Cables and 17th Edition regs

[Farmer Giles]

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?

[Andrew Gabriel]

If you are using 1.5mm cable on a 5A or 6A fuse/MCB, the cable is sufficiently overrated that you can ignore the problem.

[Farmer Giles]

Thanks for that. I'm also going to replace his shower with 9.5kW one. In doing so I will replace his existing 6mm cable with 10mm - will I need 'Prysmian' cable for that?

[Farmer Giles]

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 Prysmian stuff?

[ARWadsworth]

Is there any reason why you cannot keep the shower cable above the new loft insulation?

Adam

[js.b1]

You are referencing "Need Prysmian" & "Insulation"... you mean using

6242BH?

6242YH is BS6004 70oC PVC insulated.

6242BH is BS7211 90oC XLPE insulated. The "H" means Harmonised colours re Brown/Blue replacing Red/Black for FTE.

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 be 400mm.

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.

[js.b1]

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 wall. 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 insulation. Particularly when you price out a reel of 10.0mm 6242BH :-)

[Farmer Giles]

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?

[Farmer Giles]

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.

[Farmer Giles]

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 9.5kW shower.

[js.b1]

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 practical

Remember not to plug the eaves with insulation, the roof must retain cross ventilation.

[Farmer Giles]

I can run it across, and clipped to, the roof rafters - is that permitted?

[js.b1]

Yes, although most people just lay it on top of the insulation. Just ensure the route renders it out of reach (2 inch) of anyone doing tiles etc.

[ARWadsworth]

Yes.

Adam

[Farmer Giles]

Splendid, I have my solution - either lay it on top of the insulation or run it across the rafters. Many thanks to you both for the great advice/help.

Very much appreciated.

[ARWadsworth]

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.

Cheers

Adam

[Farmer Giles]

Yes, that seems a good idea. Particularly in view of the fact that the run is about 10 metres, and we need to buy 25 metres of cable.

Don't give him any more ideas in that direction! ;-)

Will do. Thanks once again, Adam, for your superb advice.

[ARWadsworth]

Lots of wholesalers will cut you a length of 10mm cable. It could work out cheaper than buying a 25m drum.

OK! And don't forget to post back after you have installed the loft insulation for him:-)

Cheers

Adam

[Farmer Giles]

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!