Insulation/Builder sabotage

[Jon747]

Good evening everybody. I've often read through these forums but never had call until today to sign up and beg for some advice! Any help would be much appreciated!

I have been working on a new domestic installation. First fix was completed before christmas. Today I returned - cut my downlight holes and discovered that the builders have filled the first floor void with 200mm of thermal insulation. The question was asked beforehand and I was informed that the void was not to be filled - this has been conveniently forgotten now though. The building is now plastered and decorated.

I have worked out a way to rewire the hob circuit, oven circuit, kitchen sockets and utility sockets circuit so most of the high load circuits have been taken care of. With the clients consent the insulation has been removed from where cables are grouped on return to the consumer unit.

For the rest though I have to work on the assumption that they are reference method 103 and that at least in part they will be grouped together (passing through joists and the like). This is of particular concern with two circuits:

A first floor sockets circuit (supplying three bedrooms, 12 sockets only, no heating/installed loads) -2.5mm ring final
A ground floor sockets circuit (supplying lounge and dining room, 5 sockets, no heating/installed loads) - 2.5mm ring final

Reference method 103 - 2.5mm current carrying capacity = 13.5 amps
Derating factor for grouping- 3 cables 0.70 = 9.45 amps

Would 16a ring final circuits be acceptable/safe in this instance do you think? Do you think I could argue that 16 amps is enough capacity to supply three bedrooms?

I would be much obliged for any help/advice. Worrying myself sick over this.

 

[davesparks]

Why are you applying a grouping factor to domestic socket circuit cables? In fact why would you apply it to any domestic cables? They are not going to be running at anywhere near FLC for any length of time are they.

Also method 103 is for cables in an insulated stud wall, you are talking about cables in an insulated ceiling void, method 101 would be far more appropriate.

 

[Jon747]

Reference methods 101, 101 and 102 require the cable to be in contact with the plasterboard or the joists. Given that I haven't seen how the insulation is installed across the whole building I think 103 - i.e. surrounded is prudent. I was applying grouping because BS7671 says that when enclosed in thermal insulation you should!

 

[Engineer54]

I'd go along with davesparks remarks here. Domestic installations don't pull enormous loads at the best of times, even with storage heater heating....

 

[davesparks]

Reference methods 101, 101 and 102 require the cable to be in contact with the plasterboard or the joists. Given that I haven't seen how the insulation is installed across the whole building I think 103 - i.e. surrounded is prudent. I was applying grouping because BS7671 says that when enclosed in thermal insulation you should!

I will admit I don't know the regulations off by heart and may be wrong but I am pretty sure that grouping factors are not affected by the presence of thermal insulation, that attracts it's own separate de-rating factor.

 

[telectrix]

and you seem to have forgotten that these 2 circuits are rings, so your calculated Iz will be for each leg.

 

[driverman]

How was the insulation installed that you have seen / removed? Was it wrapped round the cables or installed on top / below the cables. If causing you concern put ring on 20amp MCB / RCBO. 16amp would also be OK if you knew that the expected load would be OK. As stated above your calcs are incorrect.

I wouldn't worry about the grouping factor in your situation.

 

[telectrix]

as above. if you have concerns, put the RFCs on 20A breakers.

 

[Geordie Spark]

as above. if you have concerns, put the RFCs on 20A breakers.

.......... or make good use of your wrecking bar ........... :devil2:

 

[telectrix]

typical geordie. hit it or squash it. :49:

 

[driverman]

Have you already rewired both cooker circuits, kitchen & utility rings? Have you done calcs for those circuits? Cooker circuits OCPD devices could be down-rated if known load. Seems a lot of work to achieve very little. Whilst I appreciate you want it to be correct and tick all the boxes etc, I'm wondering if you are over thinking this a bit.
A tip in my opinion would be in any other new build /rewire that you do, would be to install radial circuits for the heavier loads ie washing machine,dish washer,tumble etc.

 

[marc8]

With regard to your downlights being covered with insulation I always put down in my quote in bold text any covering with insulation could invalidate manufacturers recommendations & could cause a fire risk & they take it upon themselves. It seems to do the trick with downlights.