Large fixed loads are best served by a dedicated radial circuit, e.g an immersion heater.

I do not come across many domestic ring final circuits where the M.C.B. trips off due to overload.

sure.

this is drifting off the scenario and topic I feel

regards

A 2.5mm2 spur using T&E cable, where the cable is rated at between 13.5 and 27 Amps as per Table 4D5, is affected primarily by the installation method. If two double 13 Amp sockets are connected onto a single unfused spur then at best two 13 Amp loads will cause no problems to the supply cable. (Ref. method C). But ref. method 103# would not be a suitable means of installation and may cause cable overheating. Of course these days many appliances are rated at 2kW rather than 3kW.

2.5mm2 copper wire if used as a fuse link, is rated at about 80 Amps. But obviously the P.V.C. insulation is the weak link here. So after a length of time in overload conditions the P.V.C. may soften and cause problems.

If the two double sockets are only ever to be used for small appliances like a 100 Watt aquarium heater and a couple of air pumps then all is well. But that can not be guaranteed to be the case in a home where just anything could be plugged in. And, what about future use if the house is sold?

Z.

I have to agree with Lyle, and this is simply because experience should be the teacher, I have no particular problem in an existing installation with two double sockets in reasonable proximity on one spur. Two spurs from one point is certainly not a problem although some still code it C2, and the only reason for this is inadequate knowledge. There is STILL the belief that a 2.5 mm2 cable will melt and catch fire at 28A, and of course this is nonsense, and to cause a dangerous fire starting temperature needs about 75A. Anyone who doesn't believe me should actually try an experiment, as I say experience is the best teacher. If the cables are buried in thermal insulation which is not inflammable (Glass, rockwool etc) then operation at 50A is very unlikely to be dangerous, but foamed polyurethane is much more dubious as proved at Grenfell.

I will ask the question again, because I have never got any yes answers, has anyone ever seen a melted cable on any domestic installation, or even anywhere else, unless an external heat source was involved. I have seen one where a cable was tied to a superheated steam pipe, but even that didn't cause more than a nasty burning smell! I think the warm cable scenario has been grossly over-taught, and this is why it seems to be an issue.

The change from 2 accessories per spur to one in BS7671 was probably not necessary, and cannot have been in any way evidence based, except the usual "what if" argument which I hate. That kind of thing is not engineering, which is risk based in all cases, it is bureaucracy at its worst.

I claim the prize. I can not remember ever seeing any P.V.C. insulated cable heat damaged along its length by overloading. I have seen P.V.C. heat damaged at badly terminated cable ends at accessories though, mainly due to loose termination screws. But even then there was no fire. Most cables are well oversized for their intended use. Even an 8.5kW electric shower run in mini-trunking on a 2.5 T&E cable did not damage the cable.

P.S. An old thread on this forum mentioned melted lighting cables due to diverted neutral currents some time back.

Z.

I think we need to be pragmatic.

In reality overloads on any half decent designed RFC are scarcely a problem - but they can be.

In reality overloading of cables are scarcely a problem with spur on spur from a ring - but they can be.

In reality we do not expect someone to use 4 or more 3KW fan heaters on one RFC - but they can do.

When designing systems we are risk averse.

When inspecting an installation we a risk averse.

So we must code something that is poor practice and could, under some conditions, present a danger. It may last for a thousand years and never actually cause a real world problem. But if it could foreseeably do so we must design/code against it.

You could buy a car that is considered a "death trap", you could use it for 10 years without problems but would you risk it?

PS - on a ring I usually double all conductors at terminations to increase the point of contact in the terminal, Taking only tunnel terminals as an example, in theory the nearer we are to achieving "filling the hole" then more contact. In practice my favoured sockets took 3 doubled conductors just nicely, they got redesigned meaning that whilst still possible it was often more straightforward to leave one conductor not doubled - I`d make that the spur. Not all manufacturers terminals suit this method though. I hate the flat plate jobs that are now popular with our terminals. Yuk!

There is no prize Z and commenting on loose connections is cheating!

Your reply Ebee is interesting, because you are aware that there is not a problem with cables, running at a little more current than the BBB says, but you think that coding based on that knowledge "is a risk to you?". I do understand why you might say that, and you are afraid that "some bright spark (or perhaps not)" might be prepared to quote BS7671 at you, perhaps in court, without any real reason behind his expert(?) view. In reality that is a very tiny risk, and even if it did happen, your expert (there are a couple here) might well be prepared to point out the realities to the court. It is very important to realise that almost all works carried out by the entire construction industry can be found "non-compliant" in some way, many of these non-compliances being actually dangerous, and a number of them criminal. The wording in BS7671 about "the inspector" or "duty holder" is that he gives an opinion, strangely not that he is an upholder of absolute regulatory compliance. The latter would of course be impossible because we have such an intertwined series of standards, statutory instruments and advice that almost no one actually could achieve the above in any reasonable timescale and without a complete set of all the installation details and "as built" drawings. The inspector must form a reasonable view of safety, and not invent his own rules to enforce his feelings of risk to him. This is why I think that a much higher degree of professional standards is required by inspectors, which would bring with it increased trust, which at the moment is pretty low.

The nice little books of codes for alleged defects provided by the schemes are a way to cover up the inadequacies of inspectors used by their members. They are in no way absolute, and the only reason to offer them is because the inspector cannot decide for themselves! Whether these same inspectors could calculate the admissible Zs of a circuit, or any other design parameter is very open to question. But they can decide that a cable size is inadequate, or a risk or whatever. There are quite a lot of 9.5 kW showers on 4mm cables installed in the country, yet these cables stubbornly refuse to fail or give any trouble at all. I know exactly why, many may know why but would happily give this a C1. I am happy to put cooking appliances on circuits with the CPD at a fraction of the worst case loading, yet they never trip. Again I know exactly why, but we often get questions implying that this is wrong. I am still surprised by the number of "electricians" who think that household supplies can supply the DNO fuse rating, and worry that this is insufficient for their supply of breakers installed. Again I have seen this coded as a C1. Crazy....

Wot, no prize? Yah shucks! A recent visit by me to a holiday chalet owner concerned a new cooker. The cooker circuit is supplied by a C20 M.C.B. The chalet owner was concerned that the cooker should be run through a 30/32 Amp M.C.B. The Crabtree consumer unit is as far as I know is obsolete. I do not stock parts for this model. I wired up the cooker and we tested it. It is a standard 4 hobs, grill and oven type of cooker. With everything turned on full the cooker worked for about 30 seconds then tripped the M.C.B. But we decided that the cooker would not be used in that way under normal circumstances, but if it caused trouble in the future I would try to find and fit a 30 Amp M.C.B.

so, you pays yer money and takes yer pick.

                                                                                                                                                            Not even a small prize?

Z.