Hi!
I have a problem with derating factors for cables going through phenolic thermal iinsulation:
I have been wrestling with my 17th edition: I am rewiring a house with 17century solid brick walls (no cavity). We are refurbing it, (electively), and want to hit refurb U values for heat throughout: so are drylining external walls with Kingspan (i.e .phenolic foam): and re insulating the
attic room, which had a joke of a conversion done inthe 90s: no insulation (got building regs though !!!!)
Going a little over board with the insulation as the marginal cost to over egg it is slight: Ill never come back and do it again. so: we are putting 50mm celotex + 77 mm kingspan (i.e. 12 mm plasterboard bonded to 65mm phenloic foam if i remember rightly), "dot and dabbed" into alcoves... and behind this is an ideal place to run cables, clipped tothe external brick wall (i.e. insulation one side, solid 9 inch brick the other). Reading my regs carefully : if the lighting is done in 1.5mm: all is fine: it could be derated 50%; and I'm fine for a 6A MCB. 2.5 mm ring mains is the difficult case. nonethelss; I can apply installation method 102, p265.. and table 4D5 p282 says I'm fine...
so ... next case to be mentally ticking off is... if I go through say 100mm of this stuff anywhere; how do I derate the cable ... ? (e.g. when coming from behind the board to a plasterboard double 13 socket/ patress)
p104 reg.523.7 & table 52.2 looks good for this : "cable surrouned by thermal insulation".. BUT: it has the caveat "for insulation having thermal conductivity greater than .04 W/m/K...
BUT Kingspan/ celotex is .022 or so. Holy Crap !!does this mean that to go through as little as 1 cm I have to derate "50%" ...? this would be mad: if i do that: I can't use a 32A MCB: (2.5mm -->13.5A..., 4mm is still not enough...... so 6mm ring .. ?!?!?? crazy !)
It seems bizare the 17th edition as far as i can see almost pretends that ceotex/ kingspan deosn;t exist, yet it is now very very common as the only practical way to hit the modern thermal efficiency targets.
Any suggestions as to what regs allow me to sanely justify what I should be doing ?
As i read it; as long as the cable is "behind" the insulationl; withinsulation on one side, the standard rule of thumb of "2.5mm ring / 32 A mcb " for sockets holds. as soon as i go through a slab of this stuff I'm screwed.... ?!?!?
what would you all do ?
I have a problem with derating factors for cables going through phenolic thermal iinsulation:
I have been wrestling with my 17th edition: I am rewiring a house with 17century solid brick walls (no cavity). We are refurbing it, (electively), and want to hit refurb U values for heat throughout: so are drylining external walls with Kingspan (i.e .phenolic foam): and re insulating the
attic room, which had a joke of a conversion done inthe 90s: no insulation (got building regs though !!!!)
Going a little over board with the insulation as the marginal cost to over egg it is slight: Ill never come back and do it again. so: we are putting 50mm celotex + 77 mm kingspan (i.e. 12 mm plasterboard bonded to 65mm phenloic foam if i remember rightly), "dot and dabbed" into alcoves... and behind this is an ideal place to run cables, clipped tothe external brick wall (i.e. insulation one side, solid 9 inch brick the other). Reading my regs carefully : if the lighting is done in 1.5mm: all is fine: it could be derated 50%; and I'm fine for a 6A MCB. 2.5 mm ring mains is the difficult case. nonethelss; I can apply installation method 102, p265.. and table 4D5 p282 says I'm fine...
so ... next case to be mentally ticking off is... if I go through say 100mm of this stuff anywhere; how do I derate the cable ... ? (e.g. when coming from behind the board to a plasterboard double 13 socket/ patress)
p104 reg.523.7 & table 52.2 looks good for this : "cable surrouned by thermal insulation".. BUT: it has the caveat "for insulation having thermal conductivity greater than .04 W/m/K...
BUT Kingspan/ celotex is .022 or so. Holy Crap !!does this mean that to go through as little as 1 cm I have to derate "50%" ...? this would be mad: if i do that: I can't use a 32A MCB: (2.5mm -->13.5A..., 4mm is still not enough...... so 6mm ring .. ?!?!?? crazy !)
It seems bizare the 17th edition as far as i can see almost pretends that ceotex/ kingspan deosn;t exist, yet it is now very very common as the only practical way to hit the modern thermal efficiency targets.
Any suggestions as to what regs allow me to sanely justify what I should be doing ?
As i read it; as long as the cable is "behind" the insulationl; withinsulation on one side, the standard rule of thumb of "2.5mm ring / 32 A mcb " for sockets holds. as soon as i go through a slab of this stuff I'm screwed.... ?!?!?
what would you all do ?