Insulated plasterboard and cable derating

[Malc]

Hi,

I've house remodelling underway and I'm looking at the electrics. The walls are planned to have 30mm thermaline basic (polystryene backed) plasterboard to improve the U value of the old walls and decrease the U of the new walls. I had thought that this would be method A, "enclosed in a conduit in an insulated wall". However, I now see that the full text says cables are just below the room surface (i.e. plasterboard) with a U > 10. Clipped direct also doesn't apply via the rear wall and text for that says masonry such as brick etc. excluding thermally insulating which I think means celcons which the new wall has. Old wall has some volcanic block which might just meet the wall resistance < 2 k.m/W (i.e. 0.5 W/m.K) in IEC 60364-5-52.

Do I have this right or am I missing something? If I use method 103 then I need 6mm2 for a 32A ring or 20A radial or 4mm2 if I make it a 20A ring (and I believe many sockets can only take 2 x 4mm2).

I also need new meter tails as CU is moving. There are 2 lengths of 20mm conduit which drop into the outside meter box and open into the floor void. These travel up the cavity for 1m or 1.2m maybe and the cavity is insulated. Again I was thinking method A but revising that I think I can still do it with 35mm2. Table 4DA1 says method C is 141A for 2 cables. I'll only have one per conduit because of the size so that is near enough 200A (141 x 1.414) which with 50% derate of 103 is 100A.

Any suggestions, other than drop the thermaline in favour of plain plasterboard which I'm considering......

 

[EFLImpudence]

I had thought that this would be method A, "enclosed in a conduit in an insulated wall".

Not if you don't have conduit, it won't.

However, I now see that the full text says cables are just below the room surface (i.e. plasterboard) with a U > 10.

If it is touching the plasterboard - make sure it is - then it will be 102.
The values are more or less the same, anyway.

Clipped direct also doesn't apply via the rear wall and text for that says masonry such as brick etc. excluding thermally insulating which I think means celcons which the new wall has. Old wall has some volcanic block which might just meet the wall resistance < 2 k.m/W (i.e. 0.5 W/m.K) in IEC 60364-5-52.

Buried in masonry is method C - the same as clipped direct.

Do I have this right or am I missing something? If I use method 103 then I need 6mm2 for a 32A ring or 20A radial or 4mm2 if I make it a 20A ring (and I believe many sockets can only take 2 x 4mm2).

Don't use method 103, then.
The CCC of a ring circuit must be a minimum of 20A so 2.5mm² will be alright with method 102 or A.

I also need new meter tails as CU is moving. There are 2 lengths of 20mm conduit which drop into the outside meter box and open into the floor void. These travel up the cavity for 1m or 1.2m maybe and the cavity is insulated. Again I was thinking method A but revising that I think I can still do it with 35mm2.

Do you know what size fuse you have in the supply?
If you are worried, unless your house is massive, fit a 60A switch fuse to protect the tails.

Table 4DA1 says method C is 141A for 2 cables.I'll only have one per conduit

It won't be two cables if you have singles in two conduits.

because of the size so that is near enough 200A (141 x 1.414) which with 50% derate of 103 is 100A.

What is the design current of your installation?
It will be nowhere near 100A - the usual 25mm² will be satisfactory.

More worrying will be if you conceal the tails <50mm. from a surface, you will require a 30mA RCD to cover them which is undesirable.

 

[Malc]

Meter tails are in cavity until floor void so >>50mm. I need (want to) install a 100mA Time delay in meter box as this is TT. Safer to have some cut out I feel for long meter tails and that also should satisfy DNO and they have a short run to my RCD.

4DA1 is single core cable table so 2 cables is 2 single cores but yes it will be one single cable per conduit.

Wall construction is like this

outer block / cavity (filled) / inner block/ dabs / polystyrene+plasterboard.

The plasterboard comes with the polystyrene bonded to it and it is dabbed on the wall. Cable heat has to go forward into room through the polystyrene on backwards to outside via filled cavity. Hence 103 since A isn't quite what it says. You need a conduit because of the polystyrene of course which PVC doesn't like.

 

[EFLImpudence]

I need (want to) install a 100mA Time delay in meter box as this is TT.

Ok but that's a different matter.

Wall construction is like this
outer block / cavity (filled) / inner block/ dabs / polystyrene+plasterboard.

Clip cables to the inner block before dabbing or even chase them in.
Does that include the tails?

The plasterboard comes with the polystyrene bonded to it and it is dabbed on the wall. Cable heat has to go forward into room through the polystyrene on backwards to outside via filled cavity.

It will just go into the inner block.

Hence 103 since A isn't quite what it says.

Clipped to inner block will be 102; in conduit next to block will be A. Chased in will be C.

You need a conduit because of the polystyrene of course which PVC doesn't like.

You could always cover the cables with a sheet/strip of protecting material.

 

[Malc]

It won't be 102 because that says "with the cable touching the inner wall surface" i.e. back of the plasterboard. I did consider dabbing them it but method C say only applies to wall < 2km/W (>0.5 W/mK) and celcons are 0.15 W/mK.

 

[EFLImpudence]

It won't be 102 because that says "with the cable touching the inner wall surface" i.e. back of the plasterboard.

That usually applies to stud walls with both sides of plasterboard.
Touching the blocks will be better than that.

I did consider dabbing them it but method C say only applies to wall < 2km/W (>0.5 W/mK) and celcons are 0.15 W/mK.

Ignore the celcon if it is touching or in the blocks.

 

[Malc]

Plaster is ~0.45 W/mK thermal conductivity celcon aerate concrete blocks are 0.15 W/mK so a lot lower conductivity, below the 0.5 W/mK required (and so is plaster but I guess that is near enough to be ok).

 

[EFLImpudence]

Ok. So, you'd better calculate the temperature which all your cables are going to attain.

 

[ban-all-sheds]

I think that right now would be the time to get the electrician who will be moving the CU involved.