Conduit

[DaveH]

I'm at the stage where I need to order and install all the electrical infrastructure for my garage/workshop. This will be surface mounted metalclad sockets and switches and galvanised conduit (it's robust and I like the industrial look in the garage).

 

How many 2.5mm singles will I be able to get into 20mm conduit and how many into 25mm conduit? I understand about the conduit factoring and obviously won't exceed this but I'm talking about physical space. There will be a couple of 90 degree bends in the conduit run for the majority of the cables, with a few more in the straight run directly from the distribution board. Hoping someone will be able to share some experience of this.

 

Also, I was originally planning to use the conduit as a common earth but my electrician has said that the Zs ohms reading 'probably' wont be good enough and he wants me to use separate earths for each circuit which obviously increases the number of cables in the conduit (hence my first question). Can someone please confirm that this is the case?

[MikeGrahamT21]

I did the utility with 20mm plastic conduit, and then got an electrician in who used singles, he said it would have been a lot easier to use 25mm conduit, and thats what he would have installed. Even so, the conduit houses sockets cabling 2.5mm and a lighting circuit 1.5mm, but it took them a while to thread so i'm guessing it was tight.

 

I'm not sure if this helps, more of an observation, i put another run of conduit in for ethernet cables, and managed to just fit 4 of those down, so you don't have a lot of room to play with

[Jeremy Harris]

You can just get two runs of 2.5mm² T&E in 20mm conduit.  If you want to get a lighting run in as well, then move up to 25mm conduit.  Alternatively, forget about using T&E altogether, as it's fine to run single core wires in conduit (it's what conduit is really designed to take).  It's a heck of a lot easier to feed singles through conduit than it is T&E.

 

Do not use the conduit as the primary PE conductor, but do make sure it is connected to PE.  If wiring with single wires, then you can just size a single PE wire in each conduit run to deal with the worst case fault current.

[Onoff]

You need to get yourself a copy of the On Site Guide then check out Appendix E. 

 

There are clear tables in there that give the conduit 'factor' for that size conduit, a big number dependent on whether it's a short straight run or a long one with varying numbers of bends.

 

You then find the cable factor for whatever csa cable (assuming singles) and whether is solid or stranded. 

 

Basically you don't exceed the conduit factor with all the cable factors added together.

[DaveH]

Thanks guys.

 

As I said, I will be using 2.5mm single core for the bulk of the runs (with some 1.5mm for lighting in some of the runs). And also I'm aware of the factoring for the conduit size and also for the cable size and type. The factor for 20mm conduit in short straight runs is 460 and 2.5mm solid core is 39 which means that you could theoretically run 11 cables in one piece of conduit but I don't think you would physically fit that many in there! Just looking for advice on how many will physically fit in both sizes.

 

My backup plan if I can't make it work in conduit is to run some 50mm trunking around the top of the walls and drop down in 20mm conduit to individual outlets. Not sure I like the aesthetics of this as much though.

[redtop]

I would do the drops in 20 / 25mm conduit and use trunking around the tops; last thing you want is to be ramming as many cables as possible into metal conduit