Possible DIY mistake???

[DanDanT]

I might have made a DIY mistake...

I have recently installed under-cabinet lights in my kitchen - a 12V screw-fix kit special. Added 30A Junction box in my ring-main, then from this buried a cable in the wall to the top of the cabinet to a FCU, then on to the transformer. The thing is, I used 1.5mm T&E - I figured as there is now a 3A fuse effectively in this cable, it could never draw enough current to need more - but I now read that is wrong (see below).

Why do you need to use the same size cable running out of the junction box as that of the ring main running though it???

The drawing says it all, but the cable to the fcu MUST be the same size as that which it is connected to.

That is why the JB has 2.5 t & e running through it and to the FCU, but only 1.5mm t & e from the fcu to the light

 

[Stoday]

Because if there's a fault between the junction box and the fuse in the FCU, a 30A fuse or MCB has to blow. That exceeds the rating of your cable.

 

[GaryMo]

Because if there's a fault between the junction box and the fuse in the FCU, a 30A fuse or MCB has to blow. That exceeds the rating of your cable.

I would never do this as it's bad practice but the cable is protected (in this case).
The 3A fuse provides overload protection, this can either be at the origin of the circuit (consumer unit) or upstream (fused spur in this case) as long as you have adequate short circuit protection at the origin, for example your 32A MCB.
Would the installed 1.5mm cable be able to carry 13A depending on the installation methods used? Would the new addition meet the required maximum earth fault loop impedance values of a 13A BS1362 fuse? (3.2ohms 80% rule).
I think it would really need to as a minimum just in case the fuse ever blows and is replaced with a 13A fuse.

Think about a busbar chamber fused at 800A. Would every cable leaving the busbar have to be rated at 800A?[/b]

 

[Spark123]

Will a 1mm² CPC be adequate to provide fault current protection using the adiabatic equation? Using a 32A MCB the figures may come out OK but if the protective device is a rewirable fuse then the answer will almost certainly be a no.
Think about the bus bar and the same rule applies, the bus bar and cables need to be able to withstand the adiabatic equation taking into account the upstream protective device. The overcurrent protection may be provided downstream. Same rule for unfused spurs on a ring final circuit.