Electrical Installation Q

Adding a new out-building to my house and just pondering the electricity supply to it. (ignoring issue of whether I am allowed to do this)
Only need a couple of lights and a couple of sockets (ad hoc use only) Intend fitting a Garage CU ,,, 40A 30mA RCD – 1 x 16A for sockets 1 x 6A for lights
Current house install is Supply authority cabinet .. cabled with 32mm tails to a 100A switch fuse which in turn goes to a Henley block and feeds 2 x CU's. (wanted plenty of circuit granularity) Be interested in knowing which option to follow…
Opt A … put a DP switch alongside the switch fuse (downstream of the fuse) and feed that via armoured cable to the outbuilding, and install the garage CU … i.e. the DP sw allows complete isolation of outbuilding without impacting house.
Opt B .. add a 40A mcb into one of the existing house CU's, and then run feed to outbuilding and it's CU. Does not allow full `isolation' but mcb would allow power interruption of that feed.
Be interested in views … and whether Regs call for Opt A .. i.e. local isolation.
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Osprey wrote:

I would use a 20A MCB for the sockets, as per a normal radial circuit, assuming you are wiring it in 2.5mm cable.

If this new switch is just going to be a switch, then your run to the out-building is going to need a HUGE cable as it will be fused at 100A You would need to install a switch fuse with an appropriate fuse if you want to use a sensible sized cable.

This is what I have done, but I used a 32A MCB, and made sure it was connected to the non RCD consumer unit.

The main switch in the CU will act as local isolation, and as you will put it on a non RCD side, you wont have any problems with RCD's tripping because there is a neutral problem.
More information is needed really..
How long is the cable run between the house CU and the outbuilding CU What cable are you planning on using (Have you installed this already?) What is the out-building made of? Are there any other services to this out-building (Water/Gas/etc.)
Toby...
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Osprey wrote:

You would need a fuse/mcb at the head end to protect the submain from overcurrent or fault currents, unless it is of sufficient size that the main 100A cutout can perform this task (which seems a bit excessive for a few sockets and a light)

Full isolation would come via turning off the CU its fed from, not its MCB
How about:
Opt 3 - install third mini CU at the head end, fed via additional set of tails from the Henley. This could have a single fuse or MCB of appropriate size, and a master switch. Hence protecting the submain, and providing ready isolation discrete from any other circuits.
More detail here:
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside
--
Cheers,

John.

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My description may not have been that clear … though both comments are useful.
Current house install is Supply authority cabinet .. cabled with 32mm tails to a 100A switch fuse which in turn goes to a Henley block and feeds 2 x CU's. (wanted plenty of circuit granularity)
I want to supply a feed to an outbuilding (new garage) … intend putting a small Cu in garage consisting of 40A 30mA RCD plus 16A & 6A mcb for limited light & power. Something like .. http://tinyurl.com/6rozky No other services - & this is a permanent building. Dug a trench last night, The run to this from house is 10m long and will be in 3core armoured, not checked but assuming it will be 2.5mm2
It would seem to make sense to have this capable of isolation from house … Opt A … put a 40A DP switch fuse alongside the existing switch fuse (downstream of the fuse) and feed that via armoured cable to the outbuilding, and install the garage CU … i.e. the DP sw allows complete isolation of outbuilding without impacting house. 40A fuse limits fault current to sub cct.
Opt B .. add a 40A mcb into one of the existing house CU's, and then run feed from this to outbuilding and it's CU. Does not allow full `isolation' without affecting house CU. If I do use Opt B … then guess I would have to terminate SWA cable in box of some sort alongside CU to allow a gland and correct earthing & termination of armour wires etc.
I have sketched out the existing installation and 2 options … http://tinyurl.com/5ddcun
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If you use option B then terminate the armoured into a metal clad 45Amp switch. (eg <http://www.sparksdirect.co.uk/metal-clad-1-gang-45-amp-dp-red-rocker-switch-with-neon-p-13999.html ) Not only will this allow a good strong point for the armoured to terminate you will then have your double pole isolation in the house.
There is little to chose beween options A and B. With option A you will still have to find a 40A DP switched fuse or a 2 way CU that will take your armoured.
Personally I would also look up the difference in cost between 2.5mm and 4mm armoured. I suspect 10m of 2.5 armoured plus the wiring in the outbuilding will be big enough the expected voltage drop.
Adam
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<http://www.sparksdirect.co.uk/metal-clad-1-gang-45-amp-dp-red-rocker-switch-with-neon-p-13999.html

That is simpler (and presumably cheaper) than a switch fuse :-) not thought of that.

