branch circuits let's say 54 way enclosure, the conductors are feeding lighting and sockets outlets where the diversity factor is very low and hence it's rare that a problem shall occur due to bundling.On the other hand where the panel board is feeding sub feeder panel and diversity factor is relatively high, the enclosure is big in comparison with outgoing feeders and generally no tied bundling is needed.

But as inspector and you would work by the book I think it would be a problem and hence derating factor along the whole circuit run shall follow IEC 61439-1.

I think in France it is a good practice to terminate home runs by terminal blocks inside the enclosure, here the panel builder is responsible for his panel board conductors and installation contractor is responsible for the external conductors. and hence if some derating shall occur is for the short length inside enclosure(between terminal and C.B).

On the other hand, there is something interesting in NEC is that they allow (with 40 percent max. fill factor in wire space) for 60cm bundling with no derating or (10 percent of whole run or 3 meters whichever is lesser), noting that BS standard lighting panel boards are very similar to UL-listed panel boards and termination occurs directly on CB but using above stated exemptions will relieve contractor from derating conductors due to bundling for short length. I was searching for similar exemptions in BS 7671 standard but I couldn't find.I think sooner or later we shall see such exemptions in BS 7671.