electrics

[dtroughton]

I just had another thought. If the spur was connected through a 13Amp fused connection unit then one could connect as many sockets to it as required as the cable would be protected. Is this allowed?

 

[breezer]

no because you would still be spuring from a spur

 

[paulfromswindon]

Tricky one that. I would have to slightly disagree with breezer on this one. Most sparks would agree this is an acceptable way around the problem in certain circumstances, because it prevents the overload. I have fitted fused connection units when customers have wired spurs from spurs and it is not practical to rewire them (ie the customer won't pay for it!), because it removes the danger of cable overload. Not good practice though.

That said, the big argument against it is that you are limited on that leg to 13Amps total load, and this is only any good if you know there will only be a small load. If you want two or more sockets, this is not really the answer. Just because you only want to plug a few light loads in now, things can change.

Spurs-from-spurs are wrong because the fusing should always be rated lower than the current rating of the cable, and anything more than one spur increases the risk of cable overload. By putting a fused connection unit, you segregate the segment from the ring, and fuse it safely. It is no longer in that instance, part of the ring circuit. A ring can feed (in certain circumstances) other things if segregated in this way.

In conclusion, I would still say don't do this, and would never recommend it , but if you really want to, ask yourself whether it's the best long-term solution. The best scenario is to do the job correctly - I know loads of sparks who would do it, but only for the reason I said at the start. It's a case of 'if you can't do it right, at least make sure it's safe'!

Paul

 

[harry_the_dog]

Going back to what paul said rearlier, if i had a spur off a spur and doubled up on the cable. ie 2 X 2.5 mm cable then i can still draw up to 48 amps safley from those two spurs (assuming the main fuse would let me). it is effectivley a 48 Amp cable carrying capacity yet is still backed by a 30A fuse. What I am saying is that this seems to be safe as far as overloading the cable is concerned. What do you think paul? I know it's not legal but theoretically it would be safe right?

 

[breezer]

I know it's not legal but theoretically it would be safe right?

in theory the bumble bee should not be able to fly but it does

 

[ban-all-sheds]

IMHO, a train of sockets on a single spur fed by an FCU is just as safe as a multi-gang trailing socket jobbie with a fused plug on the end, such as is sold in thousands of shops up and down the country. In both cases the fuse will protect the cable if someone puts too much load through the sockets.

You could not sensibly campaign against the sale of 6-socket extensions vs 1 socket ones, because everyone would point out, rightly, that both have a plug on the end with a 13A fuse, and therefore both are as safe as each other.

If you came into my office at home and saw a 4- or 6-way lead plugged into the ring main, happily feeding a bunch of low power devices like modems, hubs, laptop adapters, PC speakers etc, would you really suck air in through your teeth and say "oohh - that aint safe"?

Let's assume you wouldn't, and consider the same 4- or 6-gang item screwed to the wall to make it easier to use. It's the same.

Now consider a home-made item with 2 or 3 2-gang surface mount boxes fixed to a piece of wood. It's the same.

Now consider those boxes screwed to the wall. It's the same.

Now consider those boxes wired in with 2.5mm cable and an FCU rather than flex and a 13A plug. It's the same.

 

[breezer]

sorry to disagree no its not.

the "multi socket" has arrived because most houses only have one twin socket in one corner of a room. what goes in the corner? the tv, and now its the video, the satelite box, the music system, the surround sound sytem, the computer, the printer, the speakers the scanner etc etc

all these devices use little current.

however if you have a "hard wired socket" (especially in the kitchen) you will have the toaster, washing m/c , kettle etc etc these devices use a lot of current so that is why you can not sour off a spur, you don't see any one plugging in the washing m/c on a multi socket do you? (don't answer that, some one probably does)

My point is that with so many low consumption appliances sometimes you have no choice, but it still should not be done (but we all do)

 

[il78]

Have to agree with breezer...in that the multi-point socket used within the kitchen to supply washing machines etc would cause to fuse to blow often (under full load etc).....

But I also agree with ban-all-sheds saying this is safe....If it was over-loaded the fuse would blow.....no problems caused to the user.....

I've just moved into a property and 1x single socket in every room....and i've got loads of multi-points in use......until or if I get round to installing more sockets....

 

[harry_the_dog]

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.

 

[ban-all-sheds]

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.

 

[supersparks]

Agree with ban-all-sheds, you couldent draw more off the multiple socketed fused spur than you could if you only have one single socket, 13amps no matter how many you have it would be impossible to overload the cable

AR

 

[paulfromswindon]

Hi guys, not been around for a while, thought i'd catch up.

There is nothing wrong safety-wise with wiring a leg of spurs from an FCU, and as I have said sparkys do it sometime, but it's not good practice in my view because usage can change. Just cos you know there won't be much load now, what if, say you decide to plug a 3Kw convector heater into the circuit next winter. Why have part of your installation with a reduced capacity? If you are sure of the loading, and it will save you loads of work, then go for it. As a qualified sparky, my only point is that it is always preferable to avoid get-arounds.

The argument about doubling the cables up so that the radial spurs won't overload the cabling - well, sorry, but that's silly. Why bother? No installation is so difficult that the you cannot either cut into a ring or wire a circuit back to mains if you want to add multiple sockets.

In my opinion a radial wired from a ring (or on it's own) defeats the one reason why I think a ring circuit is always better. You have ringed earth wires, so that if an earth drops out somewhere in a socket, it doesn't mean all down-line sockets are now unearthed.

Have a nice day

 

[ban-all-sheds]

...it's not good practice in my view because usage can change. Just cos you know there won't be much load now, what if, say you decide to plug a 3Kw convector heater into the circuit next winter. Why have part of your installation with a reduced capacity? If you are sure of the loading, and it will save you loads of work, then go for it. As a qualified sparky, my only point is that it is always preferable to avoid get-arounds...

All true - but I think that the original post was about a spur into a loft, and I doubt if use would change much in the area of 3kW heaters

Very good point about the earth though.

.... No installation is so difficult that the you cannot either cut into a ring or wire a circuit back to mains if you want to add multiple sockets...

Ha - you should see my house, where I've got the same problem that someone else was posting about here - solid ground floor with all the sockets wired up as spurs from a ring on the first floor. Presumably that was compliant with the regs 50-odd years ago, but now I'm stuck with the legacy.

 

[harry_the_dog]

The argument about doubling the cables up so that the radial spurs won't overload the cabling - well, sorry, but that's silly. Why bother? No installation is so difficult that the you cannot either cut into a ring or wire a circuit back to mains if you want to add multiple sockets.

Paul I realise now that this is silly however i wired it up like this having been advised to do so buy a friend. Now the plasterer has been and plastered over my cable runs. It wouldn't bee too difficult to interupt the ring and make the spur part of it (as I should have done in the first place) however it would meen hacking into my nice new plaster to put the cable into the back the last socket on my spur and i don't really want to do this. So please just tell me this is safe! albeit silly.

 

[ban-all-sheds]

It's not just the end socket - you'd need to remove one of your 2.5mm cable runs from each socket on the spur and replace it with an uninterrupted length going from the last socket back to where you break into the ring.