Yep - not done the math's yet, but 4mm2 will be about 30% higher in cost .. plus not as easy to route, but I'll have to check the voltage drop.
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Rick Hughes wrote:

Yup, that is fine, so long as you have the spare terminal capacity on the output of the existing switch fuse to get the new cable in. (this is essentially what I was suggesting as my option 3 - although taking the feed from the existing Henly. Obviously go with whichever is easiest and in the best location.

Yup, as Adam said, stick a small metal clad enclosure in somewhere with a DP switch and you have another workable solution.

I would be tempted to up the size of the SWA a bit (using two core with armour for earth would keep the overall size about the same).
If you max design load in the garage is 26A then your voltage drop on 2.5mm^2 SWA is well in spec, but it limits your future options a bit.
--
Cheers,

John.

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John ...
What is the 'current considered view on using SWA as earth ? ....
When I last asked on this site why someone was running in a separate earth, was told that it is not acceptable good wiring practise to rely on SWA for earth fault protection on consumer side of installation ?
The c/s area of armour is less than any one of the cores, and thus it is not suitable to be used as a bonding conductor. It therefore cannot be used as as a sub-main, but it can be used as a final sub-circuit. I am not sure what my use would be classed as. ?
I have 2 garages as part of the house ... and have multiple outlets there including 32A industrial ones .. the new outbuildings do not need much in way of power ...
Estimating total cable run (excluding wiring in outbuilding) as 15m and max current load as 22A (16 + 6), then using 18mV/A/M would give me 5.94V on 2.5mm2 (well under permitted 9.2V) and 3.63V on 4mm2 ...
That makes 2.5mm2 more than man enough for the job ...
However I would consider going to 4mm2 if dropping off the earth is allowable for this installation. (as per above comment) ... any comments ?
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Osprey wrote:

Same as it's always been, it's allowable, desirable and from the point of view of saving valuable copper it's best practice. For cables of sizes likely to be used for DIY it will rarely if ever be necessary to use a separate circuit protective conductor (CPC). Even if a separate CPC is used it's still necessary to earth the armour.

This is quite untrue. Good practice does of course mean using the correct size and type of glands.

That's not true for any 2-core SWA size up to and including 120 mm^2 (XLPE cables to BS 5467). For the small sizes the armour CSA is much larger than the cores - e.g. 6 mm^2 2-core has 22 mm^2 of steel armour.
Moreover the _copper_equivalent_ size of the armour (obtained by dividing the steel area by 2.255) complies with the tabulated requirements for use as a CPC without further calculation for all SWA sizes up to and including 95 mm^2. [Ref. Table 54G in BS 7671 (Table 54.7 in the 17th ed.)]
The only thing that you have to watch is that the resistance per metre of the armour may be higher than for the line and neutral conductors, and you need to take this into account when evaluating the earth fault loop impedance (Zs).

If the armour has to act as a main bonding conductor as well as a CPC (e.g. for a submain to an outbuilding with services that need main bonding) then you'll usually need 10 mm^2 copper equivalent (for PME supplies). This, by coincidence, means using at least 10 mm^2 2-core SWA.

As an aside, the voltage drop limits (guidance) have changed in the 17th ed. and are now 3% for lighting and 5% for all other uses. Also you must allow for drop in the outbuilding's wiring in addition to the submain.

Check the Zs though. The armour resistance of 2.5 SWA is 8.8 milliohm/m at 20 deg. so applying the usual rule of thumb for heating under fault conditions it will contribute about 0.18 ohm to Zs. What's the protection on the supply side? - if you have a 30 A fuse and a TN-S supply you might be getting near the limit...

