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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> CONSUMER UNITS (AGAIN) https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1105014217 Message started by Bobnick on Jan 6^th, 2005, 12:23pm

Title: CONSUMER UNITS (AGAIN)
Post by Bobnick on Jan 6^th, 2005, 12:23pm

I realise that Consumer units have been discussed endlessly, but I can't find an answer to this question - if I've missed the answer I'd appreciate it if someone could point me at it.

I'm near to completing a house rewire (DIY). Virtually all the cables are in place and run back to the meter cupboard. Before anyone raises it - Building Control have confirmed that Part P doesn't apply because the work started before January.

I have bought a 21-way MK consumer unit box, with a 100A Mainswitch, 100A RCD and a split load kit. However, when I do the calcs, the load is not far off 100A and I'm worried that I might overload the mainswitch. Would it be better to install 2 separate consumer units - one with a mainswitch, the other an RCD - with an isolator switch before the block splitting the meter tails? The existing installation has 2 old Wylex consumer units - one with 8 rewireable fuses, the other 4.

Its a large 5 bedroom house. The used ways will look like this:

First Floor Lights (1) 6A
First Floor Lights (2) 6A
Ground Floor Lights 6A
Kitchen and Utility Lights 6A
External lights 6A
First Floor Power 32A
Kitchen and Utility Power 32A
Ground Floor Power 32A
Conservatory Power 32A
Cooker 40A
Immersion 16A
Central Heating 16A
Garage 32A

Any advice would be welcome.

Thanks

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 6^th, 2005, 1:06pm

Hey Bobnick

I don't think you need worry about overloading the Main Switch, it can happen but not often, your service fuse should be in-line to protect against that sort of overload.

Stick with the board, arrange it so the loads are in ascending order, e.g. start with the biggest current's closest to the main switch, and the same from the RCD side, this is good practice.

I would reccomend that you arrange the MCB's in this order

MAIN SWITCH 100A

Cooker 40A
Immersion 16A
Central Heating 16A
Ground Floor Lights 6A
Kitchen and Utility Lights 6A
First Floor Lights (1) 6A
First Floor Lights (2) 6A

RCD - 100A - 30mA Residual current

Garage 32A
Ground Floor Power 32A
Conservatory Power 32A
Kitchen and Utility Power 32A
First Floor Power 32A
External lights 6A

Now theres a few thing's you need to know, firstly all the breakers should be B-Type and not C-Type.

Only the ground sockets need be on the RCD, all the rest are up to you, as a rule of thumb I put all socket's on an RCD, it will only trip if you have a fault.

If the garage has an RCD of it's own in the consumer unit (CU) in there (Garages usualy have a small CU of there own) then there is no need to put the garage on the RCD side of the board in the house.

If however there is no RCD in the garage then leave as is above.

You need to look at Main Earthing and size of tails, for a 100A supply you need to be looking at using 25mm sq double insulated meter tails.

For the Main earthing to the Gas and Water Pipes and any other services you will need 10mm sq single core copper earth cable, this links from the pipe, e.g. water, back to the Main Earthing Terminal (MET) which should be located near the consumer unit.

Supplementary bonding, this is additional earthing to the main bonding, this is usualy carried out in the bathroom, it links all the pipes to each other and then to an earth teminal such as in the light switch or light fitting, this is not run back to the MET, and is run in 4mm sq earthing cable, all of the bonding should be donw with appropriate earth bonding clamps to BS 951.

A few other things to consider

Have you made sure all the cables are the correct size for there application?

e.g.. 1mm/1.5mm for light circuits, 2.5mm for a 32A Ring circuit or 20A radial and so on?

If theres any other questions you may have please feel free to ask

Earth Block -

Earth Clamp -

Title: Re: CONSUMER UNITS (AGAIN)
Post by Bobnick on Jan 6^th, 2005, 2:45pm

Thanks for the prompt reply L.Spark - and for the additional advice.

