Neutral size...

[Luminous]

Something that's bugging me and would like setting straight if you people would.....
say for instance you needed to provide a new circuit eg 20a and the customer has an old rewireable board with no room left. You decide to use a couple of henley blocks and split the tails just after the meter (all on 100a main fuse). You now have a 25mm line tail going into a single breaker enclosure containing the 20a MCB and onto your new circuit. Why does this 'second' neutral tail have to be 25mm? basically its just going to connect to a 2.5mm inside the enclosure.
I'm either missing something completely, such as the line tail nor the neutral doesn't have to be 25mm or is it that I've just set myself up to be mocked for being such a complete tool.

Customer has decided to go with a new CCU so this has now become theoretical but I got to thinking about it when I went to look at the job originally.

 

[jaresquire]

Well, Luminous, the neutral may be close to ground potential but it still carries the same current as the line (hopefully) so needs to match the csa.

 

[Luminous]

I get that bit, but in effect, inside the enclosure you'll have a 25mm cable connected to a 2.5mm cable.
It's no different than the 2.5mm carrying on to the henley block.

 

[malcolmsanford]

Why does the 2nd LINE need to be 25mm in that face.

You have a 20amp protection device so in theory your never going to exceed that loading on your circuit so you could in fact run both LIVES in 2.5mm providing you can adhere to the regs and section 433.2 onward for overcurrent and more importantly here, section 434 for fault

 

[jaresquire]

As Malc said, had it confirmed by the NIC recently.

 

[Luminous]

I knew it, I bloody well knew it. I've had 2 electricians both say that the csa of the line had to be sufficient to take its previous fuse size amp. It didn't make sense to me if there was only one breaker/circuit that it was serving. I think their argument was that someone could come along and put in a bigger circuit csa and breaker.( surely it would then be their responsibility check the feed in was sufficient?)
Cheers guys.

 

[malcolmsanford]

Well just ask these Electricians (I shudder to call them this) to interpret their meaning on the section of the regs I gave you, that is if they can read.

Luminous in your scenario what would stop someone from removing a 6amp lighting PD and decide to put a 63amp one in because we are fitting a load more lights.

You have designed your installation in your OP and as long as they adhere to the regs then your in the right. Saying that though if I was doing the job, I would be perhaps feeding that enclosure with 6/10mm just for a bit of future proofing, you never know, But if you want to feed it with a 2.5 conductor and fit a 20amp MCB in it .......your entitled to do just that

 

[Luminous]

Cheers Malcolm. Perhaps I need more conviction in my own knowledge.
Funny you should say that, I was thinking of using a bit of 6mm double insulated I have in the van for the very same reason.