I have a 32" Philips LCD TV which I am planning on fixing to the wall
in my lounge. The cabling will be routed in the wall using some
conduit. The nearest power socket is about 2m away from where the TV
will be placed.
One problem I have is with the power cable which has a figure-8
(IEC-320 C7) connector. To be able to route the cable I need to have a
5m length and these seem to be hard to come by. A couple of local
electrical shops have sugested using a standard extension cable to
extend the existing cable which would work but I am not sure how safe
this is from a electrical point of view.
Perhaps the easiest way would be to use an in line IEC plug and socket at
the joint between the existing cable and the extension. Maplin sell them.
Alternately, solder the joint and sleeve, then use heat shrink sleeving on
the outside of the whole.
*Despite the cost of living, have you noticed how it remains so popular?
Dave Plowman email@example.com London SW
I had thought about that. In that room I currently have a double socket
which allready has another double socket taken off it as a spur. I
would need all four sockets for the various AV kit that is going to be
connected up so can I take another spur from the main socket or take a
spur from the spur? I am not an expert in electrics and am quite happy
to hand this over to someone who is.
No, you can't take multiple spurs from one point (it would potentially
unbalance the ring) or from an existing spur (it would potentially overload
the spur cable AND unbalance the ring). However, you can take multiple spurs
from a fused spur. This would require a 13A fused connection unit to be
introduced that can have multiple sockets come off.
+----+ +--+ +------+ +--+
| |----------| |--------| |------| |
+----+ +--+ +------+ +--+
^ ^ ^ ^
existing FCU existing spurred new
double socket double socket socket
The FCU can be inserted adjacent to either the ring socket or the spur
socket, provided cable of at least 2.5mm is used. The new socket can be
connected at any point after the FCU and doesn't have to come off the
existing spurred socket if it is more convenient to come off the FCU or off
a junction box somewhere between the two. Multiple additional sockets may be
I'm not doubting what you say, but can you explain how it could
unbalance the ring more without an FCU than with one? In both cases the
current is drawn round both "halves" of the ring to the point where the
spur splits off ...
Is it also OK to *replace* an existing single socket on the ring with an
FCU, and then run multiple sockets from the load side of the FCU?
Other than the limit of the 13A fuse, is there a sensible or permitted
maximum to the number of spurred sockets or the total cable length from
The FCU limits the point load to 13A. A greater point load than this, nearer
one end of the ring than the other can cause excessive current to flow
through the "nearest" cable to the consumer unit causing predicatable
overload on that cable. When determining whether a ring circuit is suitable
for an application, the likelihood of one side of the ring being more loaded
than the other must be considered. Taking multiple spurs from one point
would indicate that this might be a prime candidate for such lopsided
If you're looking to do a long run of cable, calculations to ensure voltage
drop and earth loop impedence may be indicated. However, due to the 13A
protection, the allowed lengths will be actually quite high, as the current
assumed is lower than for the final circuit as a whole. Such long spurs are
rarely encountered in practice.
I knew that the position of the spur around the ring would have an
effect, that's why I put the "halves" in quotes in my post. But I
thought that on a 32A ring, 13A could still represent a considerable
fraction of the available current (except in the unlikey event that the
spur is "opposite" the consumer unit on the ring). Perhaps the meaning
of "balanced" in this case is a bit looser than I took it to be?
Does 30' sound reasonable?
If so it will be easier for me to replace an unused floor level socket
with an FCU, take a cable up into the attic via an airing cupboard,
through joists across a room and back down through another cupboard in a
study for some sockets on the opposite side of my study.
If not I've got chiselling and replastering to do and might as well
extend the ring anyway.
Perhaps balance isn't really the right word at all. The issue at stake
is the need to ensure that your design is not likely to result in any
part of the cable being overloaded for long periods. Attention was
drawn to this by a change made to BS 7671 in Amendment no. 1 (Feb 2002).
What it actually says is now:
433-02-04 For a ring final circuit protected by a 30 A or 32 A
protective device complying with BS 88, BS 1361, BS 3036, BS EN 60898,
BS EN 60947-2 or BS EN 61009-1 (RCBO) and supplying accessories to BS
1363 and wired with copper conductors, the minimum cross-sectional area
of both phase and neutral conductors is 2.5 mm2 except for two-core
mineral insulated cables to BS 6207 for which the minimum is 1.5 mm2.
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.
/Reason: to require that the current carrying capacity of ring circuit
cables be not less than 20 A and to require that the load current in any
part of ring final circuits is unlikely to exceed Iz for long periods,
see Regulation 433-01-01./
The key bit being in the last three lines, starting "... and if, under
the intended conditions of use, ..." Bear in mind that Iz is the cable
rating *as installed*. Thus this is more of an issue if the cable
passes through thermally insulated structures or if grouping factors
apply than if 'clipped direct' conditions apply throughout.
For a fused spur - yes, that'll be no problem at all.
An airing cupboard is not necessarily the best place to run a cable -
high ambient temperature, possibility of being smothered, etc. - but for
2.5 mm^2 cable in a 13A spur you've got a lot of derating in hand, so
it's probably OK - just pick the coolest corner to run the cable.
[*] Full amendment available at
This was sugested to me in another newsgroup. I went to my local Maplin
store the other day as they do the same type of connector. Problem was
the connector had a rectanguler opening for the flex and the only flex
they had was round and was quite a bit thicker. Do CPC do a more
appropriate flatter cable?
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