Is there a maximum length for a spur cable?

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AJJewsbury:

It is unlikely that a spur will ever be too long in the average U.K. house.

But might be if it's running down the garden to the shed....


 

 

If it's running down the garden to the shed it is not IN the average U.K. house.


Z.

 

If we apply the old rule of no maximum number of sockets provided the area served by the ring main does not exceed 100 sq metres, then a spur within this ring main is unlikely to be so long as to cause a problem, so not official maximum length is necessary.


A garden shed should not be served by a spur of a ring main in the house. It is a separate outbuilding so needs a single feed from the consumer board in the house. If it contains a mixture of lights and several power sockets, it needs its own consumer unit within.

A fused spur (or even an RCD spur, boo hiss)  from an existing ring could serve a couple of sockets in a small shed, and maybe a light switch as well, without breaking any rules (or the bank).

For the sort of shed that you can only get one person into at a time, to  use a complete way in the CU is a bit much. however, a means of indoor isolation (the fused spur in the house) becomes very important if the shed is likely to leak or if there is a risk of electricity theft when going on holiday. Similar thoughts apply to external sockets, I imagine you'd not include the area of the back garden in the 100m square calculation, just because a lawn mower may be plugged in once a fortnight in summer. One may think about  relaxing the voltage drop thing a bit as well - if the light in the potting shed dims when the lawnmower  is on, I cannot see many folk caring.

Even if the supply were a 4mm 32A radial I might put a fused spur with a switch as a light switch, rather than a mini CU for separate lights and power if there is only one light switch needed anyway.

No need to introduce another load of connections to test and maintain.

Dennis, I am not sure that this is a "better way". There is no reason in the regs why I cannot put a spur to my shed, whether it feeds a FSU and a light or not. It should be RCD protected, as I assume we are talking about the 18th edition. It is no different to using an extension cable, and is probably rather better as one will not be fiddling about with a plug to get the light on at night. True it is not ideal, and may bring up bonding issues, although external metal pipework is unlikely in a shed. Fitting a second CU is not a requirement, although you may like it and feeding a distribution circuit from an MCB is not ideal either, and the cable needs RCD protection if hidden anywhere, so again, why not a spur? The 100m2 "rule" does not exist, and for a shed the volt drop makes very little difference to most uses. If the RCD is tripped from the shed it may be slightly inconvenient, but this would matter little to most people. Some electricians may not like the "look of this", but then we work to the regulations don't we? A simple catenery, or SWA if buried are not a problem to the ring, and we can choose the cable size for volt drop if we like. Simple and straightforward particularly if diagrammed as is suggested in the OP. There is no requirement to only use 2.5mm cable for rings, that is just convenient and conventional with BS 1363 fittings. Appendix 15 is only informative, and not a regulation, but this often slips through peoples vision without notice.

Very effective double there Mike, de G8FNR!

A garden shed should not be served by a spur of a ring main in the house.

You might be interested in the IET's take on that - https://electrical.theiet.org/media/1695/electrical-installations-outdoors-a-supply-to-a-detached-outbuilding.pdf

(it's an older article, written to an earlier edition of the regs, but the underlying principles won't have changed).


   - Andy.

Thanks Andy for the link to the IET article, and to everyone for your comments. I don't disagree with anyone.


If you want to run electricity to a small outbuilding, e.g. a garden shed, then there is no need to go over the top with the installation. At a previous house where I lived, I supplied a single garage by means of a single 2·5 T&E cable feeding a twin socket and a switched, fused spur point to operate the lights. There was a two-pole switch at point of entry to the garage.This, to my mind, met the Regulations at the time. It was very similar to method 2 of the IET article, except that the feed came from a spare way on the main fuse board. This seemed more straightforward than finding a place on the ring circuit into which to break in and install a spur point.


Method 2 is acceptable; method 1 is preferred. It is more robust and future-proof and may be more appropriate if there is a good amount of electrical equipment in the garage, e.g. someone running a small business from there. Of course individual situations vary and various treatments are appropriate. If there is no spare way on the fuse-board, it is a bit of a palava to instal a bigger one just for the sake of a socket in a garden shed.


The area served by sockets means the area within which they are actually installed. An extension lead can be plugged into any socket, whether in the shed or in the house, but this does not mean we need to consider the area of the garden.


Some years ago, at the home of a relative, a bizarre setup was installed. The house was extended and a new double garage was built close by. The electrician installed a new, larger consumer unit in the house, to provide a means to supply the the garage from the consumer unit (no spare way on the existing one). An armoured cable was run underground to the garage, where a small consumer unit was installed. This had three sub-circuits. One was for garage light, a second for garage sockets. The third fed back into the house, through a metre length of conduit at high level, to serve lights in the house extension. The electrician argued that this was more convenient than trying to find somewhere on the existing house lighting circuit to tap into.


As far as I could tell, this did not infringe Regulations, but probably the authors of the Regulations had never envisaged such a cock-eyed arrangement. I was not keen on the house lights being fused in an outbuilding. And if a conduit was used to connect to the garage anyway, what was the point of the armoured cable when this conduit could also have taken the main feed to the garage? In fact this was a case where a single feed to the garage through the conduit to supply the sockets and a fused switched spur point for the light, would have been a simpler, cheaper and satisfactory solution.


Wind forward several years. An electric door mechanism was installed. The technician praised the installation in the garage; it was easy for him to find a good point to connect up to, and the actual workmanship was good. So at least we can say that this bizarre installation was robust and future proof.