My consumer unit maze - how to connect them?

[tom.harrigan]

My(TT) electrical installation is a bit unusual. I have:

1. A small consumer unit where the mains comes in with a few sockets.

1. Another small consumer unit, which feeds the rear of the house. ( a dining room, bedroom and a shower room)

2. A small consumer unit in the garage.

1. Another consumer unit feeding the front of the house, where the kitchen, living room etc. are. ie this is the "main" one.

At present the connections are: mains---1-(temporary)-2---3 4 is not connected.

I wish eventually to achieve: mains---4---2---3 and get rid of 1.

But I will probably have to live with: mains---1- (temporary)-4---2---3 for some time.

So my question is, what is the best way to connect the sub-main in the rear of the house (2) to the "main" consumer unit at the front (4) given this will be a permanent connection. The units are only 6m apart, the cable run will be in the ceiling, not grouped, and not in contact with insulation. I had planned to connect them with 16mm T+E, but I will probably have to fit an RCBO (40A) which could trip, which would be a nuisance. So, 10mm 3-core SWA might be a better idea?

T

[Tim S]

[e-mail] coughed up some electrons that declared:

I'd go for SWA, especially as it's only 10mm2. You get a full sized CPC (assuming 3 core) plus the armour. An RCBO would be a less good idea as there is zero discrimination[1] between that and downstream RCD/RCBOs - so a fault on the rear sockets will take out the rear lights.

You may find that technically you don't need "mechanical protection" if your cable goes straight into the ceiling and over the rafters, but deciding that would need more detailed knowledge of the route. However, SWA does give a warm feeling.

[1] Your Type S main RCD for your TT does manage discrimination because it's designed with a built in time delay.

Cheers

Tim

[Tim S]

Tim S coughed up some electrons that declared:

BTW - you know you might manage with 6mm2 SWA?

70C Thermoplastic armoured, copper conductors:

Clipped direct: 49A Free air: 53A

Volt drop at 40A, 6m = 1.752V (assuming your not tight on volt drops in your final circuits).

90C XLPE (more usual these days) has an even higher rating but your terminations may not be rated for >70C. But it buys you some spare if a section of cable gets covered in the ceiling space.

Do double check these figures yourself before committing though!

This is all assuming you will have a fuse or MCB protecting the SWA submain at 40 or 45A. There's still a risk of lack of discrimination between the upstream 40/45A and downstream 32A devices (though you'll probably be OK with the 6A downstream lighting MCB - in that respect a fuse upstream can be a better bet, but hopefully overload conditions will be rarer than situations that can trip an RCD.

Cheers

Tim

[tony sayer]

In article , [e-mail] scribeth thus

Pardon me asking but isn't that all part pee stuff that you can't DIY without inspection?..

[tom.harrigan]

You are correct, you shouldn't DIY without informing building control.

T

[tom.harrigan]

Thanks for the advice. I would be better off with SWA for several reasons, discrimination being one. I appreciate I can probably get away with thinner cable but as I have no idea how long my temporary arrangements will be in place, I'll probably purchase some 16mm 2-core (rated at 110Amps), so I can connect the various units up with it and have no worries about the final configuration.

All my "ring" circuits are on 20A RCBOs, as is the garage. The only larger circuit is my cooker (40A RCBO). All my lighting on 10A RCBOs, with the smoke alarm on a 6A RCBO.

T

[Geo]

I think that is just applies in the English part of the UK...

[tony sayer]

Really?, What do they do in Scotland and Wales let alone NI?..

[Geo]

Dunno what they do in the other colonies but here is some info for mine:-

[John Rumm]

If you went for 2 core SWA (no need for three since the copper equivalent of the armour wires is more than ample for a TT earth), and you sized it such that it could ultimately carry the full current permitted by your main cutout, then you do away with additional overcurrent protection at the transition. You would probably want a type S RCD at the origin of the SWA to protect against earth faults, however since the SWA does not require 30mA protection, this could be a relatively high trip threshold device like a 300 or 100mA device.

You could consider switchfuse type housing for this - and you could then also have a single point of isolation for all of the circuits/CUs.

[Owain]

"The fact that only certain approved persons and companies will be able to carry out electrical installation work to the demanding standards of BS

7671 will undoubtedly raise safety even further in Scotland."

I'm sceptical - Part P didn't raise safety in England.

Owain

[Tim S]

John Rumm coughed up some electrons that declared:

I'd be careful John: round my way, EDF have a maximum length of 3m on "tails" which are permitted to be protected by their cutout fuse. It seems to vary from suppier to supplier. That's not in the regs, it's a supply stipulation.

When I was looking at running 100A's worth of SWA I was considering a 100A BS88 fuse between the meter and the SWA solely to satsify them.

Cheers

Tim

[John Rumm]

Indeed - hence my suggestion to 1) use SWA (rather than tails), and 2) install a switchfuse isolator.

I also like the idea of having a single point of isolation for

*everything*. So in an emergency someone can go and throw one switch.

[tom.harrigan]

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If I put a switch-fuse at the origin, and if I take a 16mm2 2 core swa cable to the main CU, and if I prove that disconnection will take 1s in a fault, is that all I need for this circuit?

Also, the main CU will be where all the earthing/bonding connections are made, not at the "origin". The reason for this is that eventually the main CU will be at the origin, if Central Networks ever get round to do the work they promised to do 4 years ago. So, where should I connect the armour sheath? I'll probably do it at the MET?

If you can bear one more question, what should I do about the bonding/ earthing of CU in the extension? I was going to run a 10mm2 3-core swa to it, and just connect the earth core and the armour to the earth terminal in the CU (there is at present nothing to bond - just future proofing). At the main CU, I was going to connect the earth core to the MET, and the armour in the CU. Does that seem OK to you?

T

[Tim S]

John Rumm coughed up some electrons that declared:

Sorry - got lost in the threads.

I had Mr EDF (well both of them) round today, to pull and replace the cutout fuse so I could get my new CU up.

I wanted to stick one of these in the meter box:

chap wasn't qualified to reconnect, so couldn't advise me if the second bloke would go for it. So I went for it anyway.

Fortunately, in the previous day and in the 4 hours between chaps I went to town on making everything neat, tie wrapping the earth bonds, forming the tails nicely and writing out a convincing EIC.

So the conversation with the second chap went roughtly: "Ooh - we don't like you doing that... Can I see your EIC? What's your registration number?".

"Don't have one - am qualified to EAL Level 2, and it's a Building Control Job - and here's the Megger I used."

Me "If it gets in the way in future, just chop it off, I left spare on the tails"

Him "We'll OK, I let it go this once"

Me :->>>

Ended up having a very friendly chat. DIY +1!

He even moved the meter to make more room for my switch, and upgraded the meter tails which I'd only noticed mid job were 16mm2 (pulled out of a Henley block). Just goes to show - neatness counts for a lot. Also helps if your CU doesn't blow up when he puts the fuse back :)

A good day :)

Cheers

Tim

[dennis]

It wasn't intended to.

[tom.harrigan]

This is precisely the thing that drives me freekin mental! It is either technically acceptable to put an isolator in you meter box or it isn't. What the hell has it to do with whether you passed a multiple-choice questionnaire or not?

If the isolator is in an external box, then it could possibly be argued against, depending where you are. Otherwise, I was convinced the DNO was supposed to fit an isolator?

T

[Tim S]

[e-mail] coughed up some electrons that declared:

Like most things...

Alas no. It's down to the DNO. I spoke to EDF technical and they said they never fit a meter box isolator in the SEEBOARD area, but in the LEB area, the 3rd party meter operator might.

Other DNO's do - sometimes for a 100 quid on request.

Many have been known to if a supply connection is requested but no meter tails are provided when the bloke gets there.

Personally I think it should be mandatory at no charge. Gas has a c*ck before the meter. Water has a c*ck in the pavement, which I can operate. Phones are safe enough just to prat around with them (even if you're no supposed to). Why are we in the jurassic age with electricity supplies where to work on my CU incomers, I either dice with death or lose power for most of a day?

Bah

[tom.harrigan]

Why did you need an EIC anyway? My whole village is supposed to be getting underground supplies soon (by soon I mean 3years ago - hence my problem). I guarantee no one has an EIC (whatever that is).

T

[ARWadsworth]

Electrical Installation Certificate. Sometimes (but not often) needed by the DNO before they will connect power to a new installation.

A bit silly as in reality as you need the power to complete the certificate.

If your village is converted from TT to PME the supplier will not connect the earth up if the bonding is not up to PME standards. They will leave the supply as a TT supply (with PME available at the cutout)

Adam