sub main feed

[telectrix]

job came in today. granny flat already has 32A ring and 6A lighting. cust. has had a shower room built on. will have a 9.5kW shower and 5 downlighters. no problems there, and room on house CU to fit MCB and RCD. problem is, the sub main is fed with 6mm SWA. obviously inadequate. cust is OK with upgrading SWA.
BRB says 10mmSWA buried and in free air can carry 60A. this is ( taking diversity into account ) just about enough. 16mm would be ideal, but a pain to install due to bend radius and glanding. anyone think 10mm will do? or should i go for 16mm

 

[davelerave]

i would be thinking 16's if you mean a sub-main for separate board in granny flat

maybe i'm misunderstanding -but i wouldn't rcd the sub-main if that's what you intend

 

[telectrix]

the feed is to an existing CU in flat. i can feed from either RCD in house board, or put the RCD protection in the flat. The house board at the moment ha 6 circuits, all on one RCD ( 16th Ed. Split Board) but can be made into Twin RCD board. existing 6mm to flat was from RCD protected MCB in main CU, but disconnected wthen building mods, started on flat.

 

[Ponty Massive]

If your running a new feed to the cu in the flat from the house DB, wouldn't the cable need to be RCD protected as well?

 

[davelerave]

not that familiar with uk rules but

i think the swa sub-main means no rcd required on it -so it should be avoided as it could create a hazard in the bathroom esp. for elderly

 

[robsparky1975]

whats the existing load on the 100a fuse in the house with diversity applied?,what is the lenght of run?

 

[telectrix]

SWA don't need RCD as its part surface , part buried. The " flat" is just one room with shower room attached. RCD is essential. either one end or the other.( socket, shower , and buried cable in walls) i've got it all sorted except the actual size of SWA. My loading with diversity applied is 56A which is just about the limit for 10mm. My plan so far is to upgrade the main CU to 2 RCDs as there is room to do this, and feed the SWA from the protected side of the board, replacing the existing 32AMCB with either 50A or 63A. just need to decide on 10mm or 16mm SWA

 

[telectrix]

whats the existing load on the 100a fuse in the house with diversity applied?,what is the lenght of run?

got to check house load with diversity yet, it's not high. SWA run is 15m. surface cleats and underground.

 

[SimpleSimon]

How often is the 'flat' in use? Would 10mm be enough if the flat was often used as drawing 56A wouldn't always be pulled?

 

[telectrix]

exactly my thoughts. the only high load is the shower. no heating, no appliances, just will be TV, kettle, lights etc. i think 10mm is adequate as shower is only occassional use for 5 - 10 mins.

 

[SimpleSimon]

That's the thing I would think, over the lifetime of the cable it won't be drawing all of that at once so you should be able to get away with it.

 

[davelerave]

SWA don't need RCD as its part surface , part buried. The " flat" is just one room with shower room attached. RCD is essential. either one end or the other.( socket, shower , and buried cable in walls) i've got it all sorted except the actual size of SWA. My loading with diversity applied is 56A which is just about the limit for 10mm. My plan so far is to upgrade the main CU to 2 RCDs as there is room to do this, and feed the SWA from the protected side of the board, replacing the existing 32AMCB with either 50A or 63A. just need to decide on 10mm or 16mm SWA

yes- on the sub-board

30ma is a hazard on the main DB in this case

 

[J@mes]

I'd still go 16, what if the customer in the future wants a small electric heater in there or something like that? Unless there is already a gas heater/ central heating in the building.

 

[davelerave]

yes -i would use 16's

 

[telectrix]

central heatin piped fromn house. i could even down rate the ring main to 16 or 20A MCB. it's one of those borderline cases. just that 16 will be a pig.

 

[WayneL]

not that familiar with uk rules but

i think the swa sub-main means no rcd required on it -so it should be avoided as it could create a hazard in the bathroom esp. for elderly

What does this mean?

 

[Mark_Burgess]

I too would go with 16mm and be the XPLE not PVC as it carries more current and is quite flexible.

 

[davelerave]

What does this mean?

i do get a bit twisted with the uk rules(not too familiar)

anyhow in principle when fitting rcd's avoid causing a nuisance or hazard-so here in this granny flat job

sub-main is swa so no rcd required
if lights require 30ma protection rcbo them so a fault on the ring main doesn't knock out the lights when an elderly person is showering for example-that is the way i would do it anyhow

not much extra cost involved

unless i'm mistaken(could well be!)

 

[WayneL]

With you now, Dave - wasn't put clearly.

 

[Janjua]

exactly my thoughts. the only high load is the shower. no heating, no appliances, just will be TV, kettle, lights etc. i think 10mm is adequate as shower is only occassional use for 5 - 10 mins.

Why not run a separate 10mm SWA for the shower circuit alone?