zoothorn

Electrics: Zoot's gaff.

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G'day folks,

 

my extention's started! so.. maybe I can ask help to form a plan for my electrics. Carrerahill's kindly outlined one (via PM) which no doubt is ideal but I'm not quite up to speed on some of the 'radial, rings' yet to picture it. What I need simply is: 1x overhead 'rose' light in upper room, 2x double sockets. In lower room, 2x double sockets, 2x strip lights (above workbenches @ side, but low ceiling).

 

Consumer unit's in ideal spot, in my sit'room corner cupboard (modern rcd/ many fuses etc).. so I guess its drill thru wall behind > into extention? My builder's leaving me to do insulation, pB, electrics. So once he's built walls, door/ window, roof.. I can take my time on internal xyz.

 

Thanks, zoot.

 

 

 

 

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Radial: Circuit which runs out and daisy chains around consumers, i.e. sockets or lights the circuits ends at the last unit - common on lighting circuits and small power runs.

Ring: Circuit which runs as a ring - so in the case of a typical power ring it would leave the DB on a cable, go to the first socket, which then goes to the next socket and so on, then the last socket in the group has a wire that runs back to the DB to create the "ring".

 

The following depends on your DB - if it's old with cartridge fuses then this would be the time to upgrade. 

 

I'd add a new extension lighting radial or two if you have enough ways left in your existing DB, alternatively and arguably a tidier and certainly safer option if not a nightmare to do so would be to tie the ground floor ext. lighting into the existing ground floor lighting and same for first floor. Go for 1.5mm for the sake of an extra few pence per meter. 

 

Then add a ground and first 2.5mm ring if you have the ways, but a single "Extension ring" to cover the whole lot would probably be fine depending on the rooms. If you are lucky another option is to tie into the existing rings, depends where the existing rings run and how easy it would be to extend the rings to the new sockets then continue back the DB.

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Personally, I would use a radial for the sockets too. With a ring there are a lot more ways in which poor design or a loose / overtight connection, damage to a cable etc can leave it in a state where it's working but dangerous (fire risk, primarily). The testing is also more complex.

 

Depending on load, a radial may require slightly thicker cable (but half as much of it), but is a lot more bulletproof.

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Posted (edited)
21 minutes ago, andyscotland said:

Personally, I would use a radial for the sockets too. With a ring there are a lot more ways in which poor design or a loose / overtight connection, damage to a cable etc can leave it in a state where it's working but dangerous (fire risk, primarily). The testing is also more complex.

 

Depending on load, a radial may require slightly thicker cable (but half as much of it), but is a lot more bulletproof.

 

Not hugely convincing points and on par, if slightly increased in some aspects, with the alternative in terms of potential risk.

 

If he wants to run in 2.5mm and use a 20A breaker (which is very limiting) then I guess fine, realistically however, this becomes a 4mm radial - 2 x 4mm conductors into a terminal poses more risk of strained terminals due to the heavier wire and reduced compliance to take a bend when being forced into the back box, this increases the likelihood of a wire sitting permanently under tension or sheer - a little hot/cold cycling and they are more likely to loosen as the stranded make up of 4mm allows the strands settle. So you end up with as many termination risks however arguably worse than using 2 No. lighter conductors that terminate nicely and comply with being formed into shape to recess into the backbox.

 

MEIT did a big report on cable termination around about the time push-fit terminals were becoming more commonly used - the conclusion was that smaller solid core wires caused less risk than larger stranded wires. There was also a shift toward going radial again but this was ousted on various grounds as not being the best solution. 

 

 

Edited by Carrerahill

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@Carrerahill All fair points and I wasn't aware of the MIET report.

 

Nonetheless, I'm still not a fan of the failure modes for ring circuits especially for DIY design & install. For benefit of others (this won't be new to you) the cable protection depends on a reasonable division of current between the two halves of the ring. If the ring is broken, or has local areas of higher resistance due to damaged conductors, it can appear to function but be dangerously overloading one leg. This can continue for long periods without detection causing insulation breakdown (or more rapid overheating).

 

With a radial a loose connection can still cause local arcing, which can often be smelt, but a damaged or disconnected conductor will usually cause the circuit to fail and/or trip.

 

Granted, my background is in theatre/event/industrial where almost everything was radial so I probably have an innate dislike of ring.

 

To be honest in the modern world I don't think a 20A radial to a few 13A sockets is all that limiting (outside perhaps a kitchen or workshop or other special locations). There are very few high-current devices these days. 

 

But even if it had to be on 4mm then personally speaking that's probably the way I'd go.

 

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I'm with @andyscotland on this; ring finals seem like a fail-dangerous setup. I'm not planning to have any and for most radials I'll go for the minimum plausible current for the breakers, never more than 20 A. There are off-grid reasons for going that way but I think I'd follow that plan even in an on-grid house.

 

I have a notion in the back of my mind that BS1363 sockets are supposed to always be able to supply 13 amps. I.e., you shouldn't put them on circuits with less than a 13A fuse or MCB. E.g., not on a lighting circuit even if you plan to only plug a light into them, hence the use of round-pin plugs for remotely switched corded lamps. Obviously there's diversity allowed so three sockets together don't have to be able to supply 39A off a 32A ring final. Or am I imagining that and it would be OK to have BS1363 sockets on a 6 or 10A radial?

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2 hours ago, Ed Davies said:

I have a notion in the back of my mind that BS1363 sockets are supposed to always be able to supply 13 amps. I.e., you shouldn't put them on circuits with less than a 13A fuse or MCB. E.g., not on a lighting circuit even if you plan to only plug a light into them, hence the use of round-pin plugs for remotely switched corded lamps. Obviously there's diversity allowed so three sockets together don't have to be able to supply 39A off a 32A ring final. Or am I imagining that and it would be OK to have BS1363 sockets on a 6 or 10A radial?

 

A lot of sparks would frown at it but AFAIK there's nothing specific in 7671 to stop you.

 

The round pin lighting sockets were really more to avoid a nasty surprise when you plugged the iron in and plunged the room into darkness. A socket on a lighting circuit might also have thrown up issues like RCD protection under previous regs, less of a problem now RCDs are more widespread.

 

Off the top of my head there's various general regs that would apply - safety on power failure (the iron example), nuisance tripping, clarity of what circuit actually supplies the socket (to allow for maintenance isolation) etc. Most of those can be dealt with one way or another.

 

But in general so long as any cable is appropriately protected upstream for its current carrying capacity I don't think there's an insurmountable issue. It's ultimately not that different to populating a consumer unit with 200A worth of MCBS on a 60A incomer.

 

 

 

 

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@Ed Davies Or, better example, it's not uncommon in a venue to find a permanently installed 63/3 or 125/3 commando socket or even a set of 300A powerloks off radials with much lower ratings. Sometimes you even find one of each all on the same 63A circuit.. They're provided because touring dimming & distribution kit commonly has those plugs and the load/diversity is then managed downstream.

 

Of course that environment usually counts as being under competent supervision, but it's definitely possible.

 

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