I'd go for at least 4, and possibly 6 for this. Incremental cost is small and extra capacity always useful in the future. And as I said, if you're exporting a PME earth and anything needs main bonding at the far end you must use 10, or run a separate bonding conductor.
See also John Peckham's article here http://www.gadsolutions.biz/electrical/regulations/swa-as-cpc.html
--
Andy

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Andy as usual - many thanks
I looked back at original comments here regarding not using earth wires .. and it was threaded to bonding conductor requirements. So 4mm2 it is, off to TLC this morning then.
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On further Q ... if you could help
Existing house installation is on a PME (TN-C-S), but still classed as temporary supply (in final permanent location) and therefore using my own supplied earth rod. (been on this supply for 8 years )
Should I be exporting this earth to the outbuildings .. or should I insulate armour wires at outbuilding end and install a local earth rod ? At some point in future house will have it's acceptance certificate and I will request Supply Authority change supply over to a permanent one, at which point they will switch me to their earth.
If I do use a local earth rod at out buildings do I then disconnect this when I go on permanent supply, or is it just left in place as another 'multiple earth point'
There are no other services in the outbuildings.
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Rick Hughes wrote:

With everything on one 30 mA RCD too, I guess...

In which case you should design the outbuilding installation on the assumption that the house will eventually be PME.
The first decision is will the outbuilding use the exported house earth or be a separate TT installation. The basic question is "are you likely to come into simultaneous contact with the exported 'earth' (which will rise in potential during a fault) and the local ground, or metal things in contact with the local ground". Don't export PME if you're likely to use Class 1 (earthed) equipment outdoors, or to a workshop with a bare concrete floor, or to a metal greenhouse, and certainly not to a caravan or boat (the last two being illegal under the ESQC regs). TT is the safer option, provided your earth electrode remains in good condition (and remains connected!) and provided you test the RCDs regularly.
For (eventual) exported PME option connect the armour solidly at both ends. Under the temporary arrangements this will give you TT throughout. There's no need for a separate earth rod at the outbuilding end, although you can add one if you like belt and braces. Assuming the plan is eventually to feed the submain via a fuse or MCB as a non-RCD-protected distribution circuit, you'll need one or more RCDs in the outbuilding (probably just a single 30 mA RCD if it's only a a few lights and sockets as described). Under the temporary arrangement you'll have two RCDs in cascade - so no discrimination - but this will eventually resolve itself.
If the outbuilding is to be separately TT-earthed then, as you say, earth the armour at the house end only. Isolate and insulate it at the far end using an insulating gland (expensive) or use a big plastic stuffing gland and heatshrink sleeving. Earth the installation to a decent earth rod, preferably two 4ft. sections to get down to a good depth below any likely frost. The earthing conductor (rod to disboard), if normal 6491X green/yellow wire without mechanical protection, should be 16 mm^2 to comply with regs. RCD(s) as above are required and the discrimination issue remains until the house is converted to PME.
[Exported PME)

Since you're using a fairly small cable I'd disconnect it. If the cable were big enough to satisfy the 10 mm^2 copper equivalent requirement for a main bonding conductor it would be safe to leave it in place.
--
Andy

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Andy ,, Seems to be raising more questions than I thought. Some answers to questions raised.

No, the existing house install has 2 CU's ... One of which is 30mA RCD protected ... ring mains & radials The other a Split load .. providing some non-protected (freezer, alarm, smk detectors etc.) and the rest via a 100mA RCD for lighting circuits.

Outbuilding has a bare concrete floor but wooden fame, tiled roof .. no other services ... just a couple of sockets & lights. So will be running as advised 2C SWA cable out to the building ...
If I fed this via 30mA RCD protection form the house .. (either existing CU or a new feed to a new small RCD CU off the 100A switch fuse) ... then am I Ok to export the earth to the garage ? Under fault conditions the SWA cable is RCD protected .. not just outbuilding wiring. I could then just use a small CU in the garage with no local RCD.
Although I can happily install a local earth rod & cable at outbuilding ... I have a roll of 16mm2 earth cable spare.
I have put what I think are the 2 options on this sketch http://tinyurl.com/59pw8h
... is either wrong or either preferred ?
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Rick Hughes wrote:

The RCD does not really have any bearing on the risk in this case - since under fault conditions it will not open the earth connection. The concern with PME is that in the event of a fault such as a disconnected PEN conductor (i.e. combined earth and neutral) provided by your supplier, your earthed metalwork could rise in voltage. This in itself is accceptable if you can maintain an equipotential bonded zone such that you can't expose yourself to a dangerous potential differences between the elevated earth and some external earth reference.
If your outbuilding allows easy access to an independent earth reference, then you either have to include that in your equipotential bonding (back to separate protective conductor or large SWA again), or switch to TT for the outbuilding.
The other consideration is the one Andy mentioned about use of class 1 appliances outside. With a garage there is always the danger that someone will plug in such a device - say a vacuum cleaner to take out to a car etc. So no matter how carefully you have preserved the equipotential zone in the garage, you can't stop someone exporting the earth to somewhere outside that zone via an appliance.

