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Shed Supply From Ring Main
PG
41 Posts
Answered
A small (7ftx5ft) shed is to be located at the rear of a house. There will be a 2ft gap between the shed and the rear of the house.
The shed is to be used as a utility room, initially, with a tumble drier and freezer It will also have a frost protection heater and lighting.  If we allow a bit of power for future or garden tool use then there could be a max demand of about 18A. There are no other services planned (no water, gas or drainage). The 230V supply is TN-C-S
The house has a lightly loaded RFC on the inside wall by the shed and the proposal is to break into the RFC and loop the RFC through the shed. The RFC is supplied from a Consumer Unit that is located near the front of the house..
Not exactly a standard arrangement, probably wouldn’t even consider it if the shed was a few more yards down the garden but I haven’t managed to locate a statement in the Wiring Regs that precludes this approach. I wouldn’t have thought twice about extending an RFC through a conservatory.
 I wondered if the Forum thought that 465.1 (Emergency Switching) could come into force, say for isolating the shed in the case of a fire but I suppose that the shed isn’t too different from another room?
 
14 Replies
mapj1
3153 Posts
Regs wise quite OK;  but one thing-  how permanent is the shed ? Some are like a battleship, others more flimsy.
If there is a credible risk  that the shed may be removed again,it might be better as a spur or two that could also be more rapidly be isolated if the shed  roof leaked or something.

Given the proposed loading you may sometimes want more than 13A, but the provision of 2 spurs or doing the full ring loop is almost the same, but  once the loop is hooked out it is harder to undo, and if it has to be done in a hurry, it may lead to a broken ring.
Another  (but highly non standard) approach, and one that may therefore have a few folk sucking their teeth, could be a 4mm unfused spur from the 2.5mm ring- again the loading on the rest of the ring is the same as if you had extended the ring, but the outdoors  can be isolated more readily if there is a problem.
M.
Now, why would anyone used to us here suck their teeth Mike, this is probably the best approach from the standpoint one may want to remove it later. I would certainly not fit a switch either, the freezer may otherwise be switched off, and no one notices. In fact, perhaps a switch with a neon might be best? A fault would then become more obvious, I assume the ring has an RCD/RCBO of 30 mA.?
Sparkingchip
3814 Posts
2.5 mm SWA out of the back of a socket into a small consumer unit in the shed with a 16 amp MCB in it to feed a 2.5mm radial circuit within the shed for sockets and a switched 3 amp fused connection unit to supply the shed lights.

Done and dusted.
Chris Pearson
2255 Posts
Sounds like a formidable trip hazard might be created. How will the cable get from the house to the shed?
Weirdbeard
243 Posts
Sparkingchip:
2.5 mm SWA out of the back of a socket into a small consumer unit in the shed with a 16 amp MCB in it to feed a 2.5mm radial circuit within the shed for sockets and a switched 3 amp fused connection unit to supply the shed lights.

Done and dusted.

Hi Andy, how do you connect an SWA out of the back of a domestic socket and it be available for future inspection?

Sparkingchip
3814 Posts
Weirdbeard:
Sparkingchip:
2.5 mm SWA out of the back of a socket into a small consumer unit in the shed with a 16 amp MCB in it to feed a 2.5mm radial circuit within the shed for sockets and a switched 3 amp fused connection unit to supply the shed lights.

Done and dusted.

Hi Andy, how do you connect an SWA out of the back of a domestic socket and it be available for future inspection?

Might have to be wired through a box on the external wall as getting a gland into the back of the socket will be a challenge, though not impossible.

Sparkingchip
3814 Posts
Chris Pearson:
Sounds like a formidable trip hazard might be created. How will the cable get from the house to the shed?


Under a slab?

PG
41 Posts
Cables (2 x 2.5 T&E) out of the back of the indoor twin 35mm deep socket box, through the wall, into a plastic conduit box and 25mm pvc conduit dropped 18" underground to shed position.
Sparkingchip
3814 Posts
In this particular case the spur from the socket ring into the shed could be less than 3 metres, so it seems acceptable to put the overload device within the shed.
Chris Pearson
2255 Posts
PG:
Cables (2 x 2.5 T&E) out of the back of the indoor twin 35mm deep socket box, through the wall, into a plastic conduit box and 25mm pvc conduit dropped 18" underground to shed position.

Concerning the socket box: is the intention to joint one leg of the ring in there, or go back to the previous socket in the ring?

Is PVC conduit suitable for use as an underground duct? How will water be excluded?

I do not have any problem whatsoever electrically with extending the ring, and Mike's upside-down lollipop is just as good; but what troubles me a little is whether there will be adequate protection from external influences.

Sparkingchip
3814 Posts
mapj1:
.
Another  (but highly non standard) approach, and one that may therefore have a few folk sucking their teeth, could be a 4mm unfused spur from the 2.5mm ring- again the loading on the rest of the ring is the same as if you had extended the ring, but the outdoors  can be isolated more readily if there is a problem.
M.


As a rule of thumb for rings, unfused spurs should not exceed 1/8 the cable length from the spur to the furthest point in the ring.

The unfused spur cannot be an extension onto a ring if the rule of thumb is complied with.

PG
41 Posts
Chris Pearson:
PG:
Cables (2 x 2.5 T&E) out of the back of the indoor twin 35mm deep socket box, through the wall, into a plastic conduit box and 25mm pvc conduit dropped 18" underground to shed position.

Concerning the socket box: is the intention to joint one leg of the ring in there, or go back to the previous socket in the ring?

Is PVC conduit suitable for use as an underground duct? How will water be excluded?

I do not have any problem whatsoever electrically with extending the ring, and Mike's upside-down lollipop is just as good; but what troubles me a little is whether there will be adequate protection from external influences.

The ring is joined in the socket box
I think PVC conduit will be a reasonable water proof duct. Installed without joints so water ingress shouldn't be a problem. I'll add some sealant between the conduit box and the brick wall.

Farmboy
222 Posts
Weirdbeard:
Sparkingchip:
2.5 mm SWA out of the back of a socket into a small consumer unit in the shed with a 16 amp MCB in it to feed a 2.5mm radial circuit within the shed for sockets and a switched 3 amp fused connection unit to supply the shed lights.

Done and dusted.

Hi Andy, how do you connect an SWA out of the back of a domestic socket and it be available for future inspection?


if you look at some of the You Tube electricians, the current trend would suggest the armour's cut off at the supply socket, so just the insulated copper cores in their jacket enter the box, with the armour earthed at the load end.

mapj1
3153 Posts
As a rule of thumb for rings, unfused spurs should not exceed 1/8 the cable length from the spur to the furthest point in the ring.

I am quite happy to spur from the far point in a ring, with more than a zero length cable,  if the ring is not already excessively long, and in any case doing it in a higher cross section cable would allow you to get further before Zs and Vdrop catch you out.
Some of these rules of thumb if  quoted without their additional caveats and  applied willy nilly can be misleading. That is probably one of them 😃

T and E in some protection would probably do if the route is on show, but a box beside the indoor socket, and bring SWA into that and work on there, in the dry, would be my personal preferred, but only that, just  a personal house style. My experience of joints outside SWA or anything else, is that they mean trouble, and should be indoors if possible.
(could look almost like  the photos here)
Mike.
 

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