This is a question for the electricians here.
In an earlier thread of mine, 'Earthing Problem', the matter of rewiring
the lighting circuit at my son's recently acquired bungalow came up, because
it was found that there was no cpc to the lighting circuit. I am now about
to begin to do this, but I have some concerns about the regs regarding
running cables under insulation in the loft. My son has just mentioned to me
that he intends to double the thickness of the insulation up there -
presently 3 to 4 inches. I was wondering what the implications might be in
this case with regard to cable type. In the earlier thread, 'jsb1' mentioned
the use of 'Prysmian' cable where it is to be 'surrounded by insulation'.
Because I had intended to run the cable just along, and attached to, the
joists I have just bought 'ordinary' pvc 1.5mm twin and earth. This question
of the extra insulation has caused me to think that this cable might not now
be suitable. Any advice anyone, please?
I'm still struggling to find out what kind of cable I need for a 9.5kW
shower - with a run of about 10 metres across a loft with at least 200mm of
insulation. Will 'ordinary' 10mm pvc T&E suffice, or do I need to use this
I can probably run most (if not all) of it across the floor joists. But I'm
not completely sure that when extra insulation is installed - which is
intended - whether or not the floor joists will be covered too.
Can you lift the cable so it sits on top of the insulation ?
Yes the insulation will run across the joists, generally you fill the
gap (100mm say) then run insulation perpendicularly across the joists
for the remaining depth which brings you to 300-320-380mm or whatever
they want these days.
Is there a clear and obvious path for the cable?
- You could run elevated battens with the cable clipped to that, as a
route or near the insulation-free eaves
- You could use PIR insulation for one area, thereby reducing the
depth but not insulation (as one would do for boarding out).
- You could run the cable internally, although that may not be
Remember not to plug the eaves with insulation, the roof must retain
You could leave plenty of slack on the shower cable if you are installing it
before the new insulation goes in.
That will help when your son installs the new insulation as he can pass the
new insulation under the slack. Any excess slack can be taken out when the
new insulation is in place.
Top tip for your son. The easiest way to install new loft insulation it is
ask your Dad to do it:-)
And good luck with the lighting rewire and the new shower cable.
Remember to stick a double pole switch somewhere for any outside lights you
are rewiring. You have already said in your previous posts that your son has
a new 17th edition CU and the last thing you want is an outside light
letting in water to cause all the circuits on that RCD to keep tripping out.
I've bought it already, I'm afraid. It's not too bad, though, because having
that amount will give me more options. I can also replace and re-route the
cooker cable, which is stretched pretty tightly across the loft.
I'll probably post back, but it certainly won't be to say I've installed his
loft insulation - he can do that himself!
You are referencing "Need Prysmian" & "Insulation"... you mean using
6242YH is BS6004 70oC PVC insulated.
6242BH is BS7211 90oC XLPE insulated.
The "H" means Harmonised colours re Brown/Blue replacing Red/Black for
If the cable is run against insulation such that one SIDE contacts
plasterboard then normal cable is fine, because it can lose heat to
the non-insulated side. If however the cable is run THROUGH insulation
then 6242BH is used which has a higher temperature limit and thus
maintains more Currency Carrying Capacity (CCC) after derating for
insulation & temperature.
6242YH v 6242BH for example...
- 1.5mm - 20A v 24A clipped direct
- 2.5mm - 27A v 33A
- 4.0mm - 37A v 45A
- 6.0mm - 47A v 58A
- 10.0mm - 64A v 80A
- 16.0mm - 85A v 107A
Screwfix sells 6242BH in 1.5mm, 2.5mm & 6.0mm. It may be Prysmian, but
you can use anyone's BS7211 and to be honest you may prefer to do so
because Prysmain's 6242BH has an extremely stiff cable sheath &
extremely stiff conductor insulation - it is very hard to work with
compared to other generic brands I have handled.
For light final circuit cables through current insulation it is
typically advised to use 1.5mm 6242BH - that has sufficient CCC for
all possible scenarios. For a shower you may need to use 16.0mm 6242BH
which is unwieldly and may not even fit the wiring accessories.
Note you can NOT assume a wiring accessory can handle 90oC, that is to
say you can not fuse 2.5mm 6242BH at 32A - it must be fused to 20A (or
25A if clipped direct and you can find a 25A breaker/fuse). The
purpose of the 90oC limit is to permit a "higher starting point" for
grouping, insulation & temperature derating calculations so you stay
above (say) 16A per leg of a ring final circuit.
Now, if you are NOT running cable THROUGH a body of insulation you do
not have to derate so severely. Indeed some people run the cable on
top of the insulation so it is at least visible and derate for the
cable penetrating down thro the insulation depth. I think current regs
for loft insulation are 380mm or such like, so the calculation would
You need a copy of the IEE OnSiteGuide - that will answer your
questions re calculations (cable thro insulation). Amazon UK sell it
cheaply, well worth buying a copy.
Thanks for that very comprehensive reply - although I'm not sure that I'm
not even more confused now!
The cable I was thinking about was Prysmian 6242YH. I can run it largely
clipped to the joists - and where I can't directly in contact with the
plasterboard, so will 10mm BS6004 PVC suffice?
Correction, from OSG 17th...
p52 & 53 are of particular interest to you.
Installation Method for BS6004 70oC (6242YH).
C = Clipped Direct
Cable CSA 1.5mm = 20A, 10.0mm = 64A, 16.0mm = 85A
100 = Cable CONTACTS plasterboard ceiling or joists covered in thermal
insulation NOT exceeding 100mm.
Cable CSA 1.5 = 16A, 10.0mm = 45A, 16.0mm = 57A
101 = Cable CONTACTS plasterboard ceiling or joists covered in thermal
insulation EXCEEDING 100mm.
Cable CSA 1.5 = 13A, 10.0mm = 36A, 16.0mm = 46A
103 = Cable SURROUNDED by thermal insulation with cable NOT touching a
Cable CSA 1.5 = 8A, 10.0mm = 23.5A, 16.0mm = 42.5A.
Domestic lighting using 6A MCB/RCBO...
- 1.5mm 6242YH is sufficient under install method 103 (8 Amp)
- Assuming the cable is not grouped with other light/power circuit
- PVC is ok - a 10A lighting circuit would require 6242BH (XPLE)
Domestic 11.5kW shower using 50A MCB/RCBO...
- 16.0mm 6242YH is insufficient under install method 101 or 103
- 6242BH (XLPE) using 10.0mm cable would be necessary
So generally you try to prevent cable being covered with thermal
Particularly when you price out a reel of 10.0mm 6242BH :-)
Indeed. The cost factor has not really been considered, but - as you say -
the cost of a reel of 6242BH is quite an incentive to get round its use if
at all possible!
Looking at the above, I cannot see that 6242YH clipped to the joists with a
50A MCB (which is what I have) should not be perfectly acceptable for a
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