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6A lighting cct to supply a 13A socket

Farmboy
So, we all know about feeding lights from a socket cct, but would you do it the other way around, running on to a single 13A socket from a 6A lighting cct (which is wired in 1.5mm2, to run a few LED's) to supply a lawn mower and similar items? Somebody was given a quote to install the socket.


F
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    I find it most interesting that many seem to find the ability to "gold plate" simple regulations. There is nothing to say that any circuit CPD may not be less than the possible load! If the cable is of a suitable size for the CPD then everything is perfectly safe. If someone later comes to change the CPD for a larger one then that is his problem. There is nothing that I can think of that says that lighting and power circuits are in any way different, it is just conventional to separate them, and to use accessories such as light switches with a matching current rating. There is nothing to stop you fitting a light bulb to a 100A circuit as long as the cable can withstand the possible short circuit condition. This may be achieved by a further fuse or simply wiring in rather large cable, and using a 100A switch (to withstand the short circuit condition). Where accessories have reduced fault ratings of course you must take action to prevent danger, usually bu fusing down to suit. Just think about it, does a radial circuit with 20 off 13A sockets need a CPD rating of 260A, of course not?
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    A few subtleties to consider...

    There is nothing to stop you fitting a light bulb to a 100A circuit as long as the cable can withstand the possible short circuit condition.



    If by a "light bulb" you mean something that fits into a the the usual sizes of BC or ES lampholders, then BS 7671 stipulates a maximum of 16A (559.5.1.204).

     

    Could you have a 20A/1.5sq.mm. (method C)  lawn mower circuit with some lights on it?



    You might also have to consider the (probably 6A rated) lightswitches on the circuit as well as the cables - even though they're not in the path to the lawnmower (or whatever) they and so overload isn't a worry, the still need to be protected from faults, using a higher rated OPD could undermine that. The rewritten section 536 in the 18th makes that requirement more explicit that it used to be.


      - Andy.
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    There is nothing in the regs to stop a 13A socket being fed from a 'lighting' circuit fused at 6A. Indeed, as others have said, in the loft for a TV amp, or similar dedicated small load, like a fan or a window winder it is a quite sensible solution.

    Is it a good idea for a socket for general use in the public area of a block of flats ? , No, not really, the chances of it being tripped by carelessness are too high. Even for the intended use, lawnmower, it seems a bit light - a large flymo will blow a 5A fuse if it stalls on long grass or similar. I'd be looking for a radial or ring to spur from.
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    AJJewsbury:

    A few subtleties to consider...




    There is nothing to stop you fitting a light bulb to a 100A circuit as long as the cable can withstand the possible short circuit condition.



    If by a "light bulb" you mean something that fits into a the the usual sizes of BC or ES lampholders, then BS 7671 stipulates a maximum of 16A (559.5.1.204).

     

    Could you have a 20A/1.5sq.mm. (method C)  lawn mower circuit with some lights on it?



    You might also have to consider the (probably 6A rated) lightswitches on the circuit as well as the cables - even though they're not in the path to the lawnmower (or whatever) they and so overload isn't a worry, the still need to be protected from faults, using a higher rated OPD could undermine that. The rewritten section 536 in the 18th makes that requirement more explicit that it used to be.


      - Andy.

     




    Now now Andy, did I suggest any of those things?

    The basis of the question is about circuit ratings, perhaps you need to study the bigger picture.

    This is not about anything but correct use of the regulations, and these are not there to make any safe circuit design unusable. Whatever load is applied in the OP example is quite safe, it may just trip the CPD, which is the only point in question. I have no idea where lawnmowers and stair wells came from, and as for electric vehicle charging? Strangely no mention of RCDs....or even TT..........

    Regards

    David

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    Except in this case the original poster has given us specific context/

    Connecting a load greater than anticipated could well be expected - it's a socket to be installed in a bin shed for a block of flats - and the sparky who gave the quote to install it is going to take the supply from the lighting cct supplying the lights in the communal stairway ?.




     




    would you do it the other way around, running on to a single 13A socket from a 6A lighting cct (which is wired in 1.5mm2, to run a few LED's) to supply a lawn mower and similar items?




    To fit an identical socket in the roof void of the same block of flats for a TV amplifier or something would be a very different matter. It's not really a question of the wiring regs , but one of a professional judgement of what is needed for this very specific situation.


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    Now now Andy, did I suggest any of those things?



    I thought that 100W on a 100A circuit was yours (or someone else using your account?). The 20A circuit was geoffsd's (I don't think I suggested it was yours). I did say they were subtle points rather than fundamentals - but still something to keep in mind I think. All the same the point that it's not just cables but all conductive parts of the circuit that need to be protected from faults (e.g. lightswitches & lampholders) is a valid one I think.


    There is nothing in the regs to stop a 13A socket being fed from a 'lighting' circuit fused at 6A.



    How about 433.1 - and small overloads of long duration? We can't completely rely on overcurrent protective devices to protect cables - a small overload may overheat the conductor and so degrade the insulation to some extent without tripping the MCB. In conventional circuit arrangements we don't totally rely on the OPD but also attempt to restrict the load - e.g. by limiting the floor area served.


    OK a 6A MCB and 1.0mm² usually has a good margin available, but that's not necessarily the case, and a simple mower isn't likely to be a problem (unless it's one of the old electric flymos and being used to mow half an acre or more) but if the socket could regularly be used for something more powerful, or there's insufficient spare capacity on the circuit, there could be issues.


      - Andy.

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