PVC Channel

[onzadog]

So, leading on from my earlier question where I learned that I can't use the cavity between breeze block and brickwork, I'm now going to chase them into the plasterboard. I've got four lengths of 2.5mm T&E which should happily fit in the 150 mm vertical safe zone where the two exterior walls meet. However, being a belt and braces kind of guy, I was thinking of using PVC channel as well (makes any future work easier). Is there a rule about how many cables I can put in there? I know conventionally conduit should only be 40% full for a cooling air gap but if it's okay to plaster over these cables, how much air gap do they really need?

Again, many thanks in anticipation of your help.

 

[breezer]

trunking is only 35% full (or is it 45)

 

[davy_owen_88]

trunking is only 35% full (or is it 45)

It's 45%

I know conventionally conduit should only be 40% full for a cooling air gap but if it's okay to plaster over these cables, how much air gap do they really need?

The air gap is there to allow the heat to dissipate when the cables are not in direct contact with a thermally conductive material. You can go without the airgap when you plaster over cables because the plaster acts as a heatsink, something plastic capping/conduit/trunking does not.

Remember that the 45% rule includes the cable sheath, so you can't just assume 2.5mm² cable is going to take up 6.5mm² of space.

 

[Jim2287]

i thought trunking is 45% space factor ... meaning u can fill 55% of it

 

[breezer]

mind you, how many times have you seen trunking that is 110% full, yes its that full, there is no lid and its all hanging out

 

[Jim2287]

yeah true, but he doesnt want to do that

 

[onzadog]

So, it seems that trunking isn't belt and braces after all. If I'm going striaght on with the plaster, how deep do I need to go? Just into the plaster or do I need to start hacking away at breeze block?

 

[davy_owen_88]

You need to have a fairly decent layer of plaster over the top for a good finish that isn't going to crack if you tap the wall. The depth of chase will depend on the thickness of plasterboard/plaster.

 

[onzadog]

I've got four lengths to recess and it's half inch plaster board so I guess from what you're saying, I'd best get the dust sheets out!

 

[davy_owen_88]

The thickness of plasterboard + the adhesive + skim coat should be adequate for 2.5mm² cable.

 

[onzadog]

Thanks for that. Guess I'd better make sure I get it all laying flat then!

 

[Lectrician]

The space factor has jack to do with dissipeating heat - thats the grouping factor.