Shed wiring - OK

[romic]

Sorry in advance for a common post.

I've Googled and checked and it seems to be OK, but could somebody just have a read through and confirm if the set-up below is as per the regulations.

Old set up: Remote brick built shed, originally wired up by a previous owner. Fed via a 13 fused spur from the downstairs ring main and terminated in a two way fused CU in the shed. Power use in minimum - occasional hand tool, LV garden lights. Internal 13A socket accessible for garden use as required.

Proposed new set up:

3core 4mm SWA - earth is taken through the yellow wire - with the armour sleeving bonded to the yellow wire at both ends.

Two way CU in the shed with mcbs and protected by integral 30mA RCD. Sockets (probably three) in the shed, either as a ring main or as a radial circuit, plus an external socket mounted externally on the shed wall for garden use.

- 2.5mm T&E fed from a 16A mcb

Shed lighting plus external light

- 1.25mm T&E fed from a 6A mcb

In-house connection is to a 32A mcb on a split CU and is protected by the integral 30mA RCD in the CU (there are two split CUs in the house, this CU only feeds shower and outside lights, so if it trips it's not a problem).

SWA terminated in the shed and indoors in a metal knockout box using suitable glands. Metal box is also earthed (I assume that a plastic box could be used instead if required?)

6mm T&E is used to complete the circuit from the SWA to the CU at both ends.

I realise that this means that the shed light will be lost if the RCD trips in the shed. I could solve this by using a separate CU in the shed for lighting only (I have a spare CU) or is this not permissible as the light would still be lost if the 32A mcb tripped in the house CU?

An alternative would be to use the existing supply from the fused spur on the ring main to feed a separate CU in the shed just for the lighting, using a 5A fuse in the spur instead of the current 13A one. However, I would have liked to have got rid of that cable altogether.

I assume that if a separate CU was used in the shed for the lighting, that this would also require its own RCD?

Thanks

Roger (my reader sometimes loses mail/newsgroup messages

- if you think you should have had a reply/comment, please e-mail me again. Ta!)

[romic]

PS

Does SWA have to be buried underground, or can it lay on the top?. I could just run it along the top of the ground near the bottom of the fence. It wouldn't get disturbed there. I could then use cleats to secure it to the external house and shed wall as necessary.

Roger

[John Rumm]

What type of earthing do you have at the house?

You may not want to export the house earth if it is either a TT setup, or if the shed is a long way from the house. If this were the case then you could connect the earth to the shield at the source end, but terminate in a plastic CU at the shed end and provide a local earth via a rod.

1mm sq will be more than adequate for the lights on a 6A MCB.

What sort of work are you planning to do in the shed - how critical is the loss of light on a RCD trip going to be?

If it is an issue you could fit a battery backed emergency light (about

30 quid from TLC).

Since it is protecting the shower, changing it for a higher trip rating or a time delay version is not really an option. So that kind of negates any benefit of the one in the shed.

If you are not going to export the house earth then plastic somewhere (i.e. either the box or the CU) will be needed to isolate the shield. You can usually take a SWA straight into a CU - it should have a 20mm knock out that will take a gland)

It is not the house MCB that is likely to trip, but the house RCD. A imbalance fault at the shed could trip either the shed, the house or both RCDs

Battery backed light in the shed sounds like a whole lot less fuss.

[romic]

The shed is about 25 feet from the house. The total length of SWA/T&E from CU to CU is probably around 20m or less

Sorry, forgot to mention that - it's the standard TN-S with the earth supplied by the electric co.

Not a lot - it's the odd bit of drilling or (electrical) soldering and that's about it.

OK

Thanks for the advice

Roger

[Miketew]

[ Remote brick built shed, originally wired up by a previous owner

Fed via[/i][/color]

The shed is about 25 feet from the house. The total length of SWA/T&E from CU to CU is probably around 20m or less

-

- Proposed new set up:

3core 4mm SWA - earth is taken through the yellow wire - with the armour sleeving bonded to the yellow wire at both ends.-

Sorry, forgot to mention that - it's the standard TN-S with the earth supplied by the electric co.

-

Two way CU in the shed with mcbs and protected by integral 30mA RCD. Sockets (probably three) in the shed, either as a ring main or as a radial circuit, plus an external socket mounted externally on the shed wall for garden use.

- 2.5mm T&E fed from a 16A mcb

Shed lighting plus external light

- 1.25mm T&E fed from a 6A mcb-

Not a lot - it's the odd bit of drilling or (electrical) soldering and that's about it.

- If it is an issue you could fit a battery backed emergency ligh (about

30 quid from TLC).-

OK

-- In-house connection is to a 32A mcb on a split CU and is protecte by the integral 30mA RCD in the CU (there are two split CUs in the house, this CU only feeds shower and outside lights, so if it trips it's not a problem).-

SWA terminated in the shed and indoors in a metal knockout box using suitable glands. Metal box is also earthed (I assume that a plastic box could be used instead if required?)-

I realise that this means that the shed light will be lost if th RCD trips in the shed. I could solve this by using a separate CU in the shed for lighting only (I have a spare CU) or is this no permissible as the light would still be lost if the 32A mcb tripped in the house CU?-

Battery backed light in the shed sounds like a whole lot less fuss.

-- Your original prposal with swa sounds ok dont forget to bond th sheath at both ends in your metal adaptable boxes to the yellow cor used as earth ( needs yellow / green sleeve on that) If your swa is 4m which is a 32A max cable you can also use 4mm twin and earth but the 6m is ok, just that someone in the future may assume swa is 6mm as well. T emergency light is called a non maintained eml, this only comes on wit supply failure, go to any wholesaler and it should cost about £16.50

-- Miketew

[Christian McArdle]

I'd prefer to see 2 core SWA, with armour isolated at shed end, with shed TT earthing by earth rod. I would also use 6mm minimum. 4mm is only just on the limit for 20m, leaving no voltage drop left over for wiring within the shed, if you design for the full 32A, which may be required in the future.

Use a 20A MCB. That makes it one of the standard socket circuit types.

I've never seen 1.25mm T&E. Use 1mm or 1.5mm, whichever you have lying about.

Feed it from the non-RCD side. In the event of an earth trip, you don't want to have to return to the house to get the lights restored. There is no requirement to RCD protect the SWA cable and the shed itself has its own RCD protection.

That sounds fine.

If you want separate RCD protection for the lighting circuit, you have several options, in descending order of desirability.

1. Use RCBOs in a non-split isolator incomer CU.
2. Use a split load with time delay 100mA RCD incomer and 30mA instant for sockets.
3. Use two CUs, each with its own RCD.

Yes, avoid this idea.

Yes, especially with TT earthing.

Christian.