You lose the discrimination in the garage (which may or may not be an issue), but more to the point you don't add much protection to the submain since it will have adequate earth fault protection by virtue of its PME head end.

First one has the problem described wrt to the exported earth and maintaining equipotential zones.
The second would be my preferred of the two... (if you want to get posh, you could skip the RCD in the garage CU and use two RCBOs instead).
--
Cheers,

John.

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John Rumm wrote:
[Big snip - all agreed]

Don't think I've ever seen a Class 1 vacuum cleaner. Most portable appliances you'd want to use outside are Class 2 in fact, so this consideration is perhaps a bit theoretical. Nevertheless it is a consideration.

Which may or may not be significant, depending on circumstances. The bare concrete floor though inclines me to prefer the second (TT) option. There's also the disadvantage that an RCD trip in the garage means a walk back to the house to reset it.

Yes _BUT_ as drawn it's not acceptable with the house on a temporary TT supply. An earth fault on the submain cable is unprotected (Zs much to high to blow the 32 A fuse) and if it occurred all the earthing would sit up at 230 V waiting to bite you. There needs to be a temporary RCD at the house end (ideally a 100 mA delayed action one).
--
Andy

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Andy Wade wrote:

Was about to say "I have", but that was a Hoover Senior about 20 years ago... and it was not exactly new then! ;-)

Yes good point. I used to have an outbuilding supply like that (well I suppose I have again now - having fixed it once at the old house, I get to do it all over at the new one) and it was a PITA if you did get a trip.

Yes well spotted - I was forgetting we were describing the future supply rather than the current situation.
(I would still go TT on the outbuilding, and if need be run from a non time delayed RCD at the head end and just accept the lack of discrimination while the supply is still "temporary". Assuming the switch at the head end is in a normal DIN enclosure then swapping the switch for a RCD would probably be simplest)
--
Cheers,

John.

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Andy Wade wrote:

How about a waterproof gland onto a plastic CU in the garage, that way there is no way to make touch contact with the PME earthed armour of the cable?
--
Cheers,

John.

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John Rumm wrote:

That's what I meant really - where the waterproof gland is an all plastic one, something like http://www.lapplimited.com/14/en/brands/skintopR/index.html . And I'd still put some heatshrink over the cable to dress the end where the armour has been cut off flush with the sheath (this is inside the CU, of course).
A brass SWA gland wouldn't be acceptable since it's quite easy to pull the PVC sleeve down and make contact with the metal body which is connected to the armour.
--
Andy

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Andy Wade wrote:

Those are cheap enough at TLC IIRC...

You could substitute heatshrink for (or place under) the PVC boot - then it won't be sliding anywhere in a hurry!
--
Cheers,

John.

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Guys appreciate your help (and patience) ... let's see if I have understood.
I earth the armour wires of cable at house end, and totally insulated armour wires at outbuilding (plastic gland) Install an earth rod and use that to supply earth to the out building. (making that a TT installation) I fit a 30mA RCD in outbuilding to protect all circuits there (or at least AC outlets)
I then feed the cable at house end via a new sw fuse and 100mA RCD .. to provide fault current protection of the cable itself. or I could use an RCBO to combine fuse & RCD protection. http://tinyurl.com/5zd73z
When Leccy board do finally change me to PME proper, I am assuming I can leave the install as is ?
Just for a further option could I take the feed at house from a spare 30mA protected way in consumer unit .... and avoid need for the 100mA RCD. ... if this is OK (and can't see a reason why not) ... do I still need a separte RCD in outbuilding ? I can live without the discrimination. http://tinyurl.com/5kcsjc ... on the this option I have drawn a DP sw ... this is to give me a place to easily isolate the 'cable' and also easily terminate the armour wires, I would use the suggested option of.... http://tinyurl.com/57jmud
In both cases am I right that I still need the RCD in the outbuilding to provide local protection to sockets ?
I had some problem with eSnips .. so also uploaded a combined sketch ... http://tinyurl.com/6ovfum
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