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Topic Title: Garage wiring
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Created On: 06 May 2010 09:58 PM
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 06 May 2010 09:58 PM
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SparkJ

Posts: 35
Joined: 27 December 2006

I am installing a Garage Consumer unit with an RCD and 4 ways. Garage is 5 metres away from house.

House has TT earthing and modernish Wylex split load consumer unit. There is a free way in the non RCD protected ways.

I plan to run three core 6mm2 SWA cable from house to garage consumer unit. At the house end the plan is to install a 40A MCB for the garage circuit and terminate the SWA in a metal adaptable box adjacent to the consumer unit and then connect 6mm2 twin and earth to the SWA and terminate it in the consumer unit.

At the garage I plan to terminate the SWA in a plastic adaptable box adjacent to the garage consumer unit, insulate the earth core and terminate the live and neutral in the consumer unit. Then install an earth spike outside the garage connected to the consumer unit earth bar.

My questions are:-


Should I fit a 30ma RCD in the garage circuit near to the house consumer unit?

If so how do I address the contention issues of either of both RCDs in the circuit tripping?

Should I connect the banjo on the SWA gland to the earth core of the SWA at the house end?

Is there a more efficient wiring solution?
 06 May 2010 11:03 PM
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DOUGIE1000

Posts: 4168
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think your getting confused a little with exporting tnc-s. Pme.

Run swa from its own 30ma rcd or rcbo. 40 amp is there a need for such high ie workshop. Normaly a domestic garage on more than 16amp is more than enough.

Anyway run from rcd and opd in house to a main switch consumer unit with mcb for circuits as for the earth use the swa and at garage install another earth rod if you wish, two rods wont hurt.

Your main earth from the rod can be as small as 2.5mm so using the 6mm and the steel will be bigger than the orginal main rod.

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 06 May 2010 11:24 PM
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PG

Posts: 185
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Unusual to have a non-rcd section of a CU on a TT system but;-
1. Fit 30mA RCD near the CU or better still an RCBO - now the whole circuit is protected against earth faults.
2. You do not need an RCD at the garage end but if you have one then there is no safety issue if one or both of them trip.
3. Connect the metal jb to the house CU earth bar with the T&E CPC.

PERSONALLY,
a) Table 4D5 - 6sqmm T&E clipped direct is 47Amps and OK but I would miss out the T&E by glanding into a jb and bringing the sheathed cores into the CU. together with an additional connection for the armour continuity.
b) I wouldn't bother to install a spike at the garage end but would simply maintain earth continuity throughout (from house CU through garage CU to final circuit).

Regards
 07 May 2010 09:32 AM
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dickllewellyn

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"b) I wouldn't bother to install a spike at the garage end but would simply maintain earth continuity throughout (from house CU through garage CU to final circuit). "

Does the garage not form a seperate equipotential zone? I was always lead to believe you could distribut a PME supply to a seperate outbuilding, but for a TT supply the outbiulding would need to form it's own TT system.

In this instance I would always gland and bond the SWA at the supply end, and connect a CPC to the apt. terminal. This then gives a submain circuit complying with all regulations. At the garage, I would strip back the SWA and tape over protruding steel wire to ensure isolation, and enter without gland into a plastic adaptable box. In the box I put the CPC into a connector so it is available for testing purposes on the submain, I would then take 6mm double insulated tails into the consumer unit and install an Earth Stake with 6mm G+Y thus forming a self contained TT system within the garage.

As a submain itself does not require RCD protection, I would be inclined to connect to non RCD side of the board, but being a TT system add a 100mA TD RCD at origin.

Do feel free to pee on my fire, I may well bee creating much more work for myself than I need to!?

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 07 May 2010 01:14 PM
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AJJewsbury

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I was always lead to believe you could distribut a PME supply to a seperate outbuilding, but for a TT supply the outbiulding would need to form it's own TT system.

I think the usual advise is the other way around - especially if the garage has any exposed-conductive-parts and you don't fancy running 10mm2+ G/Y or has a damp floor that can't be readily bonded.

And what's wrong with 2 core SWA (purpose designed for single phase circuits), especially if you're not needing to bond to PME standards?

- Andy.
 07 May 2010 08:29 PM
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dickllewellyn

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Aggreed, if distributing from PME you would need to ensure the earth can support any bonding duties required of it.

As for what's wrong with 2 core SWA, absolutely nothing. I just find it easier to get 3 core, and I don't have to prove the armour is acceptable as a CPC!

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 08 May 2010 10:56 AM
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Zs

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Interesting thread.

I have a similar case on the go but on a larger scale. 3PN supply, 200A cut outs. it's a farm using only single phases these days.

He is having photo voltaic installed and the PV company have (rightly) asked for the existing electrics to be made good before they install. FYI national grid are offering 41p a unit at the moment and he gets a grant of about £5,000 towards putting it in.

The main house is the issue; a good 120 cable meters away from the supply head, which is in a barn. House is currently supplied with a 60A cut out and a 4mm earthing conductor. He's an old man, no longer farming, living in one room of the house and uses about 10Amps for a couple of sockets and the kettle. The existing supply is 16mm and given voltage drop I can understand their point of view.

Z readings are sky high. ( Main earth to the TT is also 4mm....farmers and their sunday afternoon dabblings eh?).

In order to make the house safer I intend to downgrade the cut out to suit the existing conductor size and his usage, and add a rod or two at the house end.

