Spay Foam in Stud Wall

[abacusf]

Hi Guys,

Quick sanity check please.

I'm wiring a conversion which has spray foam insulation on all the outside walls (around 50-75mm thick) between studs. I've not come accross this stuff before.

My ring main will be clipped direct under the suspended floor, but will run for approx 500mm in the foam. I've dug a channel in the foam for the 2.5mm t and e. This will then be plasterboarded over.

Now I've decided that as the cable will prbably not be touching the plasterboard, and its enclosed in thermal insulation I should derate the cable to 13.5A, so I'm going to use a 20A MCB. Does this sound reasonable?

So in summary I've decided its Method 103, but its not really as the insulation doesn't serround the cable, but its not 102 because the cable doesn't touch the plasterboard, thoughts?

thanks

 

[Jonspark]

IS this a ring or radial final circuit?

if its a ring i'd just but it on a 32a rcbo

 

[Spoon]

Can you not put the 500mm run in conduit, then it would be ref method A? Better than method 103.

 

[abacusf]

Can you not put the 500mm run in conduit, then it would be ref method A? Better than method 103.

Method A is for singles as far as I'm aware, not T&E

 

[abacusf]

IS this a ring or radial final circuit?

if its a ring i'd just but it on a 32a rcbo

Thanks for the replay.

Its a ring.

Why would an rcbo be better than an MCB with 30ma RCD?
Would you derate to 21A then and treat it as method 102 ?

thanks

 

[Darkwood]

IS this a ring or radial final circuit?

if its a ring i'd just but it on a 32a rcbo

How would an RCBO get around any regulations for de-rating cables due to thermal constraints?

 

[Jonspark]

How would an RCBO get around any regulations for de-rating cables due to thermal constraints?

? strange,never said it would.

 

[Darkwood]

? strange,never said it would.

Sorry if I misinterpreted it just way it came across as an answer to the OP question....curious as to why you put if it wouldn't comply?

 

[Andy78]

Method A is for singles as far as I'm aware, not T&E

No, it's for multicore too.

 

[Engineer54]

If this a general domestic socket ring circuit and not a kitchen ring, then the chances of that cable being subjected to anywhere near it's current rating is slim to remote. Personally i wouldn't bother applying any de-rating factor....

 

[Darkwood]

If this a general domestic socket ring circuit and not a kitchen ring, then the chances of that cable being subjected to anywhere near it's current rating is slim to remote. Personally i wouldn't bother applying any de-rating factor....

Couldn't agree more but you either go by the book or you don't ...and who's to say the boiler won't fail meaning they plug in a few convection heaters and do load the circuit to the hilt!.. but yes know where your coming from.

 

[Engineer54]

Couldn't agree more but you either go by the book or you don't ...and who's to say the boiler won't fail meaning they plug in a few convection heaters and do load the circuit to the hilt!.. but yes know where your coming from.

For the sake of 500mm, i doubt if it'll make the slightest bit of difference, in your above scenario to be honest...

 

[Darkwood]

For the sake of 500mm, i doubt if it'll make the slightest bit of difference, in your above scenario to be honest...

As the OP has cut a channel anyway must agree and the operating temp# of the cables insulations is 70c then we know common sense prevails here but was just saying and if we are to believe the issues with said domestic rings in thermal and assume they are not worse case scenario's then 500mm is 500mm and is a weak link....

 

[abacusf]

Thanks for all the replys. I think I'll treat it as 102 and derate to 20A and use a 32 a mcb as normal. On the other circuits I'll pack the cable out to touch the plasterboard.

I don't really understand why touching a non heat conducting surface can affect the current carrying capacity though (as opposed to not touching, not as opposed to buried in insulation)

 

[Tuttle]

Thanks for all the replys. I think I'll treat it as 102 and derate to 20A and use a 32 a mcb as normal. On the other circuits I'll pack the cable out to touch the plasterboard.

I don't really understand why touching a non heat conducting surface can affect the current carrying capacity though (as opposed to not touching, not as opposed to buried in insulation)

If plasterboard was non heat conducting then there would be no need for insulation behind it :smilewinkgrin: