Extension - Sub Mains DB?

[brizospark]

I went to see a potential job today where there is two be a two floor extension built onto a medium sized detached house. Extension rooms to consist of lounge, en-suite (no electric shower), 2 x bedrooms and dressing room so potentially no serious load.

Customer has had 17th ed. DB installed previously, all looks good, only 1 spare way in board though.

Only real problem is that the DB is at the opposite end of the house to which the extension is to be built and it is a bit of a nuisance of a run to get to.

My thinking was that instead of installing larger board or installing separate board next to existing board it would be easier to run a 10/16mm cable to a sub mains board located within the extension itself. Not really seen this done in a domestic property that many times but whats you guys thinking on this, good practice? Also any advice on best way to provide o/l, rcd protection in this instance would be much appreciated as I really dont want to protect the whole new board via 1 x 30mA RCD as nuisance tripping may be a problem

Thanks in advance lads

 

[GLENNSPARK]

take a sub out 10mm 3 core armoured will be enough.....
whats the length of run & earthing arrangements please?
you can either break out of the tails into a switchfuse ...then into an adaptable/IP box for the armoured to gland off into....but i doubt you will need this arrangement as you have a spare way in main board ....so use that.....gland the armoured off into the bottom of board 1...give it 50A (it`s unlikely to ever get there but the cable will cover it so...)....bottom end again its straight into the bottom of DB1...
in fact no...give it 45A....

 

[GLENNSPARK]

we need the earthing arrangements as this will dictate what`s going on top end (in regards to RCDs)....

 

[Guest55]

i'd just extend the existing lighting and socket circuits to cover the new rooms

therefore no subs , no extra CU , no new circuits , no spare ways needed.

;-)

 

[GLENNSPARK]

if its a TN then you may have to configure the main board with the spare way unprotected for the swa sub....RCD bottom end for finals...

- - - Updated - - -

i'd just extend the existing lighting and socket circuits to cover the new rooms

therefore no subs , no extra CU , no new circuits , no spare ways needed.

;-)

well so would I but he was asking about subs biff...so i told him...

 

[brizospark]

Biff, Not keen on extending existing circuits as there is an additional floor to be built so would prefer to split the circuits up. Glenn the earthing arrangement is standard TN-C-S with Ze of 0.11

 

[brizospark]

If I was to just extend circuits then ring circuit would have 30 x double sockets, this to me is very bad practice. I would like to do the job properly and have the circuits split up into reasonable sizes. The last thing I want is something like the original part of the house having a problem, tripping RCBO and then being called because extension sockets not working. COme on who wants their whole house socket outlets covered by one circuit. Same goes for lighting circuits

 

[GLENNSPARK]

Biff, Not keen on extending existing circuits as there is an additional floor to be built so would prefer to split the circuits up. Glenn the earthing arrangement is standard TN-C-S with Ze of 0.11

well then you come off an unprotected way in the main board.....RCBOs in db1...you can use one of the cores to extend the equepotential zone with 10mm 3 core swa.....
give em 16 A for the downstairs sockets in the extention, 16A for the up sockets and 6A for up lights and again 6A for the down lights....
so a 6 way board for db1 will do it....

 

[brizospark]

I was thinking - galv 4x4 under existing board. 10mm SWA into galv box (earthed) and into spare MCB at DB (non RCD protected side of board) other side SWA into metal consumer unit. 5 x RCBO's - upstairs sockets, downstairs sockets, upstairs lighting, downstairs lighting, smoke alarms (6of) this is a sizeable extension.

 

[brizospark]

well then you come off an unprotected way in the main board.....RCBOs in db1...you can use one of the cores to extend the equepotential zone with 10mm 3 core swa.....
give em 16 A for the downstairs sockets in the extention, 16A for the up sockets and 6A for up lights and again 6A for the down lights....
so a 6 way board for db1 will do it....

Surely 32A rings for socket outlet circuits? Approx 10 x doubles per floor!

