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.
If this is going to be a seriously fixed installation, I would consider running 2.5mm T&E cable to a 13A socket behind the television as a fused or unfused spur.
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.
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?
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.
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 added.
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?
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 loading.
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
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