2.5mm on 32A MCB

[westward10]

I seem to remember someone starting a thread, annoyed at people stating something was 'Good practise', but here's a personnel statement that something is 'Poor practise'. You can't have it both ways

Ouch

 

[Dave OCD]

I'm not sure there is a regulation stipulating max length. I think the 3m length reg you are thinking of is specifically for pendants with smaller flex.

So a scenario: you need to install an extractor fcu and the ONLY circuit you have accessible to take it off is the cooker on a 32A breaker. Does that mean you take it from the cooker point in 4mm+ to the fcu, or would you take it in 2.5?

With this scenario I'd say it would be absolutely fine to run a 2.5, the current of an extractor fan would be minimal and a fraction of what 2.5 can carry so overcurrent protection isn't an issue. And the fcu would have an appropriate fuse [probably 3 amp] to protect the fan and its internal circuitry. As far as terminating a 2.5mm cable in the same terminals as 6mm there's no problem as long as it's done carefully.

 

[Deleted member 93172]

In my opinion for whats it worth, I am sure you cannot reduce the cable size without down rating the mcb, therefore in this instance a fused spur should be connected in circuit between the last 4.0mm fed socket and the 2.5mm fed socket.

 

[DPG]

In my opinion for whats it worth, I am sure you cannot reduce the cable size without down rating the mcb, therefore in this instance a fused spur should be connected in circuit between the last 4.0mm fed socket and the 2.5mm fed socket.

Not entirely true. Otherwise busbar systems wouldn't work. Daz

 

[Deleted member 93172]

Not entirely true. Otherwise busbar systems wouldn't work. Daz

Ah, but busbar systems have fused tap off boxes to feed equipment

 

[Midwest]

As somebody said before, its shown as an example in figure 15B, so why is C2 or wrong or poor practise?

 

[Wilko]

As somebody said before, its shown as an example in figure 15B, so why is C2 or wrong or poor practise?

Fig 15B sure does show it, no question. My take is ...
Fig refers to 433.1, but 433.1 doesn't lead me to think 4mm to 2.5mm is ok. But reading on to 433.2.2, it seems to say ok < 3m, with a couple of conditions.

 

[westward10]

The op asked for opinions, mine you have two final circuits from a single protective device one of which reduces in csa. This circuit which reduces in csa is ad hoc with absolutely no thought for BS7671, the chance of further add ons with the same sized reduced cable is high.

 

[DPG]

I don't agree with not having a circuit such as this because people might add further sockets on in the future. If you go with that theory then you would never have spurs from sockets at all, because someone might come along and spur from the spur. No difference - both cases have a length of 2.5mm feeding a socket. Daz

 

[westward10]

Th

I don't agree with not having a circuit such as this because people might add further sockets on in the future. If you go with that theory then you would never have spurs from sockets at all, because someone might come along and spur from the spur. No difference - both cases have a length of 2.5mm feeding a socket. Daz

The chances of this so called spur being installed as per BS7671s "Guidance" is zero, I would feel it is open to further abuse, that is how I view it.

 

[Karin]

Fig 15B sure does show it, no question. My take is ...
Fig refers to 433.1, but 433.1 doesn't lead me to think 4mm to 2.5mm is ok. But reading on to 433.2.2, it seems to say ok < 3m, with a couple of conditions.

interesting, 'cause its clearly okay in App. 15. perhaps the intention is 433.3.1 (ii), where a single outlet is deemed "not likely to carry overload current" ?

 

[Midwest]

I don't see the argument, not to do this, on the basis someone else might add to it in the future. No decent electrician should/would do, only perhaps some DIY'er. But DIY'er do all sorts of crazy stuff in their houses, so perhaps they shouldn't have 'leccy in their places.

 

[David M]

Is it me or is this getting all out of proportion?
No its not necessarily how any of us would install it, but its not likely to be an issue either. Its highly unlikely the circuit would be overloaded during normal use, and any electrician worth his/her weight would not install another spur off that socket.
I would maybe just make a note on the report and possibly give it a C3/C4 if I felt the installation required it.

 

[westward10]

Is it me or is this getting all out of proportion?
No its not necessarily how any of us would install it, but its not likely to be an issue either. Its highly unlikely the circuit would be overloaded during normal use, and any electrician worth his/her weight would not install another spur off that socket.
I would maybe just make a note on the report and possibly give it a C3/C4 if I felt the installation required it.

By coincidence just finished doing a school EICR, existing circuit has a spurred two gang and just recently another has been connected from it. All fine according to "electrician" who also left a MWC on site.

 

[Midwest]

By coincidence just finished doing a school EICR, existing circuit has a spurred two gang and just recently another has been connected from it. All fine according to "electrician" who also left a MWC on site.

That's down to that bad boy then. Most things plugged into sockets nowadays aren't 2kw electric fires.

 

[rich bevan]

surely a 32a mcb feeding both is potentially overloading the 2.5 cable working on roughly 10a per mm of cable.

 

[rich bevan]

Just wondering what folks would do if you came across this on your travels. 4mm radial sockets circuit, on 32A MCB. Two branches from the CU, one supplying 6 double sockets, wired entirely in 4mm, so fine. The other branch consists of a 1m run of 4mm to a double socket, then around 5m in 2.5mm cable on to a second double socket.

The question is, who would be happy to leave this arrangement in place, and who would downgrade the breaker to 20A? I would say it's ok, as the 2.5mm can't be overloaded as it feeds only one double socket, just like an infused spur off a ring.

what if somebody plugs a 4 gang into the last socket??? easily overloaded then sir

 

[hightower]

what if somebody plugs a 4 gang into the last socket??? easily overloaded then sir

You mean a four gang which has a 13A fuse in the plug?

 

[kingeri]

No

what if somebody plugs a 4 gang into the last socket??? easily overloaded then sir

Not really, still fused at 13A. The most that could be drawn would be 26A. CCC of 2.5mm clipped direct is 27A.

 

[David M]

By coincidence just finished doing a school EICR, existing circuit has a spurred two gang and just recently another has been connected from it. All fine according to "electrician" who also left a MWC on site.

Things can happen, no doubt. But it is very unlikely that during normal use in a domestic property that the socket in question would be overloaded. If some idiot decided to extend off that socket then that's their responsibility. No one can foresee all events.
Like I said non of us would install the socket in this manner, and yes in exceptional circumstance there could be a chance it gets overloaded. As I said I would asses the install, make a note on the report and then perhaps code it accordingly if I felt it was going to be a potential problem.

Its easy to fall into the nanny state mentality, stifled by bureaucracy. This doesn't mean ignoring potential hazards, but rather accessing the situations to determine the severity. But we cannot wrap people up in cotton wool, they have to take some responsibility for their own welfare.