Hello... and... SHED Questions.

[Moody Toad]

Hello everyone.

I've read a great many threads on here this afternoon and have found it most informative.

I appreciate everything that everyone has said about searching the forum, and about the new regulations, and about competence (or lack of it). I do, however, have one or two questions that I would appreciate direct answers to. Questions that I have been unable to find answers to on this forum.

I have a new shed - 17' x 11'...

I wish to have an electrical supply to it...

I won't be doing the technical stuff myself, but I will be doing the donkey work... digging trenches, etc...

Although I'm not actually doing the technical stuff myself I would like to understand things a little better...

My main question relates to the actual supply to the shed. I've been told different things by two supposedly qualified people and I was wondering if someone could clarify for me, please...

The supply will come from a spare way in the consumer unit of the bungalow. 6mm T&E will go up into the loft, through the loft to a junction box where it joins 6mm 3 core SWA. This runs down the wall of the bungalow and through a trench to the shed approximately 40 metres away, and terminates in a suitable consumer unit. The conflicting information I've received is this... one person says that the supply then needs to return to the bungalow consumer unit creating a 'ring', and the other says it just terminates in the shed CU. If it has to return to the bungalow CU that will obviously be a considerable amount of extra SWA.

Can anyone tell me which is correct, please?

Thanks very much,

Paul

P.S. There were other questions, but I can't think what they were now. Perhaps they were answered during the course of my thread reading this afternoon.

 

[Adam_151]

there would be only one reason you would do it as a ring, and that is to get extra current carrying capacity by having two idential cables running together, its acceptable (sometimes used as a work around in tricky situations) but it you are putting it in from scratch, then using a thicker cable would be a better solution to acheive the same aim.

(BTW, 6mm radial will be suitable for 32A - which should be adequate for most purposes)

Either the person wanting to make a ring:

1) dosen't know his job
2) wants to use up a load of swa and have you pay for it
3) wants to create a high power submain and doesn't want to work with swa thicker than 6mm²

Go with the spark who only wants to fit a single cable

 

[Moody Toad]

Thanks for your reply Adam.

I've kind of settled on 6mm SWA, and I appreciate what you said, but let me just say what I 'possibly' intend to use the power supply to the shed for and then maybe yourself or someone else could advise whether I should move up to 10mm or not...

The will be at least 2 flourescent lights, probably 4 (maybe more) double sockets used for TV, PC, heating, and maybe a security light on the outside.

Any thoughts?

Thanks again,

Paul
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moderator

please note 7

 

[Spark123]

What distance does the cable run through the house and what size fuse will be supplying it? Assuming an overall 50m run with a 40A MCB supply, you would be better going for 10mm² to compensate for volt drop. You may even find you need 16mm² if the run is much longer.

 

[Moody Toad]

I reckon the overall distance from CU to CU is 45m - 15m through the loft, 30m outside. Haven't made a final decision on the size of the MCB yet, so any advice on that would also be greatly appreciated.

See... I told you there were more questions...

 

[Adam_151]

Just ran it through the cable calc on the tlc site, and for that length 6mm² on a 32A breaker is either going to be either 'pushing it' or out of spec in terms of volt drop (I calculated the entire run as T&E, SWA will be slightly different)

as 25A arn't too much of a common size, the next step down from 32A is 20A, which is going to be pushing it if you are going to be using heaters in the shed, my advice would be 10mm² on a 40A breaker.

Oh and another thing, you will need to have 30ma rcd protection on the sockets in the shed, there is no way you could argue they won't be reasonably expected to supply portable equipment outside of the equipotential zone

 

[Moody Toad]

Sorry, forgot to say that my intention is to have RCD protection in the shed. One thing that bothers me after reading the threads this afternoon is the CU in the bungalow. Most recommendations on this forum say connect to the NON RCD side of the house CU. I'm not sure if the CU in the house is split or not. How can I tell?

Thanks for the cable calculations. How much of a PITA will it be to connect 10mm SWA in a junction box in the loft? Can anyone point me at the kind of junction box that will best serve this purpose? I might as well know what materials are going to be needed for the job. Also, what about connecting 10mm SWA to the CU in the shed? Will that happen? Or will I need to change down to T&E in order to get it in?

Sooo many questions now...
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moderator

please note 7

 

[Adam_151]

Do you have a main switch as well as an RCD in the CU?, if you do then its split load (you could take a picture of the cu and upload it to a free image host and post a link in here if you want us to have a look at it)

for joining T&E to SWA I should imagine it would be an adaptable box of suitable size, possibly metalclad with suitable connectors placed inside, crimps or service connector blocks for example.

I should imagine 10mm² swa can be connected directly to the shed cu as long as a metal clad one is used

 

[Moody Toad]

I'm not actually at the bungalow at the moment, but when I'm next there I'll check out the CU. If it's not a split CU will I have to replace it before having the shed power installed?

 

[BenStiller]

Can anybody tell me why he was advised to connect to the non rcd side of the CU?

 

[Moody Toad]

Ben, it wasn't advice directly to me, I read it in a number of threads this afternoon. IIRC opinion was a little divided on the subject.

I just thought of something else I wanted to ask...

The 'spare' way in the bungalow CU WAS used for power to the garage although no electricity has been used in there for a number of years. Am I allowed, as a DIYer, to remove the cable that still remains, or is that a no-no under the new regulations too?

 

[BenStiller]

moody I suppose it does not matter what end the rcd is on, so long as it is rcd protected

 

[Moody Toad]

Hmmm... can't remember exactly what I read but I suspect that theory of yours may be incorrect Ben.

 

[Adam_151]

Its so a fault at the shed end cannot interupt power to the house, it doesn't matter which end of the submain the rcd protection is as long as it exists, and as long as the submain can meet the disconnection time needed.

If you plan to dig for oil in your back garden or something or have some other reason to feel that the swa is particuly at risk, then you can fit an rcbo at the house end to protect the the shed

 

[mapj1]

Normally earthing the armour is enough to protect the cable against a pick axe - the MCB or fuse will blow as the earthed (earthed by the aromor wires) tool cuts the live core..
Put the RCD at the shed end -it is far easier to reset.
How much heating - unless its a lot, a 30A supply wil be ample - a 3bar fire is only 13A. The lights will be a few 100watts, and negligable, so a 32A MCB at the house, and a 6mm supply would cover it. How worried are you about lights in the shed flickering if full load current is drawn from the sockets? If you are then 40A and 10mm is the next size up.
Please use 3 core SWA, so there is a copper earth core in paralel with the outer steel.

Is your supply TNC-s?
If so there will be a need for a local TT earth rod too.