 

[GLENNSPARK]

I was thinking - galv 4x4 under existing board. 10mm SWA into galv box (earthed) and into spare MCB at DB (non RCD protected side of board) other side SWA into metal consumer unit. 5 x RCBO's - upstairs sockets, downstairs sockets, upstairs lighting, downstairs lighting, smoke alarms (6of) this is a sizeable extension.

no...take the armoured directly into the bottom of the main board....
if you cant get in to drill it then get a bit of 22 copper pipe....get it hot and sssstt....straight through the bottom of the main board...(i assume its PVC?)
there was this fella that i used to do some work for.....he had a liking for jointing a SWA sub on a metal through box under the floor....then taking a 10mm flat twin into the board....
dichead...

 

[GLENNSPARK]

Surely 32A rings for socket outlet circuits? Approx 10 x doubles per floor!

what for?...its just an extention for christs sake....

whats all this about how socket finals have to be rings....
you look at the likely demand....and go with that....

 

[MDJ]

What's wrong with a 10 or 16 T&E if running a sub internally?

 

[GLENNSPARK]

What's wrong with a 10 or 16 T&E if running a sub internally?

yep Mike...although if buried under the floor surface then as its strictly speaking not visible would require additional protection by means of RCD top end...
go for the 10mm 3 core SWA for this....

 

[Engineer54]

If as has been stated, this is a typical medium sized house, there should be no need for a remote separate CU.

 

[GLENNSPARK]

If as has been stated, this is a typical medium sized house, there should be no need for a remote separate CU.

i think its access eng....
but i reckon its a bit extreme....

 

[brizospark]

Correct access is the issue here I would much rather run one cable from the other side of house then work from sub bard rater than potentially 8 cables. Plus would only help with volt drops and achieve better Zs readings

 

[Engineer54]

Correct access is the issue here I would much rather run one cable from the other side of house then work from sub bard rater than potentially 8 cables. Plus would only help with volt drops and achieve better Zs readings

If I was to just extend circuits then ring circuit would have 30 x double sockets, this to me is very bad practice. I would like to do the job properly and have the circuits split up into reasonable sizes. The last thing I want is something like the original part of the house having a problem, tripping RCBO and then being called because extension sockets not working. COme on who wants their whole house socket outlets covered by one circuit. Same goes for lighting circuits​

Where are you getting 8 cables from?? I thought we were talking about a lounge and a couple of bedrooms one with an en suite??

You say if you extended the original ring you'll end-up with 30 outlets, are you saying this house has currently only one ring circuit for up and downstairs?? ...And one lighting circuit if you're saying that the same applies to the lighting as well...

There are very few circumstances where a remote separate CU is actually warranted in a typical sized house that's being extended. Remote CU's are normally installed in such circumstances, where it benefits the electrician rather than the installation or the customer!!​

 

[telectrix]

yep Mike...although if buried under the floor surface then as its strictly speaking not visible would require additional protection by means of RCD top end...
go for the 10mm 3 core SWA for this....

have to disagree with you there, glenn. 522.6.101 refers to cables buried in walls, not under floors. in that case they would have to be > 50mm deep anyway.

 

[Guest55]

16mm sub mains !!??
for an additional load that will barely exceed 12A by my reckoning ??
doesnt matter how many frigging sockets your adding , bedrooms hardly pull more than 6A each under normal use.
**** me , you'd think domestic work was rocket science reading this thread

 

[brizospark]

Where are you getting 8 cables from?? I thought we were talking about a lounge and a couple of bedrooms one with an en suite??

You say if you extended the original ring you'll end-up with 30 outlets, are you saying this house has currently only one ring circuit for up and downstairs?? ...And one lighting circuit if you're saying that the same applies to the lighting as well...

There are very few circumstances where a remote separate CU is actually warranted in a typical sized house that's being extended. Remote CU's are normally installed in such circumstances, where it benefits the electrician rather than the installation or the customer!![/INDENT]

There is currently only one floor which is served by one ring, same for lighting.

 

[GLENNSPARK]

have to disagree with you there, glenn. 522.6.101 refers to cables buried in walls, not under floors. in that case they would have to be > 50mm deep anyway.

but i`m looking at it from the point of view of penetration by nails, screws & the like...this applies to scant walls....and as i see it also to floors where the unskilled/uninstructed bla bla bla...

 

[Engineer54]

So it's not a house, it's a bungalow at the moment?? So is it being converted to a house now, or just converting the loft into a bedroom and extending the lounge and a bedroom. To me, this is even less of a reason to start installing sepaate CU's