Allow me to summarise: Copper is expensive.
Radial to two rings
- Thread starter Dazzevans
- Start date
Allow me to summarise: Copper is expensive.
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Allow me to summarise:
When put in context of the total cost of employing an electrician to replace a CU and rewire some circuits the difference between the cable costs for a few 2.5mm² rings and 4mm² radials is not significant enough for it to be a design criterion.
When put in context of the total cost of employing an electrician to replace a CU and rewire some circuits the difference between the cable costs for a few 2.5mm² rings and 4mm² radials is not significant enough for it to be a design criterion.
Answer - price of copper
£6,146.83 per tonne yesterday.
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The thing I don't like about 4mm 32A radials is IIRC they are pretty marginal on the cable rating. They are just about in spec if clipped direct but any insulation, grouping factors or similar will push it out of spec.
You could go 6mm but that has issues of it's own (notably that even MK sockets are only speced for two cables of that size which is going to make it rather awkward to add any branches later).
You could go 6mm but that has issues of it's own (notably that even MK sockets are only speced for two cables of that size which is going to make it rather awkward to add any branches later).
I'm sure that's right. However, you might have added that the implied argument being put forward by mbga9pgf (and I've often seen others say the same) actually appears to be back-to-front.....
...the amount of copper in a 2.5/1.5mm² cable (total 6.5mm²) is considerably more than half that of 4.0/1.5 mm² cable (total 9.5mm²). It therefore follows that if one assumes that a ring final will involve approximately double the cable run of a corresponding radial circuit, then the amount of copper will be appreciably greater for the ring. To take the simple example of a 'straight' circuit (all sockets in a straight line) with the furthest socket at 20 metres, a 2.5mm² ring circuit would involve about 2.3 kg of copper, whereas a 4.0mm² radial circuit would involve only about 1.7 kg.
In terms of the amount/cost of copper, radial circuits therefore will usually win over ring ones - whereas many people seem to try to use the cost of copper to support the use of ring circuits. However, although it may not be such an issue for professionals, I would counter that to some extent with the hassle factor of having to make many 4.0mm² terminations in standard fittings/boxes (which, for me, is serious hassle! - hence perhaps a decisive factor) - particularly if one needs to terminate three 4mm² cables (i.e. for branch/spur).
Another factor to be considered, of course [basically due to the fact that (.5mm² x 2)> 4.0mm²] is that, although compliant, a fully-loaded 4.0 mm² radial circuit is (at least if the load is close to the 'furthest socket')much closer to cable Iz than is the equivalent fully-loaded 2.5 mm² ring circuit - so some might take the added 'comfort factor' of 2x2.5mm² into consideration.
Kind Regards, John
This assumes you loop back to the CU in one straight line, with no sockets on half the cable, which in my case, is not necessarily true. Surely, it depends on the layout.
Simply put, imho, there is no "right" or "wrong" way, it depends on the circumstances. There are arguements for and against both designs.
In my personal setup, it makes sense to have a ring final rather than a radial, but thats down to there being sockets all the way round the loop and not just on the first half.
My argument has no element of relative copper usage - in your case you could still have a 4mm² radial following the same route with just the last leg back to the CU missing, therefore using a lot more copper than a 2.5mm² radial.
It might still be a better design - it's the ring vs radial decision which is important. My view is that even if you only have 1m less with a 4mm² radial than a 2.5mm² ring the cost of the cable is not significant enough for it to be a factor in what is an engineering decision.
It might still be a better design - it's the ring vs radial decision which is important. My view is that even if you only have 1m less with a 4mm² radial than a 2.5mm² ring the cost of the cable is not significant enough for it to be a factor in what is an engineering decision.
Would you agree though there is nothing fundamentally wrong with a well installed and tested ring final cct? if theres no break in the ring during install, no continuity issues, the wires are correctly terminated at the sockets, I really cant see a problem with them. If anything, its an intelligent design solution.
Indeed it does. In fact, if a radial circuit is arranged physically in a ring layout, with the final socket close to the CU, then converting that wiring routing to a ring would involve virtually no extra cable length - in which case a 2.5mm ² ring would obvioulsy win over a 4mm² radial circuit in terms of copper weight/price. However, the main reason that's true is that to arrange a radial circuit in a physical ring is usually a bit daft, for the very reason that it uses virtually as much cable length as does a ring. If one were starting from scratch, it would be often/usually (but not always, and depending upoin access issues) be possible to design a radial cable layout (with branches if necessary) which would involve much less cable than if the radial circuit were arranged more-or-less in a physical ring - and that's when the radial circuit will usually win (in terms of copper weight/cost) over a ring. What you can't get away from, if you have either a true ring or a 'radial circuit in a physical ring', is the fact that the total length of cable has got to be at least twice (usually appreciably more than than) the distance from the CU to the physically furthest socket.
I agree totally. I was merely trying to illustrate that the frequently heard argument that rings result in less copper usage is not generally/necessarily true, provided that the circuit layout (radial or ring) is designed to be cable-efficient.
As you have said, in terms of cable usage, which method is the more efficient will depoend upon the circumstances. The other considerations in the ring/radial debate are, of course, nothing to do with cable, per se, and I'm sure will continue to be vigorously discussed and debated 'for ever' - at least, so long as both types of circuit continue to be allowed by regulations!
Kind Regards, John.
I agree totally that it should be an engineering decision - although it's obviously an issue about which you would be hard pressed to get a consensus amongst engineers as to which type of circuit should be used!
I'm not sure that I understand your "even if you only have 1m less...". It would surely be a situation in which cable cost was very different with the two designs that people might be tempted to bring non-engineering considerations into the equation?
Kind Regards, John
They might, but they shouldn't.
As a percentage of the total bill, 20m of 4mm² vs 21m if 2.5mm² (for example) is insignificant.
As a percentage of the total bill, 20m of 4mm² vs 21m if 2.5mm² (for example) is insignificant.
Again, I agree totally. It certainly isn't me that brings up 'the price of copper' whenever a radial vs.ring discussion/debate arises! As you say, I think it's just a red herring. There are definitely some engineering issues to debate in relation to the radial vs. ring question, but price of copper (or, indeed, price of anything) is not one of them!
The problem/hassle of terminating 4mm² cable is certainly a very significant issue for me. Although primarily a non-engineering consideration, it does become an engineering/safety issue if 'the problem' results in an increased risk of unsatisfactory/ unsafe terminations, which I imagine it must.
Cheers, John.
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