External Conduit

[Martman]

Can anyone tell me the current regulations regarding the running of 1mm and 1.5mm and a separate earth on an external wall.
Can I use PVC glued at the joints?
Should I use inspection bends or angle box’s or use a bending spring to bend the PVC?
How many cables can go in a 25 mm Conduit
What’s the rules on metal box trunking like 4x4 inches?
The cables will be going into the roof space how far should the trunking continue?

Thanks for your help
Mart

 

[ericmark]

The guidance for using trunking is in the guide to books not the 17th Edition book it's self and the type of singles used will also effect what can be done you can get double insulated singles.

Because we don't have to follow the British regulations we could for example follow the German regs where they are allowed to free air the cables on the bends.

So to quote rules as such is hard. However outside with plastic we have to consider expansion in the heat which since we should use black for UV rejection in the sun they can get quite hot. There are two glues one does not set and there are special connectors which are extra long for use with the non setting glue.

How many bends and how sharp the bends will alter how many cables one can pull in and to produce sweeping bends with a bending spring does take a little practice. In theroy you should install the conduit then pull in the cables but I have seen the cables pulled in before fully installed many times.

Using something like yellow 77 can make pulling cables in far easier. Do not use detergent as it's made from benzine and if there is a fault can explode.

I do not like conduit because it can so easy direct water to where we don't want it. I would always try to use SWA outside. Any box can leak and you have to consider if holes should be drilled to drain any water ingress or if to seal.

The rules in the guides do limit the number of cables but cables in contact with each other can transfer heat better than a cable in still air so much will depend on what current each cable is carrying as to in real terms how many cables a conduit can safely carry. The figures in the guide are just that a guide and one has to use experience to decide how many cables it can really carry. Also how installed.

If you tape all cables together and pull them in as one group then the chance of burning a cable while pulling it in is much reduced. But miss out one cable and pull it in after and very easy for the new cable to damage existing cables insulation and outside with water ingress this can cause future failures. Insulation testing with conduit is a must.

As I said before I don't like conduit outside. I have put up miles of conduit with alarm cables in them and the cables are quite slippery and go in easy. But try pulling twin and earth through conduit and it's a nightmare.

I don't have up to date guide to BS7671 so I'll let some one else quote the figures but only a guide remember.

 

[PrenticeBoyofDerry]

Some useful information here:
http://www.tlc-direct.co.uk/Book/4.3.3.htm

 

[ban-all-sheds]

If you use PVC conduit outside it will get soft in hot weather, and it will sag between the supports and it will look cack.

 

[Martman]

On the estate different methods have been used, cables are run from the outside in cupboard consumer unit up the wall into the roof space one property has steel conduit next door to that they have metal box trunking and a separate PVC run.

The run is about 3M on an outside wall straight up with a bend/angle box into the cupboard. It will come out of the cupboard at around 2M above ground level

No one has said that you can’t use PVC but as yet I have not been able to find anything about how many cables you can run in 25mm PVC conduit, is a case of how many will fit will do?

 

[ban-all-sheds]

I have not been able to find anything about how many cables you can run in 25mm PVC conduit,

Which means you don't have any of the books you should have if you want to do work like this.

Why is that?

is a case of how many will fit will do?

No.

See GN3, or the On-Site Guide, or another reference work covering electrical installation design.