Wiring for garden office

[electromuppet]

Hi,

I've been trawling these forums trying to find the answers to my questions and while there's plenty of advice for wiring sheds, I can't find anything that quite answers all my questions. I'd appreciate any advice.

Basically, Ive just built a wooden office shed type building in the garden, while preparing the ground I buried a length of 20mm PVC covered steel Kopex conduit running about 10m towards the house CU. I chose the Kopex over SWA cable as I need to run a thin fibre optic cable inside together with the power. (Laying SWA now would be a bit of a problem).

I plan to terminate the Kopex on two IP66 Wiska junction boxes, one on the outside of the house and one on the shed and run through 6mm 2 core+earth between them and connect through into the buildings, seperating out the fibre cable which is non conductive. The Kopex is buried 45cm deep and has a yellow "cable below" tape above it.

Inside the house, the 6mm cable will terminate on a new 32A RCBO and in the shed to a new CU with a 60ma RCD and a few MCBs.

I plan to run two circuits, for 4 and 2 sockets respectively and another circuit for a couple of lights, everything cabled to the shed CU with 2.5mm TC+E.

The shed is insulated and lined so I'm guessing there's no need for conduit behind the wall? the sockets will be on plastic bench level trunking, together with a few CAT6 outlets.

I will be getting a qualified electrician in to check it over and hopefully sign off on it, but I'd like some assurance I'm not doing anything fundamentally wrong before then.

Any advice would be much appreciated.

Cheers, Greg

 

[sparkwright]

Some double checking required first.

Did you actually mean 20mm conduit?

Did you actually mean 45mm deep?

 

[electromuppet]

Ah sorry, yes it is 20mm Kopex but it's buried a little deeper - 45cm!

 

[sparkwright]

And to make sure, have you not got the cable in the conduit yet?

 

[electromuppet]

No, the cable is not yet in the conduit and I've since learnt that might not be the easiest thing to achieve.

Incidentally, the Kopex glands are the non-swivel type for the JBs, I'm guessing I should earth the conduit? (assuming the whole thing isn't a total no-no of course!)

 

[sparkwright]

No, it's going to be a nightmare pulling that through - it take it there is no draw string already in it?

 

[flameport]

20mm PVC covered steel Kopex conduit

run through 6mm 2 core+earth

Even 6mm² singles will be exceptionally difficult.

Unfortunately you have installed the wrong type of conduit in a size which is far too small to be of any practical use.

 

[electromuppet]

I do have one of those flexible line things for pulling cables and the distance between JBs is actually probably nearer 6m than 10 so hypothetically, apart from making life difficult for myself is there anything else wrong? If it won't go through, I'll have no option but to replace it I guess, but I'd rather not contemplate the work involved in that right now. I was looking for some round solid 2C+E cable, thought that might be easier than the flat stuff.

 

[sparkwright]

All you can do is try, but I presume the inside of the conduit is corrugated, rather than smooth?

 

[electromuppet]

And I guess I can't use stranded cable if I it ferrules on the ends?

 

[Robin0577]

I have a few observations to make on this, generally negative I'm afraid.

I will be getting a qualified electrician in to check it over and hopefully sign off on it, but I'd like some assurance I'm not doing anything fundamentally wrong before then.

No legit electrician will sign off work that he/she hasn't been involved with from the start. They would be taking responsibility for design and constriction they didn't do or even supervise.

Basically, Ive just built a wooden office shed type building in the garden, while preparing the ground I buried a length of 20mm PVC covered steel Kopex conduit running about 10m towards the house CU. I chose the Kopex over SWA cable as I need to run a thin fibre optic cable inside together with the power. (Laying SWA now would be a bit of a problem).

Good luck getting 6mm2 T&E down that, even 4mm2 would be a struggle

Inside the house, the 6mm cable will terminate on a new 32A RCBO and in the shed to a new CU with a 60ma RCD and a few MCBs.

No point having 2 RCDs, one in the house and one in the shed. There's no way to predict which one (maybe both) will trip in the event of a fault. As far as I'm aware there is no such thing as a 60mA RCD

I plan to run two circuits, for 4 and 2 sockets respectively and another circuit for a couple of lights, everything cabled to the shed CU with 2.5mm TC+E.

Why 2 socket circuits? You don't strictly even need a CU in the shed end at all if you have RCD protection at the house end. You could just continue with a 32A radial circuit and use a 5A fused spur for the lighting. If you have 20 or 32A MCBs at the shed end, chances are they won't discriminate with the 32A in the house anyway.

Oh and yes, the conduit absolutely must be earthed, as it is acting as a substitute for the armour of the SWA that would normally be used. E2A - Scrap that, as RF has just reminded me, flexible conduit isn't good enough anyway, it would have to be the rigid stuff.

 

[electromuppet]

Yeah, it's corrugated which will add to the pain.

 

[sparkwright]

Yeah, it's corrugated which will add to the pain.

Then you're in **** Street.

 

[electromuppet]

Thanks Robin, I was planning to leave everything exposed so any electrician could check everything properly. I do telecoms cabling offshore so I'm confident I can do a fairly tidy job. I would have the electrician handle the cabling from the house JB into a new RCBO though so there would still be some work for them.

 

[RF Lighting]

Even if you can get the cable through the Kopex, it is still not compliant. BS7671 specifically lists flexible metallic conduit as not being suitable for use as a CPC as it can not be reliably earthed, and as a result it can not meet the criteria required for cables buried in the ground.