H
Does this look right?
- Thread starter ChiTao
- Start date
If it is not allowed, then surely there cannot be any explicit prohibition.
More importantly, how do you interpret 433.1.1 to two 2.5mm² conductors in parallel?
I'm temporarily away from my BYB again, so I can't quote chapter and verse, but the regs do address the question of conductors in parallel.
However, that relates to conductors of equal (or very nearly equal) length and therefore would only be applicable at the mid-point of a ring. As we know, the closer a load gets to one end of a ring, the greater becomes the difference between currents in the two legs, and as the load approaches the end of one leg, the current in the other leg approaches zero - i.e. it approaches the equivalent of a single 2.5mm² cable.
Kind Regards, John
Conductors in parallel must run together.
Yes, I seem to recall that. In any event, as I said, if the two conductors are different lengths (as in ring final, other than at the midpoint), then one of the conductors leg of the ring can carrying anything from ~0% to ~100% of the total load current, depending on the relative lengths [position of load(s), in the case of a ring]. 433.1.204 implicitly makes the assumption that, if the circuit is 'fully loaded' the positioning of loads will be such that no more than two-thirds of the current will flow down one arm (CCC=20A, OPD=30A) - but that obviously is just an 'assumption'.
Stillp's question suggests that he was hoping that the presence of two 2.5mm² legs of a ring would increase the effective CCC - but, as I've said, in the 'worst case' virtually all of the load current would flow through just one of the 2.5mm² legs.
Kind Regards, John
And, as JW2 recalls, be of pretty much the same length.
So a ring final is very much not a circuit with parallel conductors.
Well, in a literal sense it obviously is (a circuit with parallel conductors) - but it is not the scenario addressed in BS7671 of parallel conductors which are 'almost the same length & running together'.
Kind Regards, John
But it is considered to do so in BS7671 - as you went on to say, "433.1.204 implicitly makes the assumption that, if the circuit is 'fully loaded' the positioning of loads will be such that no more than two-thirds of the current will flow down one arm (CCC=20A, OPD=30A)". Looking at the topography of the three RFCs in my house, that seems a realistic assumption.
Do the electrons know that?
Yes, and that will very often be the case - but it's only an 'assumption' and it might only take one double socket very close to the CU to change things (if you believe that a double socket can supply >20A!). It's true of nearly all, but not quite all, of the ring finals in my house. The one exception which comes to mind is a (reasonably lengthy) ring final which has a double socket less than 1 metre from it's CU in my utility room.
Of course, in practice the majority of ring finals probably have Method C cable installation, hence a CCC of 27A, so could never be overloaded by a 26A load (alone), no matter where located on the ring. However, with a CCC of 20A, as permitted by the regulation, I could undoubtedly 'overload' that very short bit of cable with that one double socket if I really wanted to!
Kind Regards, John
No, but you can't use the rules for conductors in parallel for a ring circuit.Do the electrons know that?
Of course not, but the conductors are, electrically, in parallel, and the authors of BS7671 seem to have decided that it is acceptable to assume that the shorter conductor may be considered to carry not more than 75% of the total current.No, but you can't use the rules for conductors in parallel for a ring circuit.
But what is the probability of that happening, and for long enough to cause a problem?
J
jg321
All of my downstairs sockets are on one ring final, which is kitchen, lounge and front room (plus the odd one or two in the hallway). CU under stairs, with a short run of cable to all the kitchen sockets, then out to the lounge, front room, and back to the CU from the other side of the house - meaning all of my kitchen is one the shortest leg of the ring, by about 1/3rd.
Poorly designed? Yes, most certainly. In practice though, doesn't appear to cause any major problems.
I'll look to getting something more sensible in when the kitchen gets redone next year. Depends on the electrician of course but I'd be happy with a ring final for the kitchen (or maybe 4mm radial), and a 2.5mm (with appropriate OCPD) radial for the remaining sockets downstairs which typically have very little load.
Cheers,
Jon
Poorly designed? Yes, most certainly. In practice though, doesn't appear to cause any major problems.
I'll look to getting something more sensible in when the kitchen gets redone next year. Depends on the electrician of course but I'd be happy with a ring final for the kitchen (or maybe 4mm radial), and a 2.5mm (with appropriate OCPD) radial for the remaining sockets downstairs which typically have very little load.
Cheers,
Jon
They seem to have decided that such will usually be the case but, given that the regulation says nothing about the topology of a ring or the distribution of loads, they can't be sure that it will always be the case.
The strange thing is that they do 'say' that "the current in any part of the circuit should be unlikely to exceed for long periods the CCC of the cable", with some suggestions of some steps which can be taken to try to achieve that (which is all very reasonable) BUT, for some reason, they have chosen to 'say' that in ('Informative') Appendix 15, rather than in the regulation. They could (and some would say "should") have included that in the regulation.
Kind Regards, John
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