Ring Finals Vs Radials

[ericmark]

433.2.1 is what stops a lot of configurations. It does allow 433.2.2 which gives us the 3 meters limit for an unfused spur.

I have argued many times about the idea of feeding a final ring with a larger cable to reduce volt drop or improve the loop impedance and I can't see any reason why when a kitchen in the opposite end of house to the consumer unit why a 16mm cable should not feed a JB which then feeds the final ring except it does not comply with regulations.

So where a cable goes through thermal insulation we should use a thicker cable at that point so the current capacity stays the same! I think the comment "Get Real" should come in here as we all know that is not going to happen.

In a large warehouse I found a 6mm ring feeding multi-drops in 2.5mm to each socket. The JB (Conduit box) was in the roof space as was the 6mm ring and only the 2.5 spur dropped down to each socket which kept the loop impedance within limits also volt drop. However since each drop was about 5 meters I would think to the book it did not comply.

I also looked at a very nice job of wiring a Cat 1 listed building in mineral insulated 1.5mm ring and the earth loop impedance with the outer being earth was within limits. However the volt drop was not. The question is does it really matter? It did what was required and all the cable was hidden behind mortar so it was hard to tell there was any wiring just seemed as if some one had fitted wireless sockets.

Some times I think there are just too many rules and not enough common sense? We are suppose to be tradesmen and although guide lines are nice we should be free to do what makes sense.

So lets consider a large site and a security guards shed with one light and a convector heater. Options:-
1) 16mm cable to keep volt drop to 3% limit.
2) 4mm cable and HF lighting unit which will still work at 200v and RCD protection so everything works OK.
Why should the site electrician not be permitted to use option 2)?

We go to college and learn all about Kirchhoff's law. What is the point if we are not allowed to use it?

 

[EFLImpudence]

I think you are allowed to use knowledge (common sense).

It's just that there are specific regulations for ring final circuits because of the undersized conductors.

If 6mm² is necessary for derating factors then, likely, a ring isn't going to be the best option.

 

[JohnW2]

I have argued many times about the idea of feeding a final ring with a larger cable to reduce volt drop or improve the loop impedance and I can't see any reason why when a kitchen in the opposite end of house to the consumer unit why a 16mm cable should not feed a JB which then feeds the final ring except it does not comply with regulations.

Do I take it that you're talking about a 'banjo'/'lollipop'design, with a single, say, 16mm² cable going to a JB, with a ring (wired in 2.5mm² or 4mm²) starting and finishing at that JB? Whether or not that is compliant is, as you know, open to debate. However, I can't see anything in 433.1.103 which stops you having the first and/or last legs of a 'regular' ring final wired in fatter cable than the rest of the ring, provided it has a 30A/32A MCB - which could achieve what you describe above.

Kind Regards, John.