How to avoid insulation under the floor?

[DMRW]

Domestic work not my usual field however a "friend" is having their extension done and asked me to help with wiring etc.
I was double checking what size cable for the cooker point and he said that above the ceiling he has 6" joists and will be having 6" of insulation squeezed in. How do I get around the severe de-rating factor that brings? Don't fancy making 16mm of into a cooker point! It appears to me part P and the new part L regs don't agree.
HELP

 

[Darkwood]

If you cant avoid the insulation then im afraid you have 3 options ...

-MICC .... a dying skill but would see vast reduction in size
-Do the run in 16mm then derate from cooker switch to outlet Your front end device will still be protecting the smaller size cable but i would note it on the cert'.
-Tell your friend your busy and unfriend . him on FB

 

[Octopus]

What has Part P and Part L got to do with it?

 

[ample current]

Would it be possible to run the cable under the insulation as apposed to in it and de-rate the cable using reference method 101

 

[DMRW]

Thankyou for your answers.
Mi would be perhaps a little unorthodox for a domestic extension although a little tempting. And with regards to unfriending him. I'm on loan to an estates department and am sort of paired up with him as he was historically the lamp changer. Might make work a little awkward. Running in 16mm and derating not a huge fan of that but could be the solution.
Murdoch - I know part p is mostly about registration and competency but I assume please correct me if I'm wrong that it states work should be carried out in line with bs7671? Which gives the derating factors etc. And is part L not all to do with energy efficiency and insulation values?
Ample current - running the cables under the insulation gives me two concerns
1st wouldn't that be very similar to running in insulation anyway. Its going to be squeezed in as the joists are a few mm smaller than chosen insulation.
2nd getting through the joists has to be done 50mm from top or bottom bringing my cables out the joist into the centre of the insulation.
Opinions?

 

[jaypp]

if you haven't done cable calcs yet, what length of run, rating of appliance, grouping etc.? is it flat roof extension, any other poss routes?

 

[DMRW]

It is a dorma bungalow being extended on the side with garage utility etc downstairs master bedroom upstairs in the roof. Haven't been to job yet but have plans. From plans and discussions with client it seems every suggestion I put to him has insulation added on top. Oh I plan to make that a room at the end. Or its involved in the extension I'll have to insulate under the floor there too.
Length of run obviously depends on final route but the best route discounting insulation is 15m. Its for a general cooker point. Which means I want it as high a rating as possible. The board I'm coming off can have either a 40a or 50a breaker fitted. 50a supply to cooker will be unachievable so was hoping to be able to use the 40a breaker.

 

[DMRW]

Also regards grouping I believe I should be able to keep it separate.

 

[jaypp]

i think 16mm is good for around 20kw using info supplied (won't take long for someone to correct me if wrong!) which seems rather alot. did you apply diversity? hope u get it sorted.

 

[Geoffsd]

10mm is good for 32A with maximum derating (103).

 

[diditrain]

How much head height have you got? Could you clip to the underside of the joists and then batten out across the bottom of the joists.

 

[Engineer54]

Go and LOOK at the place yourself first, before you start making assumptions of what can and can't be done. I'd be surprised if the vast majority of this cable run, can't be run outside of any insulation in a bungalow type installation. Maybe even take a few photo's that you can post up here...

 

[diditrain]

He could always use some cornice trunking