The earthing to the incoming gas and water was the only thing in the house that was done correctly when I bought it!

As for the rest of the installation, I've stuck to the guidance in the On-Site Guide. Where I was in doubt I've probably over-specified (A professional failing - I'm an Aeronautical Engineer). My wife isn't thrilled about sockets "halfway up the wall" (450mm), but she's getting used to it :).

Thanks again.

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 6^th, 2005, 3:07pm

Well if your sticking to the On Site guide (brown/blue) you can't go wrong, this is just the simplified domestic version of the regs BS7671

You do tend to get used to 450mm, we had allot in high up in the lounge (around 1000mm) when it was wired in the late 70's as the furniture was right below this, subsequently the rest of the house is about 450mm.

Do you know what type of Earthing system it is, e.g. TN-S, TN-C-S or TT?

Title: Re: CONSUMER UNITS (AGAIN)
Post by Bobnick on Jan 6^th, 2005, 5:04pm

I'm using the Blue version of the OSG - From what I can make out from the IEE website, there isn't too much difference between this and the Brown (for standard Domestic installations). I'm sure someone will respond soon if that's not the case!

The earthing system is TN-S - the earth comes via a clamp on the incomer sheath. The only earthing block is inside the consumer unit (which is OK according to the OSG), but I might change that because the earth cables for the incoming services currently have their own clamps onto the incoming supply cable.

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 6^th, 2005, 5:26pm

Hi again

The only difference between the Blue version and the Brown version is that the brown version is updated to Amendment 2, this contains the new cable colours, where they were previously Red (Live/Phase) Black (Neutral), they are now Brown (Live/Phase) Blue (Neutral) the same as flexible cable colours.

You should really upgrade the earth block (MET) if you can, it's the normal way of running the earthing now.

Quote:

but I might change that because the earth cables for the incoming services currently have their own clamps onto the incoming supply cable.

I'm not entirely sure what you mean by this, But if you follow the pictures for the TN-S setup in the On Site Guide your good to go, make sure it is the TN-S earthing your looking at and not the other systems as they have a different earthing system.

Title: Re: CONSUMER UNITS (AGAIN)
Post by Bobnick on Jan 6^th, 2005, 5:44pm

Yes, that's all I could see when I checked on the IEE site.

Sorry if I didn't make myself clear about the earthing.

Because there is no earth block outside the CU, the earth cables from the incoming services are clamped to the sheath alongside the main earthing connection.

When I said "I might change it" I meant that I would install a separate MET - from what you say, that's exactly what I ought to do. With that change (assuming I have 16mm earth cable from the clamp to the MET and from the MET to the CU earthing bar), the installation will be exactly as per Fig 4a on page 27 of the OSG.

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 6^th, 2005, 6:12pm

Yep

You should only have the supply earth clamped to the sheath, never heard of the water & gas bonded to the sheath next to it ::)

You have the right idea tho, try and find out if the supply earth cable to the CU is 16mm as it needs to be for 25mm sq tails

You should only need a 4-way, 16mm in and out at either end, and 10mm water and gas on the other two.

Hope it all works out for you

Regards

LS

Title: Re: CONSUMER UNITS (AGAIN)
Post by Lectrician on Jan 6^th, 2005, 7:37pm

It is common practice in domestic installs to utilise the earthing bar inside the CU as the MET.

Most CU's even have 3 or 4 terminals labelled 'B'. I would take your 10mm bonds direct to the CU bar, and a 16mm from there to the supply cable sheath (which may or may not have a terminal fixed to the wall for you - if it doesn't, the elec board will have to connect your 16mm earth to the sheath).

A seperate MET is also fine, although I always think they tend to corrode and go green when exposed, and if the CU isn't in a cupboard, it can look rather ugly!!

Good luck.

Title: Re: CONSUMER UNITS (AGAIN)
Post by sparkyjonny on Jan 6^th, 2005, 10:33pm

I'm not keen on visible METs - all you need is a kid with a screwdriver & an inquizative mind and you've dot no earthing!

