I read a thread recently ("T & E in steel 20mm conduit") about a chap who had ended up with 2 x 2.5mm² T&E in (about) 20mm steel conduit as part of a ring circuit on a 32A MCB.
ban-all-sheds pointed out that in this situation the current rating of the cable would have to be reduced and that as such it would be pushing it to say that it was still to code, and that to keep things comfortably within the regs, it would mean using a lower rated MCB, probably 25A.
I am wanting to come up with a cabling design for my new house that will allow me to have 2 x 2.5mm² T&E, in conduit, for ring circuits, and use a 32A MCB, and be comfortably within the regs.
Nearly all of the time the cables will be in the safe zones, and it seems that uPVC would be easier to work with than steel, so I thought it would be nice if I could go with uPVC.
So I was wondering about using 25mm round uPVC conduit.
Assuming a 1mm wall thickness, 20mm conduit would have an inner cross-sectional area of about 254mm², and 25mm conduit would have a inner cross-sectional area of about 415mm² (63% larger).
www.tlc-direct.co.uk says that 2.5mm² T&E has cross-sectional dimensions of 10.3mm x 6mm, which would have a cross-sectional area of about 55mm².
So for 20mm conduit, 2 x 2.5mm² would occupy some 22% of the space, whereas with 25mm conduit it would only occupy 13% of the space.
Would that be sufficient to make it work?
I have spoken with asterisk and it seems that the thing we are dealing with is the grouping factor. The grouping factor seems to be expecting singles in conduit rather than T&E, and it doesn't appear to take into account the amount of free space in the conduit.
I am wondering though about in what scenarios the grouping factor actually applies. Imagine you have a 500mm(!) diameter conduit with 2 x 2.5mm² T&E, randomly positioned, running through it. It would seem that the cables would be considered to be independent and no special factor would be applied. But in some a technical sense, the grouping factor might still apply. For practical purposes, would anyone apply the grouping factor? If not, then where do we draw the line?
Below is a drawing showing a cross section of the 20mm and 25mm scenarios. Everything is drawn to the same scale. The conductors are 2.5mm² for the phase and neutral and 1.5mm² for the earth.
Could it not be argued that for the 25mm case we are into "sausage down a railway tunnel" territory?
Is the problem that the cables are unclipped, and therefore we can't guarantee how much space is between them, so we have to assume the worst?
______________________________
Moderator
please note 10a
ban-all-sheds pointed out that in this situation the current rating of the cable would have to be reduced and that as such it would be pushing it to say that it was still to code, and that to keep things comfortably within the regs, it would mean using a lower rated MCB, probably 25A.
I am wanting to come up with a cabling design for my new house that will allow me to have 2 x 2.5mm² T&E, in conduit, for ring circuits, and use a 32A MCB, and be comfortably within the regs.
Nearly all of the time the cables will be in the safe zones, and it seems that uPVC would be easier to work with than steel, so I thought it would be nice if I could go with uPVC.
So I was wondering about using 25mm round uPVC conduit.
Assuming a 1mm wall thickness, 20mm conduit would have an inner cross-sectional area of about 254mm², and 25mm conduit would have a inner cross-sectional area of about 415mm² (63% larger).
www.tlc-direct.co.uk says that 2.5mm² T&E has cross-sectional dimensions of 10.3mm x 6mm, which would have a cross-sectional area of about 55mm².
So for 20mm conduit, 2 x 2.5mm² would occupy some 22% of the space, whereas with 25mm conduit it would only occupy 13% of the space.
Would that be sufficient to make it work?
I have spoken with asterisk and it seems that the thing we are dealing with is the grouping factor. The grouping factor seems to be expecting singles in conduit rather than T&E, and it doesn't appear to take into account the amount of free space in the conduit.
I am wondering though about in what scenarios the grouping factor actually applies. Imagine you have a 500mm(!) diameter conduit with 2 x 2.5mm² T&E, randomly positioned, running through it. It would seem that the cables would be considered to be independent and no special factor would be applied. But in some a technical sense, the grouping factor might still apply. For practical purposes, would anyone apply the grouping factor? If not, then where do we draw the line?
Below is a drawing showing a cross section of the 20mm and 25mm scenarios. Everything is drawn to the same scale. The conductors are 2.5mm² for the phase and neutral and 1.5mm² for the earth.
Could it not be argued that for the 25mm case we are into "sausage down a railway tunnel" territory?
Is the problem that the cables are unclipped, and therefore we can't guarantee how much space is between them, so we have to assume the worst?
______________________________
Moderator
please note 10a