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I have a client whos sockets have been wired in 6mm in some places but wired in 2.5mm between somepoints. As far as i can see this is not a serious problem as 2.5mm cable is requirement any way.Always knew u could have a bigger cable than u need but never smaller than circuits needs?!!?!?!?
 
thou shalt not reduce the size of the conductors in a circuit ( forgot the exact reg,) but if it;s 6mm to start, it must remain 6mm throughout.
 
I realise that as the smaller cable acts as almost a fuse and can caused problems by heating up (so ive been told). Gonna tell the customer to take out the few links of 6mm and put in 2.5mm which is mostly in the circuit in conservatory. y they did it in 6mm i will never know as its not like a volt drop could happen from upstairs to downstairs(not to uprate cable to 6mm!!!) and only runs normal domestic appliances. Wonder if this was a DIY job or something!!Does this make sense to anyone?
 
thou shalt not reduce the size of the conductors in a circuit ( forgot the exact reg,) but if it;s 6mm to start, it must remain 6mm throughout.

I don't recall ever coming across that.

When i was at college we were taught how to reduce the size of cable on a street lighting circuit to reduce the cost while allowing for volt drop at the furthest point.
 
maybe street lighting is diff. but on a domestic, a 6mm cct ( radial ) would perhaps be fused at 40A. then it is reduced to 2.5mm capable of carrying say, 25A. PROBLEM.
 
I bet getting 2 6mm's into the socket terminals is a right laugh. Surely this must be a no no, as far as i am aware the maximum CSA cable that should go into a standard socket terminal is 10mm! so 2 x 6 = 12, this must mechanically overload the terminal and its screws.
 
I've wired ring circuits with the two legs from the mcb in 6mm then the rest in 2.5mm (the 6mm to allow for volt drop)

By the way i didn't design it i just installed it and thought it was crap myself, but it sort of made sense.
 
I believe you should give us a little more information, like what is protective device? Is the circuit ring or radial? How long is that first leg? Then maybe answers could be more accurate and more useful for you.
 
Check that the 6mm legs (sound like extensions off the original 2.5mm ring) are not 6mm due to the installation method. Alternatively it may just be that the installer has a lot of 6mm he needed to get rid of. As far as I know you can always go up in size, above the design size, if you wish for whatever reason but cant go down. I am pretty sure there is no reg that states all the wiring needs to be the same size.
 
As long as the CPD is selected to suit the smallest CSA then there is no reason why CSA cannot be reduced as a circuit progresses.

A common example is factory lighting, running say a 6mm leg from the DB to the first point to combat volt drop, perhaps reducing to 4mm or 2.5mm eventually. In this case, the current carrying capacity of the lowest CSA would be taken into account.
 
ive done a ring in 4 mm because of the length of the cable runs the calculated Zs was going to be higher than permitted
 
As long as the CPD is selected to suit the smallest CSA then there is no reason why CSA cannot be reduced as a circuit progresses.

Could not have said it better. There is no reason why any circuit could not be in different sizes along its length and long as the smallest csa can carry the load, eg 25mm to a 16mm to a 1.5mm to a 50w gu10. This is what happens from a sub station to your house. The question is about protection of the circuit. Select the correct rated fuse for the smallest csa and your ok. Closest I can find in the brb is 133.2.2 page 19 but even that doesn't give us a defined answer. Good point about putting too much stress on terminals. Sounds like this is domestic, in your case I would change it all to 2.5 (ring) or all to 6mm radial to stop any confusion for the next sparky.
 
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ok heres the deal, the upstaires ring comes from the board to 1st floor(all in 2.5mm cable) does upstaires ring and part of the leg comes down to the conservetory and does 4 double sockets there which is comprised of 2.5mm cable then the next leg to next socket is 6mm and then next leg from that socket is 2.5mm and random cable used within conservatory. Me personally as an electrician have never come across this way to wire so im advising the client to have all 6mm replace with 2.5mm in the conservatory then back to board. the ring is on a 32A Rcd. ive been advised maybe to put the conservatory on its own ring then at least everything ive done will be up to spec and at least be right cable etc. Such a strange problem just not sure which way to go for the best of the clients house.
 
If all the readings are ok (r1,r2, insulation resistance, polarity, earth fault loop impedance) then I would leave it as it is and note it on the certificate.

You didn't say what you were doing to the installation?
 

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