Long chases of 2.5mm T&E in 20mm oval conduit

[Lectrician]

The NIC strongly recommend you do fit them, but will not pick you up on it if you don't use them where they can be ommited.

A surface metalclad back box always needs the fly lead. It is only flushed boxes you can get away with out doing, and then only, as mentioned above, when you have at least one FIXED lug (not the adjustable ones).

 

[seasickstevie]

I wouldn't leave as much conduit entering the box as in the picture, as it tends to restrict space. Just a few mill is all you need.

Bit difficult to use 25mm conduit in most cases, as the majority of boxes only have 20mm knock-outs.

 

[Chri5]

If it was me I would:-

Use 20mm or 25mm ROUND conduit, not oval ega tube.

Use suitable bushes on the box.

Use boxes that are 30mm+ deep, and you can get boxes with both 20mm and 25mm KO.

If you ever need to replace the cabling, round conduit is far easier to move cables in and out.

Neither provide mechanical protection and RCD's required at board. You could also opt to run bare cables and simply plaster over.


Regarding the chopping out, try drilling round the box with some tape on the drill set as a depth marker (set as the depth of box being used) this will prevent quite so much plaster / render blow.

If you can put up with the dust, a grinder and / or a duble grinder / wall chaser will do near perfect cuts and save on plaster damage and making good costs.

 

[ban-all-sheds]

Looks like oval conduit suished at the end

Well spotted. I stand corrected!

Use these:



They're soft rubber, and easily deform to take oval conduit.

Not especially cheap, but you might strike lucky on eBay - I once picked up a box of 100 of them for less than £7 inc P&P...

 

[ColJack]

If your going to go the conduit route, why not use it in the ceiling as well, use 2.5mm 6491 (1.5mm for the CPC/earth) conduit singles, one conduit drop down the wall as opposed to two.

can't do that..

if wiring in singles, the earth/cpc must be the same size as the live conductors..
it's only acceptible in T+E because they made an exception for it.. ( something to do with the cpc being unsheathed / single insulated equivalently..)

 

[Spark123]

No, you can use a reduced size in conduit as long as it complies with the adiabatic equation s=sqrt(I²t)/k which is the same reason it is allowed in twin and earth.

 

[ColJack]

you have me at a disadvantage sir, my regs book is locked up in my bench at work ( currently on a weeks lay-off, pending redundancies.. )

I thought that reduced CPC size was only aplicable to multicore cables where it is formed by either an integral conductor, or by the sheath of a sheathed cable.. ?

 

[Spark123]

Nope, have a read of 543.1.1 when you get back to work

The csa of every protective conductor, other than a protective bonding conductor, shall be:
(i) calculated in accordance with Regulation 543.1.3, (which is the adiabatic) or
(ii) selected in accordance with Regulation 543.1.4. (which is table 54.7)

 

[ColJack]

I have my 16th regs book at home with me, and table 54G ( 54.7 ) specifies that where the live conductors are less than 16mm², then the cpc must be the same size, 16mm² - 35mm², cpc must be 16mm², above 35mm², no less than half the size..

 

[ban-all-sheds]

Read 543-01-04 above it, and then 543-01-01 & 543-01-03 2 pages back....

 

[Lectrician]

I wouldn't leave as much conduit entering the box as in the picture, as it tends to restrict space. Just a few mill is all you need.

Bit difficult to use 25mm conduit in most cases, as the majority of boxes only have 20mm knock-outs.

If using 25mm oval you would not take it into the box, you would use grommets in the traditional fashion.

If you want to use 20mm round conduit and use conduit adaptors on the galv box, you should use 35mm boxes to keep the conduit further back to allow for a decent cover of plaster. 35mm boxes have the knockouts further back. I only ever do this when the conduit forms part of a fully constructed conduit/trunk system. I dont see the point on domestics otherwise - just oval and grommets, along with the other 99% of sparks!.

 

[Click-Sure]

can't do that..

if wiring in singles, the earth/cpc must be the same size as the live conductors..
it's only acceptible in T+E because they made an exception for it.. ( something to do with the cpc being unsheathed / single insulated equivalently..)

Looks like i've got a hell of a lot of jobs to go back to and pull in new CPC's then. Still, I'm quiet at the moment and slabbing the patio can wait.

 

[Lectrician]

It is convention to use the same size singles, however, aslong as it complies to the adeabetic(spelling?) or table 54g(16th ed) it is fine.

I must learn the new table numbers!

 

[DaveyH]

Thanks for all the advice so far everyone.

Another question now though.

How much plaster needs to be covering the conduit? If I was to plaster over it now there would be about 5mm.

 

[ban-all-sheds]

Check here to see if that's thick enough to avoid cracking, and longer term discolouration of the wall due to different thermal characteristics.