Large house 2.5mm or 4.0mm

[grandfortune]

Hi all,

Used to doing small 2 bed and 3 bed new builds so cable sizing is never an issue

got a huge 7 deb house 5 floors to do in the next 6 months, had a quick look and now wondering what cable size to use, before i get the book out i thought id post here for more advice

4.0mm legs for ring then 2,5 or 4,0mm all the way, seems to be quite a bit more expensive than 2,5mm also on the lighting 1.5mm to feed the first point then 1.0mm after.

or other option is radials, seem to be getting more common now and some favor over rings.

putting a large 24 way board in for the extra rings and lighting circuits

any advise would be appreciated , also told me he wants the cables capped ggrrrr, you guys use oval tube or channel for this these days?.

Cheers
Grand

 

[Leesparkykent]

The only real way is to put pen to paper and work it out. A RFC in 2.5 the maximum run is about 98meters if you base it on the guidance of 26A from the OSG (20A shared equally and 6A at the furthest point). 0.47 Ohms

A RFC in 4mm based on a design current of 26A is about 160 meters. 0.67 Ohms

4mm 32A radial ( not many installation methods you can choose being on a 32A OCPD) based on the value of the OCPD is about 32 meters. 0.53 Ohms

4mm radial 20A OCPD and design current based on the value of the OCPD is about 52 meters. 0.87 Ohms

2.5mm radial 20A OCPD and design current based on the value of the OCPD is about 32 meters. 0.62 Ohms

2.5mm radial 16A OCPD and design current based on the value of the OCPD is about 40 meters. 0.78 Ohms

Then you have installation method and CCC of cable to be used to take into account.

 

[Ian1981]

Depending on were db is situated you might be able to use 2.5mm for the shorter distance from db ring circuits and may need 4.0mm for the further away ring finals.

 

[Scott48]

Buy bigger db use more rings in split house up and use 2.5 making 2 x 4.0mm into a
Socket is a nightmare

 

[Deleted member 9648]

Buy bigger db use more rings in split house up and use 2.5 making 2 x 4.0mm into a
Socket is a nightmare

Only for lazy gits who flush in 25mm boxes without any thought beyond their own installation. 4.0mm in a 35mm box is easier than 2.5 because it's more flexible, and a 35mm box allows for future changes of accessories to flat plate etc.
To the OP regarding lighting. I never use 1.0mm now even on small installations due to the reduced allowable volt drop on lighting circuits. 1.5mm all the way, even if the circuit is standard enough to not require any calcs.

 

[telectrix]

Only for lazy gits who flush in 25mm boxes without any thought beyond their own installation. 4.0mm in a 35mm box is easier than 2.5 because it's more flexible, and a 35mm box allows for future changes of accessories to flat plate etc.
To the OP regarding lighting. I never use 1.0mm now even on small installations due to the reduced allowable volt drop on lighting circuits. 1.5mm all the way, even if the circuit is standard enough to not require any calcs.

but with the increaded use of LED lighting, load is far less, so 1.0mm is easier to terminate directly into downlighters (loop in/out. and VD is not a problem.

 

[Ian1981]

Well not once have a done a new build or rewire and thought oh I'll make this harder for myself and fit 35mmback boxes to the sockets and chase the brick out to do it ! When it's unnecessary.

Fused Spurs perhaps but not sockets

 

[LeeH]

Cannot you use smaller sub boards to reduce the final circuit lengths?

 

[grandfortune]

builder(boss) doesn't like the idea of a sub main box,

thanks for the replys guys, done a bit of reading , looks like 40.mm ring, for top floor and may be 3rd floor, seems strange to change down to 2.5mm, will look at costings and probably stick to 4.0mm all the way. will use 1.5 for main feed to lighting then 1.0mm, never have an issue with terminating 2x 4.0mm into socket but 1.5mm is a killer as i do tend to have alot at the switches.

cheers
grand

 

[Deleted member 9648]

Well not once have a done a new build or rewire and thought oh I'll make this harder for myself and fit 35mmback boxes to the sockets and chase the brick out to do it ! When it's unnecessary.

Fused Spurs perhaps but not sockets

The end user wont thank you when they want to fit flat plate sockets. It's laziness.

 

[Deleted member 9648]

but with the increaded use of LED lighting, load is far less, so 1.0mm is easier to terminate directly into downlighters (loop in/out. and VD is not a problem.

True....but I still prefer to use 1.5mm.
The only time I go down to 1.0mm is smokes. The Aico alarms we use are a pig with two 1.5 3 cores.

 

[Deleted member 9648]

Cannot you use smaller sub boards to reduce the final circuit lengths?

A certain 'Engineer' and former forum stalwart would have torn you to shreds for such heresy! More than one DB in a domestic? The Horror..........

 

[Deleted member 9648]

To those who disagreed with post #10 can you tell me what you'd do if you went back to a job where you'd installed 25mm boxes and were asked to fit flat plate accessories which normally require 35mm deep boxes?

 

[Ian1981]

To those who disagreed with post #10 can you tell me what you'd do if you went back to a job where you'd installed 25mm boxes and were asked to fit flat plate accessories which normally require 35mm deep boxes?

What I can tell you is if I'm doing a Standard new build and its specified as standard sockets , then I'm not going to then fit bigger boxes thinking that one day someone might want to alter it.
Like wise were i wire the sockets someone might one in the other corner of the room one day but you don't fit a socket in every corner of every single room , you go by the drawings or any extras.

If the back box needs altering then it needs altering.
It's installed to the standard it needs to be.
I sometimes wire loop in loop out at switches I don't then think what if someone wants to wire a supply from the centre room light but there's no permanent mains to it.

If I went back then I'd charge for the privilege of altering it.

Not every person wants flat plate accessories do they?

 

[LeeH]

A certain 'Engineer' and former forum stalwart would have torn you to shreds for such heresy! More than one DB in a domestic? The Horror..........

Why? What was the logic?