twin and earth tails

[mickles]

Hi everyone, i recently had a look at a first fix which has been abandoned and now needs completing. this is a shop premises, with the meter located in basement. shop premises are ground floor and there is a firstfloor flat (no work to do in there). The shop consumer unit is connected to the meter by approx 10 metre length of probably 16mm twin and earth. there is also a 16mm earthing conductor.
looking through the regs, i am under the impresion that tails need to be 25mm. from a loading point of view, 16mm would be ok
interestingly the upstairs flat is also connected to the meter via twin and earth, and i have seen this done in a few flats i have had cause to visit for faults etc.....is it legal? and if so, under what authority/ regulation

 

[HandySparks]

CPDs in a typical CU protect circuit conductors that are rated much lower than the tails.
If they can protect the much lower rated circuit conductors, then they will have no problem protecting the higher rated tails.

Sure, they will protect the tails from the overload of one individual final circuit.

You appear to be under the impression that the majority of installations have demands in excess of their supplies.
If that were the case, the DNO's would be inundated with calls to replace their blown fuses.

Not at all. Only that, if in exceptional conditions, the final circuits are loaded to the point that the supplier's fuse blows, it's the suppliers fuse that's providing the ultimate overload protection to the tails, not the final circuit OCPDs.

Unless you know something that I don't, the DNO's are not inundated with calls to replace their fuses.
In fact such calls are quite a rarity.

I'm not saying that it's a regular occurrence.

If you want to add up all the ratings of the CPDs in a CU, and take that as the demand, knock yourself out.

I'm not saying that at all. However, if the sum of the ratings of the CPDs is too great, then you can't rely on them to protect the tails; and you're left with the suppliers fuse to do the job.