sub main

[jam2020]

hey guys,

ive got to put sub mains into 17 new build bungalows. the DNO supply is external about 7m from consumer unit.normaly i would put 16mm swa sub main with 80a fuse to protect it. the trouble is it is the insulation will be>100m fibre glass. what is the current carrying capacity of the swa? if i duct it across the roof space will this help? really dont want to use 25mm if i can help it. cheers guys

 

[Zeespark]

BRB will say.

AndyGuinness

 

[jam2020]

been looking at table 4d4 but only gives instalation methods c,e,d i want it for reference method 101.

 

[Zeespark]

Can you clip/cleat the joists to the roof timbers? that way I think (dont qoute Me) that that is reference method A.

AndyGuinness

 

[jam2020]

clipped direct is ref method c. but does this not appy to just twin and earth? if i put it on a cable tray so its in free air it becomes ref method e and capacity becomes 97amps

 

[Zeespark]

Reference methods, as far as I am aware, are applicable to all cable types.

AndyGuinness

 

[jam2020]

methods 100,101.102.103 are specifically for flat twin and earth cables in thermal insulation.i can only find installation methods c,e and d for swa.

 

[septiclecky]

Have got to ask why are you putting in the mains to the bungalows and not the DNO?

 

[jam2020]

asked the builder that very question!! answer i got was thats how we do it. dno supply is in a external box and cosumer unit in a cupboard inside. therfore i have to run a sub main to it. can i ask what would your opinion be septiclecky?

 

[Chr!s]

First of all id do my up-most to keep the cable away from any insulation.

Do you have to use swa ?

If you do then id contact the cable manufacturer and ask them for the de-rating figure.

 

[jam2020]

if i dont use swa then there is no mechanical protection therefore rcds are required,as it will have to drop down a wall at the point where meter cupboard is.

 

[septiclecky]

asked the builder that very question!! answer i got was thats how we do it. dno supply is in a external box and cosumer unit in a cupboard inside. therfore i have to run a sub main to it. can i ask what would your opinion be septiclecky?

Just confused from your original post thought that they wher putting a supply to a central point then you where running seperate supplies to each bungalow.

You will need to fit a seperate Fused switch at the DNO end to protect the swa if more than 3m but will need to be at least 25mm .

 

[jam2020]

no this is each bungalow. if i can't ensure there is no insulation touching the swa i think i will have to go 25mm but i would really like to know the ccc of this but just cant find it in 7671.as ive stated before it only gives ref c,d and e.any ideas where i can find this???

 

[kme]

you will NEVER find method 101 for SWA, because 101 is ONLY for T+E; which isn`t SWA. You simply use the derating calcs given at the start of appx4....

HTH

KME

 

[Zeespark]

Sorry for the mix up KME, I dont have My BRB close to hand, will keep schtoom from now on:Salute

AndyGuinness

 

[Larnacaman]

jam2020,

It is not obligatory to use the Regs cable current carrying tables, they are there as an overall guide only.

If your cable manufacturer has a BS approval for its cables, (and it probably will) you are quite at liberty to use the cable data published by the manufacturer. Design Engineers rarely use cable tables in the BS 7671...

 

[jam2020]

so whats the answer lads? im looking at 25mm now

 

[Larnacaman]

As previously suggested by others, clip or cable tray your SWA cable above any house insulation material in the loft space....

Cables have to be surrounded by insulation to be de-rated, no problem with just touching it on one side....

 

[kme]

Zee - wasn`t having a go at you mate......

The O/P appeared to be asking for the table for ref. method 101 for SWA......

 

[jam2020]

ok so if i put 16mm on cable tray above insulation cable capacity goes up to 94a ( table 4e4 colum 3) would everyone agree with this??