Separating supplies

[Qedelec]

Can anyone enlighten me.
I am installing a slot meter and CU for a customer who got stung with a huge electric bill from students.
The supply cables and cpc to the new meter will be about 4.5m and will be enclosed in 25mm²x40mm² mini trunking together with 6mm² Cooker cable, 2x2.5mm² Ring and 1mm² lighting.
I am installing a 100A DP isolator in the original CU and had intended using 16mm² for the feed.

I was told by the wholesaler that you're not allowed to fit 16mm² it has to be 25mm².
I know OSG says meter tails must be 25mm² but does this apply to extended supplies

 

[fido]

Given the length of tails you should be using 25mm. You will need to fit Henley blocks between the existing meter and CU. If there is no isolating switch on the existing meter you will need to get the electricity supplier to do a temporary disconnection while you wire up the new meter.

 

[Qedelec]

Henley block shouldn't be necessary as I said in the first post, I intended to add a 100A isolator in the original board. But I take on board what you say and will re-evaluate the job.

 

[Spark123]

If it is fused to 100A (you dont mention the knockout fuse) the you need to use 25mm² (rated 101A method 3).

 

[fido]

I don't understand how you would wire up the additional isolater inside the existing CU. Would you fit busbars from the existing isolator? If there are 25mm tails feeding this isolater, would there be room for such busbars? Or were you intending to use the isolater in reverse and take the output from the top?

 

[plugwash]

why not just run the new meter from a 50A breaker in the existing CU that way you would only need 10mm tails to the new meter which should be easy peasy to fit.

 

[Big_Spark]

Forget Henley Blocks, the meter for the students would have to be provided by the supplier and this could be expensive and a nightmare to organise.

Further, forget the 100DP Isolator, it would not comply with the regs.

What you need to do is run a sub-main from the initial board, without knowing what manufacturer this board is, I would say Install a 63A Type B MCB, then run 16mm² from this to the new Meter that your installing, this meter will then feed the second board for the Student part of the property. Remember, the Main Isolator in the Student Board should (Idealy) not exceed the rating of the protective device.

 

[Qedelec]

Thanks FWL for your input. Sounds very sensible and a lot easier.
I was concerned about getting all those cables in the existing trunking and had not considered a 63A MCB.
For your info the origin Board is an old starbreaker (pre plug-in busbar) so adding a MCB is a doddle and I wanted to keep the isolator in this board for neatness (no one likes a confusing mess of switches).

I bow to your superior experience.

P.S. Bet you Ban would have said the same.