Best way to add strip of 13A double sockets?!

[Alan Dempster]

Evenin' all,

Although a seasoned DIY sparkie, I would appreciate suggestions for adding additional SWSOs (Switched Double Socket Outlets) in this scenario. This particular install is to go behind a television, and power a chunk of AV gear:

|| cabling (buried in plaster) || _______ _________ _________ | | | | | |

--------- ------------ ------------- existing new double new double

Now the cabling runs from ceiling to floor in the lounge of my 'well old' property! The existing double socket is on a ring. I need to add two others.

IEE 16th Edition regs state that of course, you can only add one DSO (double socket outlet) as a spur. So do I:

1) Make a joint with choc-block inside the metal box of the exising DSO, then extend the ring to the new end socket, looping back through the middle one to the first

or

2) Spur off the first socket using 4.0mm cable, then spur off that (so spur cable is within spec for two double sockets)

or

3) Bugger the regs and simply add two spurs in daisy-chain format, like you're not supposed to do!

I call the installation 'strip sockets' because the faceplate of each double is literally within about 2mm of the next on the wall, allowing neat and tidy connection of up to six appliances (TV, DVD, Playstation, Terrestrial Digital (DVB) decoder, etc etc...

Of course, electrically I'd get away with 'spurring off the first spur' - but I want to keep as close to the regs as possible. Breaking the ring and installing 30 amp joint boxes is not an option here. Neither is running new cabling - cables are buried in lime-plaster than is as hard as diamond!

Incidentally, I have installed 'strip sockets' to great effect in my computer room, giving me a total of 12 sockets by way of six 13 amp doubles. Only on that occasion, I was able to pick up both sides of the ring under the floor, and connect one at either end, looping in and out of each new socket with the requisite 2.5mm sq. T&E.

I'd be eager and grateful to hear suggestions from all who have experience of doing this type of install in the past. I suppose I'm leaning towards option (1), with a suitably earthed metal back box.

Many thanks

Alan

[chris French]

In message , Alan Dempster writes

I'd be eager and grateful to hear suggestions from all who have experience

I did something similar in the 'office'.

I would suggest 3 - running them as spur with multiple outlets, with one small change to your plan. supply the spur via a FCU fused at 13 amps, this makes it reg compliant.

However, not sure i'd bother any more. I'd just have one or two double sockets and feed them into multiway distribution boards plugged into the sockets. Our TV/VCR/DVD/STB setup is fed like this. The distribution strip is fixed tio the back of the TV cabinet and it all is very neat.

[John Rumm]

Either that, or 1 from the OPs list but using crimps rather than choc blocks since these will fit in the back box much more easily.

[chris French]

Indeed.

One advantage of the fused spur route, is that if you use an switched FCU you can turn everything off in one go.

[The Natural Philosopher]

Best way is to extend one limb of the ring using crimps. Or a chick block INSIDE a socket, and run a full ring to them.

Thats what I always do.

[Mike Barnes]

Seconded. In the neighbourhood of our TV there are ten mains-operated boxes. There weren't always that many, of course, but they've arrived gradually over time. Think ahead to the time that you have ten things to plug in.

I've fixed distribution strips to the back of the stands with sticky pads (for flat surfaces) and nylon cable ties (for thin frames).

I've also made extensive use of IEC plugs - like those on the back of a PC - which save a lot of space. They're especially handy for appliances that have an IEC inlet, because you can substitute an IEC extension lead for the lead supplied with the appliance, avoiding the need to cut the existing lead. IEC distribution boards and extension cables are available from CPC, Maplin, etc.

[.]

In article , Alan Dempster writes

Why don't you just extend the ring to encompass the new sockets, not bother with spurs at all

[Dave Liquorice]

This would be my approach for that reason, Switched FCU positioned so as to be reachable... Of course some stuff you don't want to switch off so I think I'd end up with a the existing double socket feeding a trailing socket unit for the premenantly live stuff (if more than two) then a SW'd FCU to another double socket and trailing socket unit for the switched stuff.

This way you can easyly adjust the number of sockets on each side by getting the appropiate trailing extension. These trailing thingies normally have the abilty to be fixed to a surface so the whole thing can be very neat and not floating about.

[David M]

I'd definitely go with a couple of 6 way distribution boards, personally. I can't see any justification for going down the 3 double sockets root, both from an aesthetic and needless work point of view. Unless the OP has some very esoteric AV kit nothing is going to draw much power, and there is no need for individual sockets.

If this lot is visible, then 3 double sockets next to each other is going to look pretty poor. In which case if you use the dist. boards most of the wiring can be hidden.

If it's not visible then screw the dist. boards to the wall if you want.

cheers

David

[Andy Wade]

I might plump for:

4) Two separate unfused spurs in 2.5 mm^2 taken from the existing socket (with the cable for the furthest socket run through the middle mounting box, but not connected to anything therein).

Another possibility would be:

5) Instead of 3 doubles, use two 3-gang sockets (like MK K2737WHI) with the first on the ring (replacing the existing double) and the second as a spur. Since the 3-gang sockets incorporate a 13A fuse the spur will count as fused and no rules will be violated.