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 6^th, 2005, 11:18pm

Personal choice to some degree lecc, having the gas and water 10mm run into the board looks bad imo, on the same hand so can a MET.

Not sure a MET should be going green tho as it's nickel plated copper, perhaps it got scratched, you must take more care Lecc ;D

But kid's tampering with screwdrivers like Jonny suggests would quickly leave you without an earth, having it contained inside the CU obviously gives it that added protection ::)

Still this isnt a debate about MET or Earth bar's so let's NOT cloud the issue of his consumer unit incase he comes back with no lights ;)

Title: Re: CONSUMER UNITS (AGAIN)
Post by Bobnick on Jan 7^th, 2005, 9:08am

Thanks guys. I had great difficulty finding the computer with all the lights out ;D

I noticed that the CU earth bar had terminals marked "BOND", but fig 4a of the OSG showed the MET and earth bar separate so I figured that was the best way to go. The point about young budding electricians is a good one, though with the CU close to the MET there are probably even more "interesting" things to do with a screwdriver. An ugly MET I could probably live with.

If I understand this correctly, either a separate MET or use of the CU earthing bar are both OK, I just have to bear in mind the points made by Lectrician and Sparkjonny.

Thanks again. I'd better get on with fitting that CU!

Title: Re: CONSUMER UNITS (AGAIN)
Post by get_it_done on Jan 13^th, 2005, 7:20pm

wrote on Jan 6^th, 2005, 11:18pm:

But kid's tampering with screwdrivers like Jonny suggests would quickly leave you without an earth, having it contained inside the CU obviously gives it that added protection ::)

From Approved Document P

1.5 ... Accessible consumer units should
be fitted with a child-proof cover or installed in
a lockable cupboard.

Another one one of these interpretation questions - what is 'accessible' ? I don't expect an answer.

Title: Re: CONSUMER UNITS (AGAIN)
Post by zambezi on Jan 13^th, 2005, 8:30pm

installed in a lockable cupboard.

Should be fun trying to find the keys to the CU (or even the CU) in the dark or better yet, try and put a key in a keyhole while you are watching one of your children doing the robot (with their finger in a socket outlet). I know that should never happen but how many circuits/homes are adequately protected?

There again your child could be doing the robot attatched to the cu, so it is a no win situation.

Title: Re: CONSUMER UNITS (AGAIN)
Post by L.Spark on Jan 13^th, 2005, 9:38pm

wrote on Jan 13^th, 2005, 7:20pm:

I'm sure we never had this ruling before ::)

Title: Re: CONSUMER UNITS (AGAIN)
Post by BigT on Jan 17^th, 2005, 5:21am

I have noticed that you dont seem to be putting in linkable smoke detectors or a heat detector for the kitchen. I have not seen the job so dont know how many levels or floor distances, should be one at least on every floor and heat in kitchen.

Here is a site that might help you with this,

www.tlc-direct.co.uk go to the technical section or look up smoke detectors then go to the technical section.

hope this helps mate but you do need them per building regs.

regards T

Title: Re: CONSUMER UNITS (AGAIN)
Post by JohnDavies on Jan 17^th, 2005, 8:21pm

Just noticed this post - Thanks, T.

I am doing a refurbishment but had no idea - must have missed it when I read the regs. Looks as if I have to fit 3 x 240v smoke alarms now, and possibly a heat sensor, as well as having sockets half way up my walls and switches half way down them!!

The joys of modern living.....but if it saves just one life it is worth it, I suppose.

John Davies

Title: Re: CONSUMER UNITS (AGAIN)
Post by plugwash on Jan 17^th, 2005, 8:30pm

wrote on Jan 17^th, 2005, 8:21pm:

The joys of modern living.....but if it saves just one life it is worth it, I suppose.

that is a very bad mindset to get into?
you should always look at the cost/benifit of any method of reducing deaths otherwise you just end up with more and more regualtion of areas where the number of deaths is already extremely low