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 Post subject: Metal or plastic
PostPosted: Tue Aug 28, 2018 12:04 pm 
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Hi guys

I’m putting in power sockets in my shed and I want to know if there is any regulation to stop me from using plastic back boxes as opposed to metal.

I know the metal ones are harder to break If anything falls against them but no children will ever be in in the shed, or would you recommend fitting metal fittings and stop being a tight sod.

Thanks for your input and suggestions

Ronin


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 1:55 pm 
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Guess you are talking metal clad for face fitting (not recessed galvanised type). My garage has all plastic fitted by an electrician before I moved in, they seem to be ok but just as with consumer units metal preferred in my books. Don't know the regs waiting for experts to advise.


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 4:29 pm 
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Metal sockets arent that expensive, I bought some for my shed but not had the them fitted yet: https://www.screwfix.com/p/13a-2-gang-s ... clad/69127

That price includes the socket plate and back box.


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 6:18 pm 
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Thanks for the replies

I think I’ll go for the metal option as they will be better wearing.


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 7:01 pm 
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Sensible decision but be aware that whereas plastic surface boxes are normally cabled through trunking (and there are in fact particular back boxes available for the job), metalclad sockets should be cabled through conduit. This requires a connector piece inserted into the knockouts in the box, and possibly elbows and tees for joining lengths, as well as saddle clips for wall fixing. But it will definitely look the part.

Conduit & components are available from:

CPCcouk (min. order for free shipping £6 inc. vat, if you order some 3m lengths your order will be delivered ND by UPS as they can't then use RM;

TLCDirect (carriage charge unless collect from a branch near you) online;

S/Fix (buy too much & take back what you don't use);

and of course all electrical wholesalers (at a higher price than the others mentioned, in my experience).

Comes in black or white, which is a bit odd as metalclad is usually grey.


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 8:59 pm 
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arco_iris wrote:
metalclad sockets should be cabled through conduit.



If the OP was to go down that route, the sockets should also be wired in singles not Twin and earth.

Just use twin and earth and do use grommets, preferably come in from the bottom of each socket. (Stops things falling in through the grommet hole)

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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 9:41 pm 
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someone-else wrote:
arco_iris wrote:
metalclad sockets should be cabled through conduit.



If the OP was to go down that route, the sockets should also be wired in singles not Twin and earth.

Just use twin and earth and do use grommets, preferably come in from the bottom of each socket. (Stops things falling in through the grommet hole)


A. As OnlyMe will concur, T&E in normal use gives two layers of insulation (the inner cores and the outer casing) whereas with single strand (the inner cores) the conduit provides the outer - there is NOTHING to prevent you, in a simples ring main installation, putting two T&E cables in a conduit (providing three layers of insulation and so ott). Single strand comes into its own with multiple circuits in the same conduit, industrial applications, and wishing to put 2.50mm power & 1.00mm lighting through the same piping. Much tidier.

B. What would be the point of using metalclad boxes if the T&E is clipped to the wall unprotected & entering through grommets. Such an installation would an untidy and unprofessional abortion, no impact resistance for the t&e. Humph. Ugh. Yuk. Puke.

As mentioned before, black (or white) conduit, clipped back properly, connectors, tees & elbows (glued, to be pedantic) and metalclad fittings will easily, and at low cost, provide a professional & industrial standard installation in what is, to be frank, a garden shed. At minimal cost, if you use cpc as an e.g., top class result for little effort.

As I have here.


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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 9:55 pm 
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But two 2.5 mm twin and earth cables in 20mm conduit does not allow enough "air space" since you are only supposed to use 40% of its capacity.

For what its worth, you can hit a clipped twin and earth cable with a hammer and it wont go bang.
You hit it with a sharp edge and yes it will go bang, that same sharp edge will also slice through plastic conduit.

Plastic conduit and trunking is more for aesthetics / cable management than mechanical protection.

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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 10:11 pm 
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arco_iris wrote:
A. As OnlyMe will concur, T&E in normal use gives two layers of insulation (the inner cores and the outer casing) whereas with single strand (the inner cores) the conduit provides the outer - there is NOTHING to prevent you, in a simples ring main installation, putting two T&E cables in a conduit (providing three layers of insulation and so ott). Single strand comes into its own with multiple circuits in the same conduit, industrial applications, and wishing to put 2.50mm power & 1.00mm lighting through the same piping. Much tidier.



I do concur.



For this message the author OnlyMe has received gratitude : arco_iris
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 Post subject: Re: Metal or plastic
PostPosted: Tue Aug 28, 2018 10:14 pm 
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I figured you would.

We will just have to agree to disagree

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Which is correct? Metre or Meter? Click the link to find out more.

Working on anything electrical? have you got a multi meter? why not? Would you hit a nail with a shoe?

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 Post subject: Re: Metal or plastic
PostPosted: Wed Aug 29, 2018 4:51 pm 
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someone-else wrote:
But two 2.5 mm twin and earth cables in 20mm conduit does not allow enough "air space" since you are only supposed to use 40% of its capacity.


Air space is to allow for heat dissipation, correct? If the conduit is clipped to garage wall which will be cold, is loss of air space going to have much of an impact?

ah


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 Post subject: Re: Metal or plastic
PostPosted: Wed Aug 29, 2018 7:08 pm 
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someone-else wrote:
I figured you would.

We will just have to agree to disagree



There are two types of plastic for back boxes.

Moulded brittle plastic ones or PVC conduit ones.

The PVC conduit ones are as indestructible as the metal ones unless you use fire or really want to hit it with a sledge hammer.

Two 2.5mm T&E down a 20mm conduit. It's doable, but 25mm conduit would be better. Derate the cables by 20% and they are still within spec.

It's not much different to using oval plastic conduit to supply cables recessed sockets in the house.


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