[Cuprager]

Dumb question time!

I know that you are not supposed to take more than one socket off a ring as a spur but have never figured out why! Can anyone enlighten me?

[John Rumm]

Firstly there is the issue of the cable. On a ring, each socket will be fed by two cables, either of which will carry the full load current of the socket. Hence if you spur from the ring with one similar cable there is little danger of causing damage to the cable with an overload. If however you add several sockets to that spur cable then you run the risk of exceeding its capacity. That is why the regs require the introduction of a fuse into the spur if you are going to do this.

The second issue is that of load distribution. Rings work well as a result of both diversity (i.e. the statistical likelihood that only a small proportion of the actual maximum load will ever be required at any one time), and the fact the loads will be spread around the ring. A ring with all the load concentrated at one end, could for example, overload the short cable run back to the CU since it would have a proportionately lower resistance than the longer run, and hence carry more than its "fair share" of the current. So again adding lots of sockets to a single spur (even if you wire it in heavier cable so as to not allow the spur to overload), you will still be concentrating a large load on to a single point on the ring.

Other issues: fault loop impedance; a ring socket will have two earth wires connecting to each socket, while a large spur will have ultimately half the total earth conductor size/area. Hence the resistance to earth from a spur socket is likely to be a bit higher than for a ring one. The longer you make it and the more sockets you add the worse this gets. Hence you potentially lower safety since the fault current that can flow to earth is reduced, and the time to blow/trip the protective fuse or breaker gets longer.

Maintenance: there is the danger than someone will mistake a socket on a multiple spur as a part of a ring (since it will have a cable in and out and hence look like a part of the ring). Adding to the possibility that later modifications will increase the risk of overload.

Increased testing time: testing a ring is relatively simple using a low ohms meter at various points with the conductors cross connected (and disconnected from the CU!). Each spur adds complexity to the process.

[Cuprager]

Thank you for that really great explanation! Now I know...

[Andy Wade]

For "spur" read "unfused spur." There's no limit if it's a fused spur of up to 13 A (subject of course to consideration of the load likely to be connected).

Unfused spurs are restricted to one single or double socket (or one item of fixed equipment) because:

a) the rating of 2.5 mm^2 cable is assumed to be 20 A, but the circuit's overcurrent protective device is 30 or 32 A, so does not provide overload protection to the spur. Spurs rely on the plug or fused connection unit fuse(s) to protect the cable. It's assumed that the load on a double socket is unlikely in practice to exceed 20 A [*], but with two singles there's much more risk of 26 amps worth of load being connected, which could overload the cable;

b) even if you used 4 mm^2 cable to overcome the first objection it's undesirable to connect a large load at one point on the ring (especially a point near to one end. The ring circuit works on the principle that the load is will be reasonably evenly distributed along its length. Ignoring that principle greatly increases the risk of cable overload occurring.

[*] Beware of the risk of violating this in the context of kitchen & laundry appliances.

[Alan Dempster]

Hey Andy, thanks for your input!

I like it - still not stricltly 'cosher' because there's two spurs effectively from the same original DSO, but at least the cable is protected to being used within its load limit.

Nice one!

Now to struggle with four lots of 2.5mm sq. conductors in the one socket. Deep joy (in a deep box!!!)

Cheers

Al.

[S Viemeister]

(snip)

Is it considered acceptable to have both a tumble dryer and a washing machine using the same double socket?

Sheila

[Stefek Zaba]

No need to worry in practice: a woshmosh runs at its peak of 2 or 2.5kW only while it's actually heating water, typically only for 15 mins or so at the start of the cycle; while your "normal" tumble dryer is likely also to have a 2.5kW heater, and will run at less than 100% on-all-the-time even when doing wet towels on the highest setting. The peak load will be 5kW, which near as damn it is 20A (5000/240 = 20.8A, divide my 5kW by 230 if you want to pretend UK mains is 230V to get

21.7A if you want to panic, but you won't convince me a 240V-design heater will pull its full kW rating running at that slightly lower voltage ;-). So, even when both appliances' heaters are on, there's only the 20A flowing through the double socket, and no decent-quality socket with incoming conductors sensibly tightened will run anything above slightly warm...

More, and authoritative, detail over at

[The Natural Philosopher]

Sure. Why not?

[Andy Wade]

It's 100% kosher. Nothing in the rules for spurs (Appendix 8 of the IEE On-Site Guide) forbids two separate spurs being connected to the ring at the same point.

There is an overriding consideration though, in the form of Reg.

433-02-04 of BS 7671 itself, which includes the words:

" [...] Such ring final circuits are deemed to meet the requirements of Regulation 433-02-01 if the current-carrying capacity (Iz) of the cable is not less than 20 A, and if, under_the_intended_conditions_of_use, the load current _in_any_part_of_the_ring_ is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable." [My emphasis, and note that Iz is the _as_installed_ cable rating.]

That should not be a problem with double-gang sockets of reputable origin.

[Andy Wade]

Not if you know that a sustained RMS current demand of over 20 A will occur; that's when not.