Seperate CPC out to garage - duct or not to duct?

munty12 said:

I was thinking of running a new supply to the garage. Problem is it has a water pipe (only four inches of lead and the rest is poly!) so I need to earth it with a 10mm.

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munty12 said:

The garage is only 1 metre away from the house. I dug a trench one metre long across the concrete path between the house and garage (about 300mm deep)

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munty12 said:

The length of the run is only 6M in total, so I thought I'd run a 10mm 3 core SWA, protected by a 1 way RCD CU, but someone else suggested running a smaller SWA supply cable with a seperate 10mm CPC for the water main - which makes obvious sense, right?

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munty12 said:

so I thought I'd run a 10mm 3 core SWA, protected by a 1 way RCD CU

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munty12 said:

Which begs the quetion, if polypipe is considered to provide the correct protection, what's stopping me from doing away with SWA altogether and instead running the supply in twin and earth inside polypipe?

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D Skelton said:

Potentially it may need bonding, not earthing. Have you performed any tests to determine whether this water pipe is an extraneous conductive part, if so what were your results?



Are you planning to concrete back over when you're done? I assume so but if not then 300mm is not deep enough. Also, have you remembered your warning tape?



Erm, not really. If the water pipe is indeed an extraneous conductive part, I would be using the 10mm three core option, one of the three cores for a main bonding conductor (permanently crimped and extended within the garage board to the water pipe) and the armour for the earth.



I would be installing the RCD within the new garage board rather than protecting it at the supply end. I'd take the feed directly from a spare way in the existing fusebox/DB via a steel adaptable box (or directly if you have a steel fusebox/DB), if no spare way then a small single switchable fuse housing would suffice. This is of course assuming that you're not on a TT system.



Forgetting the many regulations you would be breaching by doing so, common sense dictates that polypipe isn't that great at protecting cable from a spade. I would still duct the SWA though.

I'm guessing by your questions that you're not an electrician? I don't mean this in a patronising way at all, it's just that I'm in Brighton too so I'd be happy to send one of my guys over to give you a hand if you want to make sure a proper job gets done

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munty12 said:

Which begs the quetion, if polypipe is considered to provide the correct protection, what's stopping me from doing away with SWA altogether and instead running the supply in twin and earth inside polypipe?

Much thanks for any suggestions.

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D Skelton said:

Hahahahaha!

Well, munty is more than welcome to engage my services to brush up on his tank operating skills

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Murdoch said:

Maybe Rommel should offer his services to do it properly. I'm guessing that Munty12 is a DIYer...

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D Skelton said:

Hahahahaha!

Well, munty is more than welcome to engage my services to brush up on his tank operating skills

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Is this a euphemism, or something! I'm scared now.

I'd be happy to send one of my guys over to give you a hand..

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Now I'm terrified!

Thanks for all the sound advice guys. I was worried yesterday that the thread was going to slip down further and further into obscurity without so much as a peep from anyone, so I'm grateful for all the replies. I'd spent a while writing a full reply last night, and duly managed to delete it before I posted it. DUH!

I would be installing the RCD within the new garage board rather than protecting it at the supply end. I'd take the feed directly from a spare way in the existing fusebox/DB via a steel adaptable box (or directly if you have a steel fusebox/DB), if no spare way then a small single switchable fuse housing would suffice. This is of course assuming that you're not on a TT system.

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It's a TN-S system. The house CU is plastic, with little space (it's crammed with RCBOs!)

Potentially it may need bonding, not earthing. Have you performed any tests to determine whether this water pipe is an extraneous conductive part, if so what were your results?

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There is an old CU in the garage, and I found continuity between the tap and the light switch in the garage, when there is no sign whatsoever of a CPC between garage CU and said pipe (which is a little odd perhaps because there's 3m of plastic pipe between the point where the lead pipe comes in and where the tap is - I thought plastic was insulated, and that water was deemed a poor earth conductor).

So yes, the pipe is bringing a potential into the garage.

If there is only one pipe, what is the difference between bonding and earthing?

Are you planning to concrete back over when you're done? I assume so but if not then 300mm is not deep enough. Also, have you remembered your warning tape?

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Yes I'm going to back fill it with a layer of sand, then solid concrete. Surely depth is decided by the need to protect it from accidental damage, such as a spade?

Forgetting the many regulations you would be breaching by doing so, common sense dictates that polypipe isn't that great at protecting cable from a spade. I would still duct the SWA though.

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Someone could try to chop the cable with a spade, but they would only chip the top layer of concrete and make their whole body vibrate with the effort!

Or is it that the mechanical protection is also aimed at keeping the cable safe from a digger, or somesuch?

Erm, not really. If the water pipe is indeed an extraneous conductive part, I would be using the 10mm three core option, one of the three cores for a main bonding conductor (permanently crimped and extended within the garage board to the water pipe) and the armour for the earth.

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Do I really need to thru-crimp the 10mm? Can't I simply terminate it at the garage CU, and then run a seperate 10mm CPC from the earth terminal to the pipe?

Isn't the question - is twin & earth designed/rated for burial ?

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I guess that's the short-end of it. Just how good is this polypipe? If it is not considered strong enough to offer mechanical protection from a spade - what is it considered strong enough for? What's to stop someone running the stuff on the surface alongside a hedge to a shed or somesuch at the bottom of the garden? In theory, the shed could then be supplied by twin and earth.

Thanks again.

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