Adding a new out-building to my house and just pondering the
electricity supply to it. (ignoring issue of whether I am allowed to
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
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.
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.)
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
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:
My description may not have been that clear though both comments are
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
If you use option B then terminate the armoured into a metal clad 45Amp
) 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
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.
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.
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
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
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
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.
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.
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.
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.
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
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
... is either wrong or either preferred ?
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).
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
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).
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)
That's what I meant really - where the waterproof gland is an all
plastic one, something like
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.
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
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.
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 ...
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