Back fusing/ Protecting a cable

[sparkmanuk]

If you have a busbar chamber supplied by a large cable fused at say 100A, and then smaller cables taken from the chamber that go to isolators that are rated at 30A. Technicaly the smaller cables are underated but cannot pull more than their design current due to the isolators they are connected to, is this allowed by the regs, I know this is done all the time but is it BS7671.

 

[AEC]

If you have a busbar chamber supplied by a large cable fused at say 100A, and then smaller cables taken from the chamber that go to isolators that are rated at 30A. Technicaly the smaller cables are underated but cannot pull more than their design current due to the isolators they are connected to, is this allowed by the regs, I know this is done all the time but is it BS7671.

Regulation 434.2.1 applies here. It allows the reduced cross sectional area of the conductors for a distance of 3 metres.

 

[albi]

so if you had a 300v buzz bar feeding a 100A single phase cu in 10mm tails thats ok

 

[scotsparky]

No alibi what yo have to do is adjust your cable size for the upsteam fuse.
For example a 300A busbar -----> 100 hrc switch fuse protecting a d.b then you can use a 25mm cable as the fuse upsteam will blow before to much load is drawn throught the cable.

Hope that makes sense.

 

[sparkmanuk]

I understand the theory, but if i put the point like this, If a large factory were to be wired today by a large respected industrial contractor like 'Baileys' etc would they allow it. I am asking this question because like all sparks you look at how others have installed things over the years, but I sometimes wonder if i am picking up bad habits from old sparks.

 

[bugsy]

If you work to the reg's you won't go wrong, and no 10mm tails is not the one

 

[scotsparky]

I understand the theory, but if i put the point like this, If a large factory were to be wired today by a large respected industrial contractor like 'Baileys' etc would they allow it. I am asking this question because like all sparks you look at how others have installed things over the years, but I sometimes wonder if i am picking up bad habits from old sparks.

Simple answer is yes they would do it!I have seen i tdone a lot and it is legal. Just be carefull about the upstream protective device.