Conduit with first fix wiring
- Thread starter benjy_b
- Start date
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Looking at your drawing, bear in mind that at most sockets you will have two or maybe three cables going into the box. Your conduit would need to be sized to accomodate that.
The IET's On Site Guide (you should get a copy, at least) gives cable capacities for different sizes of conduit. Per the discussion above, the capacities only deal with singles being run in conduit. Running twin & earth cable in conduit is a PITA as the slightest twist in one cable will make pulling in the next length very difficult without wearing away the sheath of the existing cable.
As you are thinking of putting in the conduit first and then designing the installation, then installing over a period of years you are going to have a hard time anticipating your changing needs.
Socket back boxes generally only have 20mm knockouts, so that is going to be your practical conduit diameter limit without also doing some serious metalwork at each socket location.
IF you decide to go down this seemingly tortuous route then, in my opinion, you are going to need to provision 3 conduit drops at each socket, and hope that your cable size is no bigger than 2.5mm²
A couple of other observations
The IET's On Site Guide (you should get a copy, at least) gives cable capacities for different sizes of conduit. Per the discussion above, the capacities only deal with singles being run in conduit. Running twin & earth cable in conduit is a PITA as the slightest twist in one cable will make pulling in the next length very difficult without wearing away the sheath of the existing cable.
As you are thinking of putting in the conduit first and then designing the installation, then installing over a period of years you are going to have a hard time anticipating your changing needs.
Socket back boxes generally only have 20mm knockouts, so that is going to be your practical conduit diameter limit without also doing some serious metalwork at each socket location.
IF you decide to go down this seemingly tortuous route then, in my opinion, you are going to need to provision 3 conduit drops at each socket, and hope that your cable size is no bigger than 2.5mm²
A couple of other observations
Why on earth do you think you would need that?
I very much doubt that it would. Insulation requirements, connection principles and much else for LV installations are vastly different to auto.
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This is merely a proposal, hence the enquiry.
What would be the normal routing of cabling in this instance? Run it through the insulation without conduit and derate accordingly?
What would be the normal routing of cabling in this instance? Run it through the insulation without conduit and derate accordingly?
Lighting - whatever you want.
Sockets - between insulation and plasterboard, i.e. not entirely surrounded by insulation. Or use 4mm^2 and put it anywhere.
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Fire & Drafts:
If you've got a piece of tube coming from the open, drafty floor cavity, surely there is a chance the draft would run up the conduit?!
Same goes for fire/smoke/hot gases?!
Transferable skills:
Why be so obtuse?! To quote myself, I said, "SOME transferable skills".
For example,
- using a screwdriver
- using various tapes
- using a crimping tool
- using a multimeter
- routing cable
- joining cable
- wiring principles
- calculating voltage drop
- calculating load
- calculating appropriate circuit protection
-
If you've got a piece of tube coming from the open, drafty floor cavity, surely there is a chance the draft would run up the conduit?!
Same goes for fire/smoke/hot gases?!
Transferable skills:
Why be so obtuse?! To quote myself, I said, "SOME transferable skills".
For example,
- using a screwdriver
- using various tapes
- using a crimping tool
- using a multimeter
- routing cable
- joining cable
- wiring principles
- calculating voltage drop
- calculating load
- calculating appropriate circuit protection
-
The conduit that would be terminated in an enclosed metal box that has a socket fitted on the front.
If you had fire/smoke/hot gases under your wooden floor then a bit of expanding foam in your bit conduit isn't going to help one iota.
I am not being negative, it is just that you are making what is a fairly easy task into a complicated saga for no benefit.
We could have some discussions re your list of skills and tools. there's a lot missing
EFLI meter, RCD tester, insulation tester for starters
I really don't see what the big issue is here. The OP wants to put some metal knockout boxes into his wall and fit a few straight drops of plastic conduit to the floor void for future use. As long as they are straight drops then there shouldn't be any issue with pulling T&E through them. Allow 1 drop for each cable in order to to accomodate 4mm².
Putting it in conduit does not mean it does not need derating.
I don't see any problem at all here. I often put conduit drops in the wall as it adds very little to the cost and makes the installation much easier to alter in the future.
OP : if you install two 20mm conduits dropping vertically straight down from your socket to the sub floor void, you'll have no problems at all.
Terminate the conduits to the back box with a female adaptor. Also I'd advise using 35mm back boxes.
OP : if you install two 20mm conduits dropping vertically straight down from your socket to the sub floor void, you'll have no problems at all.
Terminate the conduits to the back box with a female adaptor. Also I'd advise using 35mm back boxes.
There isn't one, now that the OP has actually explained what they intend to do.
I'm sure I'm not the only one who originally thought, from what you were telling us, that you were planning a much more extensive conduit installation than that, with longer runs, 90° bends/elbows etc.
Yes, but....
It will need to be de-rated even if you do use conduit, as the conduit will be surrounded by insulation. And in your first post you told us that the walls had insulation in them as well, so that's run of 700+mm through it.
There is no method for sheathed cables in conduit totally surrounded by insulation in BS 7671 - I strongly advise using a factor of 0.5. i.e. the figures for Method 103:
I hope you are not suggesting that the ring final needs to be 6mm² BAS. Designers who do things like that tend to be sent to site to help second fix 
, then they get a nasty note that the materials cost has gone over whats allowed for.
, then they get a nasty note that the materials cost has gone over whats allowed for.10mm² surely?
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No, "just" 6 milli as that is over 20A and they will be doubled up in a ring final.
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