Ring final with 4mm legs

[kingeri]

So I have been working on a four storey house, in which the wiring is pretty sound, the sparks who did the last rewire (approx 2004) seems to have loved 4mm T+E, all the sockets are on 4mm radials, with either 16, 20 or 32 amp breakers. The exception is the very top floor sockets, which he did as a ring final, with 4mm 'legs' and 2.5mm looped between sockets. I can only think he did this to combat voltage drop, as as far as I can see installation methods would have allowed 2.5 to be used throughout the circuit. Anyway, I know this is fine, just wondered if its a method anybody else has used over the years?

 

[ruston]

Yes , probably his calculations for the fourth floor have indicated that he needs a larger CSA for that floor.

 

[Octopus]

Or simple lack of commutation between the people pulling the cables!

 

[telectrix]

as here:

https://www.youtube.com/watch?v=yBBWUZfgRiw

 

[7029 dave]

Trying to use up his knock off 4mm t&e.

 

[Marvo]

Yeah, probably due to the cable he had on had rather than by conscious design. A ring final circuit will run with a far higher load on one leg if it's made up of different CSA cables. Also a bit dodgy for someone upping the MCB in future if it's not clearly labeled in the CU. Not something I'd personally install.

 

[kingeri]

Yeah, probably due to the cable he had on had rather than by conscious design. A ring final circuit will run with a far higher load on one leg if it's made up of different CSA cables. Also a bit dodgy for someone upping the MCB in future if it's not clearly labeled in the CU. Not something I'd personally install.

Both 4mm legs are the same length and follow the same route, they are approx 40m (by calculation). The loop between them in 2.5mm is around 15m, serving two bedrooms and ten double sockets. I can't see there being a problem with the ring being unbalanced here, as such. I wouldn't install it either (why not just do it all in 4mm?) though!

 

[Leesparkykent]

Assuming a load of 20A at the furthest point and the balance to the rating of the protective device evenly distributed so for a 32A protective device this equates to a load of 26A at the furthest point. mV/A/m x ib x L /1000 /4 so in your case 18x26x95/1000/4=11.115V. Would of been close to the recommended maximum value of 5% (11.5V) using 2.5mm. But TBH I could never see a load of 26A being used on the top floor of a house unless they have a cannabis factory up there :goofy:. I would of probably done it on a 2.5 RFC as would of complied for volt drop and its what I would of had on the van....Im not the biggest fan of 32A 4mm radials but accept there are times its warranted to use such a circuit. For one the maximum length would be around 32-33 meters if you took the design current to be 32A.

 

[R G]

The use of 4mm2 T&E along with 2,5mm2 T&E is nothing new , it was commonly used for spurs from the ring circuit , probably someone using the old ideas here..

 

[EastAnglian]

In new designs (i.e. new builds & rewires) I will usually run radials rather than RFCs. 4mm cables for 32A circuits and 2.5mm cables for 16/20A circuits. I believe that a larger number of short radial circuits is a better design than a smaller number of RFCs - partly because of the improved load distribution between circuits but mainly because it avoids the hidden danger of a ring with a single discontinuity in a conductor. I have discovered sufficient discontinuous rings on 32A & 30A MCBs/fuses in my time to regard these as unsafe practice.

 

[Leesparkykent]

In a new build I struggle to see how you would get 4mm T&E to comply on a 32A circuit breaker unless its supplying a fixed load that pulls no where near 32A.

 

[EastAnglian]

In a new build I struggle to see how you would get 4mm T&E to comply on a 32A circuit breaker unless its supplying a fixed load that pulls no where near 32A.

Comply with which parameter specifically?

 

[Leesparkykent]

Comply with which parameter specifically?

current carrying capacity.

 

[EastAnglian]

Yes - fair point in a new build other than 'clipped direct' reference method ...... I must learn not to generalise when posting

 

[kingeri]

How about an unfused spur wired in 4mm but serving 2 double sockets, off a ring final wired in 4mm? Found this today. I don't think its actually dangerous, but does it comply? Ring final is about 20m total length and spur about 3m.

 

[EastAnglian]

It certainly doesn't comply with Figure 15B of the Informative Annex 15.

 

[kingeri]

True enough but could it be regarded as a 4mm radial, after all you could remove one leg from the DB and that's what it would be.

 

[HandySparks]

It certainly doesn't comply with Figure 15B of the Informative Annex 15.

Maybe not, but it's as safe as a radial wired in 4mm² in the same location.

 

[kingeri]

We must have been thinking along the same lines at the same time there!

 

[EastAnglian]

Ah yes, I just re-read the original and see its a 4mm RFC - doh! In which case I would agree with the above!