I was not planning to break the earth at the house end and thus, by adding a 16mm earth and rod or two, to enhance the existing set up rather than create a new TT for the house. But have been musing about breaking it.

My issue is the route across the huge green lawns with a new cable. All the other buildings are getting new supply cables. Job done. Really difficult to bring a new cable in to the house though. In passing, the armour has not been used as earth so I will be having that linked in with the existing 4mm

So, while you are discussing the smaller scale job, any views on the same thing on a much bigger and further away equivalent?

Zs
 08 May 2010 08:34 PM
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deleted_2_tony30

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Zs, i thought the grants were finished now that FIT,s have been introduced. well worth inst5alling solar, lots more people should get MCS registered!
I would add further rods/plates to reach the required resistance and connect straight into the consumer unit, i would leave the dno one in as well.
ps how do you intend to downgrade the cut out
regards
tony
 09 May 2010 06:53 AM
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normcall

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Either mole cable in or a chain trencher for the lawn. No mess and almost invisible.

-------------------------
Norman
 09 May 2010 03:52 PM
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Zs

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Sorry Tony, that was a lazy explanation.

At the moment the wall where the incomer is is a maze of rusty old fused switches leading off to various parts of the farm. Several Henley blocks and even some earth clamps on bits of SWA armour linking the earthing together. It's a bit of a mess. And filthy too, all cobwebs and mouse poo. I think Billy off eastenders might have done it.

I intend changing all that to a new 3PN board and giving each part of the installation it's own breaker and overload protection from that. I'll keep the big switch to the bore hole pumps but lose all the rest of the old fused switches and most of the henley blocks.

RCD protection at both ends but we wait to see which end triggers first! Am hoping he won't have to walk far if the RCD pops but we never know do we?

The main switch inside the board of the actual house will also have to be be downgraded from the existing 100A one to about a 40A, but haven't done the sums fully yet. Sometimes takes a few days to order up an especially low main switch but can be done.

You are very well informed and indeed the grants are over. This one has been given special dispensation because he's been unwell and there was a delay. I'm still not convinced by the maths of eco friendly systems but in this case he is doing it because he is particularly green. Still very much a rich man's playground this energy saving lark. Not yet for those of us interested in saving money on our bills.

Zs
 09 May 2010 04:22 PM
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alancapon

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Originally posted by: Zs
. . . . The main switch inside the board of the actual house will also have to be be downgraded from the existing 100A one to about a 40A, but haven't done the sums fully yet. . . .

You have confused me now. The original 100A switch should be perfectly ok unless it is faulty. The rating gives the maximum continuous load that the switch was designed for, it has no bearing on what the actual load is (as long as it is equal or lower).

Regards,

Alan.
 09 May 2010 05:01 PM
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deleted_1_windycom

Posts: 438
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Originally posted by: Zs

Interesting thread.

So, while you are discussing the smaller scale job, any views on the same thing on a much bigger and further away equivalent?

Zs


Why do you want to add a 16mm earth ?

Have you asked DNO if possible to convert to PME ? I managed to get that done on redundant barns last year. The pole transformer was close by and it had PME (liquorice bits sticking up) so had the old 3PN terminated and a new run to each of the new barn conversions through trenches we cut using farm machinery, to keep the costs down. This was right in the sticks so you may be lucky as well.

Regards, John.
 09 May 2010 11:49 PM
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AJJewsbury

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FYI national grid are offering 41p a unit at the moment

It's actually better than that! 41.3p/kWh (for <=4KWp PV systems fitted to existing buildings) is actually for generating it - i.e. that's payable even if you use it yourself and don't export it to the grid. If you do export it, you get (at least) an additional 3p/kWh!

- Andy.
 10 May 2010 01:37 PM
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OMS

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Still very much a rich man's playground this energy saving lark. Not yet for those of us interested in saving money on our bills.


I guess that's because most of us never see the real cost of energy madam - it's been far too cheap for far too long.

I'll give you something to think about - slavery came to an end not because it was abhorrent to most right minded people but because the value of a ton of coal (and the energy within) was financially cheaper than feeding and housing 150 slaves for a few days and a lot easier to manage.

So a move to renewables should not be solely money focused in my opinion - we should act sustainably because we can - and it might just save a catastrophe.

Regards

OMS

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Failure is always an option
 10 May 2010 05:14 PM
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BS76712008

Posts: 19
Joined: 05 May 2010

Hello

I would fit an rcbo and main switch in its own enclosure in the house fed from service connector blocks (split the live and neutral) between the two, my reason is that you would therefore be protecting both the cable to garage and all other equipment that is in or fed from garage
distribution board (this would also contain a dp main switch and some smaller value mcbs (depends whats in garage) the cable to the garage would be 3C 6mm csa not 2C 6mm csa earth for garage taken from CMET in house using third core of SWA.

If a fault develops with cable to garage or other equipment on this circuit then it would not affect the rest of the installation. Best to keep out buildings sheds etc. separate - good practice. Hope this helps.


Lee.
 11 May 2010 04:35 PM
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deleted_2_tony30

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Energy independence is a great thing, just think in 20 years with no spending on energy/infrastructue combined wih soaring oil prices/gas prices the guy that paid £12-15k for a 2Kw pv installation will be laughing, he will probably make 3 times that over the life of the panels( seen the figures somewhere) compared with the initial money sitting in a bank account.

Possibly a good little earner, as well as helping to be green, its not easy you know!

tony
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