Spacing of junction boxes and fcus on a ring circuit

[chopsaw]

Does it matter how closely junction boxes and fcus are spaced on a rin

circuit? I need to supply seven sockets and four individual appliance in one room from a ring. To make a neat job I propose to mount th junction boxes and fcus on a board about five feet long in the lof above the room and run the spurs from this board to the room socket and appliances. Would appreciate any comments.

Thank

-- chopsaw

[chopsaw]

I need to supply four sockets and four fixed appliances in the same roo

from a ring circuit in the loft. I am planning to spur off the ring t the sockets from junction boxes and the fixed appliances will b supplied from fcus. To achieve a neat job I would like to mount th junction boxes and fcus close together on a board about four feet long and two feet wide in the loft. The ring cable lengths between each bo and fcu would be about 1 foot to get them all on the one board.

I would appreciate any comments about whether this is good practice.

Thank

-- chopsaw

[Lobster]

Sounds like a hell of a lot of spurs and jct boxes. Any reason why you can't extend the existing ring to include all these sockets and fcus? You'd have two cable drops down to each socket, with no jct boxes in the loft other than maybe the first and last; each socket being linked continuously to the next.

David

[Owain]

The design of the ring assumes that the load will be reasonably evenly distributed. There is guidance that spurs should not exceed the number of sockets on the ring, and if you are adding 7 sockets as spurs that may take you over. Adding them all at one point may create an unacceptable point load. Depending on what the 'four individual appliances' are, your plan may be either a departure from best practice, or completely stupid. It does not sound a 'neat' job.

Why can't you either

(a) extend the ring properly, or (b) run a new radial circuit

Owain

[jim_in_sussex]

May I make a very general observation please? Your concern sems to be first with the aesthetics and secondly with whether it is electrically safe. Making things neat and tidy often means they are safer, but not always. Would suggest looking at the job from the other direction - decide what is electrically acceptable & then find a neat solution.

I would like to mount the

It may or may not be acceptable or advisable to run the spurs as you indicate & it may be worse or better than extending the ring. Consideration of what makes ring circuits 'tick' is needed. BTW in what follows for 'socket' read almost any connection into the ring.

First a ring circuit is a way of distributing circuit outlets over a large area of potential demand whilst keeping cable quantity & run requirements down. At its very core is the same statistical idea that insurers and bookies use: you can plug in a lot of appliances, but the chance that every untoward event will happen all at once is very small. With a ring the idea is that the chance of everything being plugged in and switched on at the same time is low and hence risk of a circuit overload is low. The assumption starts to break down when very large current drawers - heeaters, washing machines etc - are used. 'Diversity' is the technical term for the estimated load & you're supposed to make a calculation (see OSG), but in practice any attempt at precise calculation has to incorporate so many imponderables that you don;t get a clear cut answer.

Ideally a general purpose ring should only be used for light or medium power appliances. High power appliances should have their own circuits, which could be radial or dedicated rings. For instance in my kitchen there is a general purpose ring with a lot of socket outlets, plus a dedicated ring which serves just the washing machine, dishwasher and two other socket outlets - one is mainly used for a kettle. The fridg is on its own dedicated socket on a radial non-RCD circuit. Check are any of your new sockets for heavy prolonged power using devices?

A ring uses 2 supply cables in parallel, each run back to the consumer unit splits the current according to the resistance of each run (Ohms law with resistors in parallel) - ie mostly in the range 25% to75% depending on the exact position of the socket in the ring. A socket too near either end can cause an unwanted current imbalance & cause the current in that part of the ring to exceed the rating of the

2.5mm^2 cable (when reckoned at the minimum 20 amps).. Thus ideally outlets ought to be further than 25% of the total ring length away from the CU. Any outlet nearer the CU than that is really only suitable for low current devices.

Most standard circuits shown in the IEE On Site Guide (OSG) are limited in length by the maximum voltage drop permitted (4%). Voltage drop over a ring is generally more favourable than in a radial using the same cable, but if you make the ring longer the worst case voltage drop will rise. Assuming a spur uses the same cable as the ring (ie usually 2.5mm^2) it could be a toss up whether inserting it causes more voltage drop than extending the ring - you have to do the sums to be sure. A spur only carries the current to the socket, limiting the voltage drop along it. OTOH an extended ring will have to carry the full ring current to the socket and back. You need to look at your proposed arrangement - short spurs may be preferable to a greatly extended ring. To appreciate the relative advantages of each option, use a spreadsheet and Ohms law to make some simple estimates knowing the resistance of the cable per metre (see OSG) & trial currents, eg

5A,15A, 30A, for various ring lengths and spur positions (remember the ring forms a parallel circuit).

Although circuits lengths are most commonly limited by voltage drop, three other factors come into play - prospective short cicuit current which determines the MCB or fusing requirements; potential shock voltage at a live to earth fault point which should be under 50v; and disconnect time ie the trip or fusing time. Technically these should be checked if you make any circuit alterations, but if you are well within the standard circuit lengths in OSG & you have short spurs you're probably OK BUT YMMV.

Next all cable in a ring must be capable (after applying all derating factors) of carrying at least 20amps: (2.5mm^2 usually passes, but not always). So a spur to a 13A double socket must be able to carry at least 20A. At first sight this looks underated, a problem that is got round by declaring (somewhere in the regs) that double 13A sockets need only be rated at 13A, not 26A.

The ring should not form a figure of eight - ie from one CU end there should only be one path back to the other CU end.

There is an arbitrary limit in the regs for ring circuits that there should not be more spurs than socket outlets (doubles counting as 1) on the ring. Check by counting your sockets.

Finally the question of your close-together junction boxes. Check above points first. Unless it is close to the CU there's nothing against having junction boxes close together (nb there should only be one spur feed per box). OTOH if all your 4 new outlets are close to the CU & draw heavy current for long spells then there's a risk that you will overload the shorter part of the ring cable.

Then there's one or two practical implementation points you ought to watch. Junction boxes are often regarded unfavourably because they are (a) frequently hidden away and difficult to access - joins made in a socket outlet are far more accessible. (b) screws often work loose within days - why is unclear, but perhaps it is partly some form of elastic or creep reaction to compression from screwing down tight alternately vibration is sometimes blamed (c) poor construction of cheaper boxes - tightening a screw down can fracture the plastic or strip a thread (d) cable joints always run a risk of imperfect coupling. Some people advocate crmping joints instead of junction boxes (e) poor layout or carelessly separated wires can be a source of insulation resistance faults.

Due to the last if you do install as you originally suggested, then avoid cutting the existing ring if you can. Otherwise insert a single piece of cable in the ring and strip back the insulation over each wire to pass through each junction box terminal, then screw down each spur wire on top. [ie don't cut the inserted cable between the ends of the old ring]

HTH Above is only a quick run through -: please post again if you have any lingering doubts

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[chopsaw]

Thanks Jim for taking the time to answer my questions in so much detail.

Very grateful. It helps considerably.

[Dave Stanton]

Did you really have to repost all that, just to say thank you.

Dave