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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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    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
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    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
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