Advice required please ,(Sub Main)

[E14RKS]

Hi hoping you guys can help,
About to take on a job in central london. at the moment there is a meter cupboard in the basement, the basement and first floor are two seperate flats, At the moment they have split the tails, then into two old 63a fused switches then what looks like 10/16 mm T&E fed out to the distribution boards. now ive got the relocated the distribution boards. my idea was to replace the old 63a fused switches but need a little advice on what to run from them to my new fusboard locations, the upstairs flat wants the DB up high so i need to chase up, am i right in saying 25mm tails cant be used as they would be sunk in the wall? so would it be a case of using SWA? if so what swa should i use bearing in mind cable runs are no longer than 3m? also the main fuse thats in place is a old bs88 60a, should i put 63a or 100a fused switches in, bearing in mind the electricity board might up it to 100a in future? appreciate any comments its just not had much to do with submains before?
thanks

 

[pushrod]

Need to use S type rcds or SWA for distribution circuits (i'm assuming you meant to say "cable runs are now longer than 3m") If you are using switch fuses they need to protect the cables you are putting in regardless of the main cut out. So In < Iz.

 

[E14RKS]

Thanks for the reply,
so as long as i come out the fused switch with swa then it can be burried in the wall?
the main fuse is a bs88 60a, so should i use a 63a fused switch?
the distance from the switch fuse to the DB is around 2.5 meters which size cable should be used? once again thanks for the advice regarding sub mains.
cheers

 

[bugsy]

two way ryfield from head 25mm swa from ryefield to each flat meter each flat independantly

 

[pushrod]

The distributor usually specifies the max acceptable length of tails and their min CSA. However, that only applies while they are being protected by their cut-out. Once you have put your own protective device in ie your switch fuse, that no longer applies. So you just need a cable with a greater current carrying capacity than your switch fuse - so 16mm² will, more than likely, easily cope unless there is some serious derating being caused by the installati0n method. You do not need the 25mm²

I misinterpreted your original post and didn't quite realise the runs were so short - using some steel conduit on the buried parts would also be a possibility and also were worth checking what the DNO would allow for the length of the tails in your situation - 3m may well be acceptable to them.

Edit ; mind your 63A supplies aren't much for flats - i take it there are no 10.5kW electric showers with electric hobs planned lol

 

[E14RKS]

pushrod thanks for the reply, really appreciate your advice, there not flats, its an exisiting house, 3 storeys and there turning the basement into a self contained apartment for there daughter, at the moment the is one incoming supply bs88 60a, the tails have been split which feeds two meters two fused switches and in turn a DB for the basement and one for the rest of the house, i know this is cheeky but if this was presented to you what route would you go down bearing in mind at some point the dno might upgrade the head to 100a, look forward to hearing back from you, steve

 

[pushrod]

Have re-read your posts and tbh still unsure of the set up - Is it one dwelling with one metered supply from the electricity company? , main supply cut-out in basement, house to be divided into a basement flat (for daughter) and ground/first floor for rest of family?

Tbh you have a number of choices and it is probably easier for you to make the best ones as you can actually see it. You just have to stick to the basic principles of making sure that you are protecting the main cut-out and any cables you put in with suitable protective devices and providing additional protection(eg swa, rcd or steel conduit) where appropriate (ie buried cable). Reading it at the moment i am not sure that is being done with 2 x 63A switch fuses coming off the 60A main cut-out?. If that is indeed the case i would apply to have the main cut-out up rated and then go to a similar rated switch fuse (wylex or similar), straight after the meter, to use as a main isolator before splitting the supply. It sounds as if at present you could have a situation where max demand could easily exceed your main cut-out.

As i say though, very difficult to know without being able to see everything properly and must admit i can't really visualise what's is going on but hope my comments have been of some help.