Preparing for power

[shedbang]

Sorry - it's about sheds. But bear with me - I have done some searching, but have emerged confused, as you'd expect. Despite having wired in the odd ring main in the past I am well aware that nowadays I am not allowed to touch anything remotely electrical. But I'd still welcome your help in preparing something for a 'proper' sparky.

I am building a shed on (reinforced) decking which backs onto a garage. Shed and garage will be about 3 meters apart. The garage has power fed by a 32A MCB from the house CU via around 30m of SWA into an RCD protected CU which supplies hot tub, garage ring and lights. I am at the stage where the decking is all removed and I am shoring up the joists (the shed weighs a tonne because she picked it, because it looked "nice" - its not your usual £200 B&Q job...). Now is a very good time to bury some SWA between eventual shed and garage as it will be nicely concealed under the deck. I'll bury the SWA under the sand under the decking (still with me?) nice and deep as prescribed and punch it through the garage wall. The shed side will be fed up into it when I build it. Then I'll phone a man with pound signs for eyes to make it all work for me.

So silly questions:

1) Is it okay to run the shed from the CU in the garage? If not, there's no point burying this cable and I will deal with it by giving up.

2) I think the house is a PME earth which I guess has been extended to the garage (it's a newish house). Okay to extend again to the shed? If not, there's no point burying this cable and I will deal with it by giving up.

3) I have some 2.5 three core SWA cable (left over from a job I did for some friends - now sadly deceased - some years ago). Cable is in good 'nic. Shed will have two lights and two sockets ('tis but a modest shed). Is 2.5mm big enough d'you think? (Hope so, or I'll need a bigger drill bit - but I won't give up on this count...)

4) Under our new laws, am I allowed to wire in the ring and the lights in the shed from the CU, or will all the fires of hell raise up and consume me if I do this?

5) I am hoping to encase the shed CU in a water feature of some sort. Will this cause any problems? Okay - this is a joke.

Any help gladly received...

 

[JohnD]

Rather than bury the SWA, you might consider burying a duct (plastic waste pipe is good) 500mm down with a rope to draw cable through.

At the same time you can bury an extra duct for the telecoms and broadband cables you might decide on later.

 

[Job_n_knock]

Cant think why someone would want Data and phone lines into their shed but ho hum each to their own. Shed eh...Just run the SWA buried as you describe. You cannot connect more than one RCD on the same circuits so your existing RCD should suffice. Just make sure both ends terminate into Earthed metal boxes. Your man with ££ sign eyes will do the rest.

 

[RF Lighting]

Cant think why someone would want Data and phone lines into their shed but ho hum each to their own.

Maybe not data and telecoms, but I have a burglar alarm network cable to my garage and shed (I also have TV to the garage)

 

[Job_n_knock]

Fair enough as I said each to their own. Id put a little CU in the shed and run a single zone alarm to it. Must be cheaper than ducting. Instead of a bellbox Id put a sound bomb in. Or make the dog sleep in the shed.

 

[shedbang]

Thanks for this. They'll be nothing exciting in my shed apart from me, so I don't need telephone ducts etc.

Do you think my 2.5mm SWA will be fine? They'll be no load to speak of in the shed so I would guess a 20A MCB would be fine in the garage.

You've got me interested now. Why can't you put two RCD's on a circuit and the SWA is 3 core. Couldn't the man just use the 3rd wire? (I note that the termination between CU and SWA in my garage is done using one of those things they use to earth to pipes on the metal of the cable and not a gland).

 

[ban-all-sheds]

(the shed weighs a tonne because she picked it, because it looked "nice" - its not your usual £200 B&Q job...).

OOI do you have a URL for it? I need to get a decent shed..

1) Is it okay to run the shed from the CU in the garage? If not, there's no point burying this cable and I will deal with it by giving up.

Yes.

2) I think the house is a PME earth which I guess has been extended to the garage (it's a newish house). Okay to extend again to the shed? If not, there's no point burying this cable and I will deal with it by giving up.

Yes. Presumably there are no extraneous-conductive-parts in the shed, such as water or gas pipes, exposed structural steelwork etc?

3) I have some 2.5 three core SWA cable (left over from a job I did for some friends - now sadly deceased - some years ago). Cable is in good 'nic. Shed will have two lights and two sockets ('tis but a modest shed). Is 2.5mm big enough d'you think? (Hope so, or I'll need a bigger drill bit - but I won't give up on this count...)

2.5mm² will drop about 1V with 20A over 3m, so it will be fine.

4) Under our new laws, am I allowed to wire in the ring and the lights in the shed from the CU, or will all the fires of hell raise up and consume me if I do this?

The most recent version of my summary.

You've got me interested now. Why can't you put two RCD's on a circuit

Well, you can, but there will never be any guarantee about which one would trip, unless one is a time delayed sort. So if you have no non-RCD circuits free in the garage CU, don't use an RCD CU in the shed. In fact - don't bother with a CU there at all - just come off the 20A MCB in the garage, terminate the SWA in a box in the shed and run a 2.5mm² radial from there to the sockets, and to a switched FCU for the light.

and the SWA is 3 core. Couldn't the man just use the 3rd wire?

Not sure what you mean?

(I note that the termination between CU and SWA in my garage is done using one of those things they use to earth to pipes on the metal of the cable and not a gland).

Yee-hah!

 

[shedbang]

Thanks Ban

Url for the shed http://www.gardensite.org.uk/logcabins/hytti.php. Helps if you have a crane or some sort of shot-putter to hand during construction.

No doubt Mr. Sparky will decide if he uses a CU or a box (I thought a CU looked much more professional...). What I actually mean't by the 3rd wire was could you use the 3rd wire in the SWA as an earth and not bother with earthing the cable?

 

[RF Lighting]

What I actually mean't by the 3rd wire was could you use the 3rd wire in the SWA as an earth and not bother with earthing the cable?

No.

The sheath of the cable must be earthed regardless of wether a third core is used, or not.

The armour is there to protect the cable, and the person who cuts through it. If someone was to cut the cable (eg with a spade), then this would cause a short circuit to earth, cutting off the supply to the cable.

If the armour was not earthed then the spade could become live, and electrocute whoever is holding it.

 

[Lectrician]

If install ducts - dont use waste pipe - use the correctly coloured and manufactured ducts and bends. Look for civil duct.

Most decent alarms use 4 cores from panel to keypad or expander - I use standard 4 core SWA.

5 pair SWA telephone cable is readily available, and CAT5e SWA is available, but you need to look hard. You can get duct-grade CAT5e easily. You can also get the green duct-grade coax.