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Unfused Spurs from a Ring Circuit

Horace Charles

I am not an electrician but have electrical knowledge and some knowledge of ring circuits and the wiring regulations. I would like to understand from expert readers some points about an unfused spur from a ring circuit.

As I understand it the regs say that an unfused spur in 2.5mm cable from the ring circuit may feed only one single or twin (or triple?) socket OR one fixed appliance via a Fused Connection Unit (FCU).

BUT if instead of feeding a single fixed appliance, that FCU fed several sockets, would such an arrangement comply with the regulations. The combined maximum current to those sockets would of course be limited by the FCU 13 amp fuse and RCD protection is assumed at the Consumer Unit.

This limited current capability of sockets fed in this way might be restrictive, but might be OK if low current loads are connected to them e.g TVs, computer power supplies, electric armchairs etc.

I would like to understand if such an arrangement would be feasible before I call in a qualified electrician to quote for it. The reason for asking is that it would allow the provision of surface mounted sockets thus avoiding the need for disruptive cable chases in the wall.

Thanks for any feedback you may offer

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  • +1

    You could use a 4mm cable then you won't need a switched fused spur so long as you are confident that the point in the ring you are connecting to is not going to get overloaded. If it is for a few convenience sockets that won't get multiple high usage appliances plugged into them such as heaters then you are fine.  Or it is pretty straight forward to extend the ring from a double socket if it is on the ring already, it's just a couple of crimps or the like behind the socket if the box is deep enough and the ring is not already too long.

    Gary

  • 0 in reply to aligarjon

    Thank you for your feedback. I will be talking with an electrician to look at the feasibility of extending the ring at the existing flush socket.

    Re your suggestion to use 4mm cable, do the regs allow an unfused spur of that cable size to feed more than one socket.? I understand it has greater current carrying capability which I'm guessing is greater than the 30 amp (or 32 amp In a new CU) protective device ?

    I see also you mention a "switched" FCU. In my original idea would it need to be switched ? I'm just technically curious and I will be pursuing the ring extension approach.

    Thanks again for your feedback 

  • 0 in reply to Horace Charles

    do the regs allow an unfused spur of that cable size to feed more than one socket.

    The regs aren't overly prescriptive - there are some basic principles that need to be followed (protection against overload, faults, providing shock protection and so on) but how that's achieved is often left up to the designer. Rings are a bit of an oddity in that it's not easy to prove that things like overload protection (20A cable on a 32A protective device) using conventional calculations, so there are a set of rules that if you follow them are "deemed to comply", to make life easy. That's not to say you have to slavishly follow those rules for rings - if you can prove the basics are complied with by some other means, that option is open to you.

       - Andy.

  • 0 in reply to AJJewsbury

    There is no overload  problem with the 4mm spur from a 2,5mm ring, even in the worst case of the full 32A going down the spur and the rest of the ring unloaded, so long as that spur point is no nearer to one end or the other than about 1/3 of the total ring length, and in practice, as there will be load on other parts of the ring, and less than 32A on the spur, it is probably OK attached to the ring almost anywhere unless the ring loading is very lopsided indeed.

    However, its another example  of a departure from the beaten path of well proven designs, and requires extra design effort and is likely again not to be favoured by those arriving on scene, when the ring lengths are unknown being hidden in the fabric of the building.

    Being a bit unusual also makes testing a bit tricky as there may now in effect be two "far points" to consider (and verify for voltage drop and fault current purposes).

    Personally I have nothing against such non-standard arrangements, but there is a school of thought that they confuse both candidates and examiners and should be avoided. Or that may have been a discussion about truly original exam questions.

    For an easy life, just extend both sides of the ring to make it a bigger ring, the to extensions can run side by side and are not so much different to 4mm in terms of  the hole size needed.

    Mike

  • 0 in reply to mapj1
    mapj1 said:
    For an easy life, just extend both sides of the ring to make it a bigger ring, the to extensions can run side by side and are not so much different to 4mm in terms of  the hole size needed.

    Indeed, so why confuse things and make a lollipop?

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