Do the fixed appliances merit a ring? They'd be quite happy on a 20 amp radial wouldn't they? I suppose it depends on whether there's heating involved (washing machine, dishwasher, cooker). In our case only one of those is in the kithechen though.
snipped-for-privacy@isbd.co.uk wibbled on Monday 19 October 2009 19:41
I considered using radials, but it's harder to work with 32A radials (cable may need to be 6mm2 depending on installation methods and you can't really get 2 6mm2 cables into one accessory).
20A radials might be fine for normal rooms, but in the kitchen/utility I have: 1 combi microwave 1 Kettle 1 washing machine 1 dishwasher 1 tumble dryerA lot of those are right next to sockets so it was easier to run a couple of rings rather than try to second guess how 20A circuits might be arranged.
20A doesn't really map, especially as some of those can be plugged in to random places. 32A worked much better, so I chose a ring as the most practical implementation of a 32A circuit.
manufacturer declares the maintenance free, does that carry any weight what so ever, would they be accepted by a BCO as not needing access?
I thought I'd seen various people slagging off similar sprung contacts which are a bit more common on European fittings?
It sounds like a sensible approach, as long as you treat the circuit as a radial. You cant treat it as a ring because you'd have more spurs than socktes in the ring. You'll need to check your circuit protection is upto the job, so you might be limited to 20A circuits. But if you're running a new cable there's little extra in running 2 or 3.
NT
Sounds like good practice. Retrofitting such an arrangement isn't needed though.
NT
Andy Burns wibbled on Monday 19 October 2009 20:27
In the absence of any standard benchmark or test (eg a BS document or equivalent), I would tend to believe manufacturers who generally demonstrate competence.
It will also be very rare to find a BCO who wants to argue the merits of BS7671 minutae, though his agents might...
On the other hand, I've bought stuff with screw terminals where it is near impossible to get a decent clamp force on the core before the screw strips, or the core disappears up the side of the screw, so you can do well adn do badly in both camps.
Cheers
Tim
Adding more sockets doesnt increase loading on the ring, and you can add as many as you want to the existing ring. A single ring house is, as you already know, perfectly workable, but yes adding a 2nd ring would surely be preferable if its doable. Otherwise its easy side to pop a breaker by switching too many things on at once, and fusepoppen becomes possible.
I cant see any mileage in messing about with the bedroom sockets as you initially suggested, because it doesnt take any significant load off the ring. Best would be to put new sockets on your new ring or 2 - wherever they are in the house.
Where you run a new ring to really comes down to budget and what damage you're willing to inflict on the decor. The most useful place to split the load would be in the kitchen, and utility room if there is one, so I'd look to focus my mind on providing a 2nd ring for there, with a 2nd ring elsewhere being a signifcantly lower priority.
Hopefully the bedroom could be supplied by more sockets from a cable laid in the loft, wheher from new ring or old.
If you then have 2 rings feeding one room its a good idea to put a notice on the fusebox stating this, people have a habit of assuming all sockets in one room are on the same ring otherwise.
NT
Well, I'm an physicist really.
Providing you can pop out the downlighters and then pull them back through the holes, that counts as accessible anyway. That's the standard way to access the transformers too.
I don't know what sort of contact they have, but to have enough pressure to be gas-tight, it would need to be very small, like back-stabs. That's OK for a 0.5A lampholder, but not a 30A ring circuit. Otherwise, if it's a large a spring loaded contact (which won't have the pressure to be gas-tight), the resistance will increase over time, which means the heat increases over time (as a second order effect), which usually weakens springs, so the contact pressure drops, which further increases resistance and temperature, and you have a classic runaway overheating joint. That's not a basis for a maintenance free connection, but rather more a connection on which maintenance is pretty much guaranteed to be required. Crimping, soldering, or brazing, as required by the regs, are maintenance free connections. Even screw terminals can generate the pressure required for a gas-tight connection, providing the metals used have similar coefficients of expansion so the contact pressure doesn't change with temperature.
That's a guideline only, and not part of the regs. It comes about because rings normally start with no spurs, and have spurs added as later additions. When you reach the point where there is more addition than original, the original installation clearly no longer meets current requirements.
In the case of designing a system to be laid out that way from the start, this concern would not apply.
Its ok, but probably hard work. A more pragmatic split might be to do a ring for the front and another for the back of the house (or left and right), or house and kitchen (or some combination etc). There is no reason the split *has* to be upstairs / downstairs. Splitting an existing ring front and back say - would only require you run a couple of additional "legs" out to the split point, and would leave most other sockets / cables untouched.
The problem with this is if something else trips the RCD whilst you are away for a few days, and the house freezes as a result of having lost its frost protection, then you could have a house which is substantially written off by a water leak.
If it has an RCD, like the fridge or freezer, it's a bad idea to share it, and none really merit a dedicated one. Having said that, I have a boiler on a dedicated 10mA RCBD, but that's because it's in the bathroom (and I had several spare 10mA RBCOs;-).
The bottom is cooler, and they don't think much about minimising the cost of repair when designing boilers.
IME with the BCOs I have had communication with regarding electrics, they are relatively ready to accept anything that makes their life easier! Usually they are not going to be experts on electrical installations and hence are less likely to argue obscure points. However each will be different.
Where did you get that figure from?
There is a maximum 100m^2 floor area that should be served by a single ring.
The actual cable limits will depend on the voltage drop or earth fault loop impedance of the circuit and the selected cable type (which limit kicks in first depends on the actual details).
Your typical 2.5mm^2 T&E with 1.5mm^2 CPC with Type B 32A MCB would allow 106m of cable (table 7.1 OSG 17th edition). Note this is longer than the 84m previously allowed in the 16th edition due to the extra voltage drop now permitted for general purpose power circuits.
This would in effect be a radial, you can have a 32A protected radial, however all the cable would need to be able to carry the full circuit load (i.e. typically 4.0/1.5 mm^2 T&E) - you can drop down to smaller cable sizes willy nilly.
Things like this get messy and difficult quickly. Generally when reducing a cable size you would need protective device at the reduction, unless you could show that both fault and overload protection are still provided at the origin of the circuit. Spurs from rings are a special case where the responsibility for fault and overload protection is split, with the latter being delegated to strict enforcement of only 1 double or single socket limit per (unfused) spur. Radials however can be extended at any position without this restriction, which would make this type of arrangement non workable in practice.
However, since the OP is planning on a number of additional sockets, its also fair to assume that 1 rings may no longer be suitable. (There is a fair chance that the limits of the existing circuit are being pushed - much would depend on the kitchen usage). So going from one to four is quite probably overkill, although one to two could be quite reasonable.
That is a good point and worth checking - especially if the CU has rewireable fuses. Since this will leave any spurs from the ring inadequately protected against fault current.
Andrew Gabriel wibbled on Monday 19 October 2009 22:25
I would have to say Andrew that I've never personally experienced any nusiance tripping of RCDs and I've lived for years in a flat with a whole-flat main 30mA RCD. Is it worth making the design more complicated (especially given the the 17th pretty much requires all circuits to be RCD protected - or - make onerous demands on the installation of that circuit) for a comparitively low risk event?
Cheers
Tim
I haven't either since changing to a split load unit some years ago. There are only four circuits unprotected - a utility radial in the kitchen, the cooker, central heating and immersion.
Should there be a "not" in there?
Owain
yup, my bad, thanks.
that should read: "you can not drop down to smaller cable sizes willy nilly"
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