I need to supply four sockets and four fixed appliances in the same room from a ring circuit in the loft. I am planning to spur off the ring to the sockets from junction boxes and the fixed appliances will be supplied from fcus. To achieve a neat job I would like to mount the 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 box 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.

Thanks

chopsaw wrote in message ...
I need to supply four sockets and four fixed appliances in the same room
from a ring circuit in the loft. I am planning to spur off the ring to
the sockets from junction boxes and the fixed appliances will be
supplied from fcus. To achieve a neat job

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
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 box
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.


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


Copyright notice: the above information is solely for private users of
uk.d-i-y and may not be reproduced or abstracted in any form without
specific permission, for any purpose by any commercial or public
organization or their staff in connection with implementation,
approval, training, certification or otherwise in relation to the
Building Regulations Part P. Copyiing or adaptation will be considered
an infringement even if a general licence issued by the UK Copyright
Agency is held.

Thanks Jim for taking the time to answer my questions in so much detail.
Very grateful. It helps considerably.

CRAP SNIPPED
Thanks Jim for taking the time to answer my questions in so much detail.
Very grateful. It helps considerably

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

Dave

--
For what we are about to balls up may common sense prevent us doing it
again
in the future!!