harry_the_dog said:
There is a difference ban-all-sheds because as you said a 6-socket extension has 1 plug on the end so all 6 can only draw 13 amps. If my spur has 6 sockets i could draw 13 amps from each one! Obviously the main fuse would blow first but the point I am making is that by doubling up on the cable i am safe because the capacity of the cable is then greater than the main 30amp fuse in the consumer unit.
No - you can't draw 13A from each socket as I'm talking about a multi-socket spur
fed by an FCU - and the fuse in that would blow at just the same load point as the fuse in the plug of a 6-socket extension lead. I could wire up a spur with 2.5mm cable, from an FCU with a 13A fuse, and put 80 single gang sockets on it to power 80 mobile phone chargers,
and it would be perfectly safe. I could plug 80 3-bar electric fires in, and it would still be perfectly safe, because as soon as I turned the second one on the fuse would pop.
I would agree that you probably
shouldn't have a spur with 80 sockets on it though
I don't think the regs actually
forbid having multiple sockets on a fused spur, do they? If they do, then I'd love to see someone present a cogent argument proving that it isn't safe to have 6 sockets protected by a 13A fuse when those sockets are fed from an FCU on the ring but it is safe to have 6 sockets protected by a 13A fuse when those sockets are fed by plugging into a socket on the ring.
I'd also like to know, if it is forbidden, why this sort of arrangement:
Fig 6.2 An arrangement for main and final circuits in a large installation
is OK, because this is nothing more, or less, than a cascaded heirachy of spurs off spurs off spurs (or radials off radials off radials, if you prefer that word). Or quite possibly a mix of
rings and radials off radials off radials. Let's not forget that everybody's house supply starts off as a 100A fused spur, and my how we then spur off that. Think about the meter tails - that is a spur. Think about the bus-bars in the CU. That is a spur. Every light circuit is a spur off a spur.
Electrically, there is nothing wrong with multiple spurs off spurs, or multiple devices on a single spur.
The points that breezer makes about why multi-way extension leads were introduced, and about kitchens, are quite correct, but, firstly the original question was about supplying a loft, and I doubt if there would be many kettles and washing machines there, and secondly I'm not suggesting that multi-drop spurs are universally applicable, any more than using a 6-way extension to supply kitchen appliances would be applicable.
A spur is just a particular type of circuit, and like any other circuit it needs to have the following characteristics:
1) A device to protect the cable from being overloaded.
2) Enough capacity to serve the loads that are likely to be plugged in simultaneously - the old diversity question.
Nobody gets upset about radial circuits - they are a length of cable with a fuse/MCB at one end, and several sockets along their length. As long as the cable is fat enough to carry the likely current, and the fuse/MCB has a lower rating than the safe capacity of the cable, and discriminates by that rating being lower than the next upstream one, i.e. the main breaker in the CU, everyone is happy. And what's the difference between a radial and a spur?
No matter how many sockets there are on a fused spur, you can't overload the spur cable, and you can't compromise the safety of the ring because that has its 32A fuse/MCB upstream of the 13A fuse in the spur.