I have a 2-wire overhead sub-main to a large shed 40-45m away. The house
earthing is TN-C-S (with an earth rod, ref other append) and the shed
earth comes from a local earth rod; there's an RCD in the shed CU. I
plan to replace the overhead feed with 3 core SWA in the ground (0.5m
deep, with warning tape above it at 0.3m deep), terminating the SWA
internally in IP65 adaptable boxes at each end and running the final
couple of metres in T&E.
The feed will come from a non-RCD protected MCB in the house CU. From
the TLC cable size calculator it seems that buried 2.5 SWA is OK up to
about 3.6kW for non-lighting circuits (about 2.2kW for lighting
circuits). I plan to either feed the SWA from a 16A MCB in the house and
to use a 10A MCB in the shed or or to use a 16A type C MCB in the house
and a 16A type B MCB at the shed. Any comments?
I believe the correct earthing in this situation is to export the house
earth down the SWA (hence 3-core, the armour will only be earthed at the
house end) and to remove the shed earth rod. Once again, have I missed
On 30/09/2017 11:36, firstname.lastname@example.org wrote:
ok. Although in reality with the current configuration, two core SWA
would be adequate in the circumstance.
Your limitation here is down to voltage drop. 2.5mm^2 SWA gives around
18mV/m/A of voltage drop. So 45m of 2.5mm^2 is going to give 10.53V on a
13A load. i.e. Just out of spec for a supply that will run lights...
(how much you care again will depend on the use case for the building)
At that distance I would run a larger CSA SWA - the difference in cost
of cable is small compared to the effort in laying it in. So allow for
some future proofing and expansion unless you are sure that the current
demands are unlikely to ever change.
As to MCB selection, that rather depends on what the power requirements
in the shed are going to be?
There is no "correct" answer as such, which option you choose will
depend a bit on circumstance. With a PME head end, if you elect to
export the earth, then you also need to export the main equipotential
zone. For that to make sense, the outbuilding should not have access to
any other sources of earth potential that can't readily be bonded into
the zone. (so for example, it would not be possible to extend the
equipotential zone into a greenhouse with a bare soil floor).
Also if extending the equipotential zone, the earth connection of your
submain will also need to function as a main bonding conductor and hence
meet the (copper equivalent) CSA requirements for that (typically 10mm^2
with PME - although that assumes size of the neutral of the supply is of
35mm^2 CSA or less, and where the supplier has not specified another
size be used).
If you are using 2.5mm^2 SWA, then its armour will typically be around
17mm^2 CSA. Divide that by 2.255 to get a copper equivalent of 7.55mm^2.
So if you add a 2.5mm^2 core to that, you just squeak in to meet the
minimum CSA for a PME main equipotential bond.
The building is mostly a store with a powered hacksaw, log splitter and
small welder. Electrical loads are modest and not going to exceed a
couple of kW.
I've already got enough 2.5 SWA to do it.
I chose 10A at the shed to ensure the installation stayed compliant and
16A at the head to protect the cable and avoid nuisance trips.
The point about the floor is new to me, and very relevant to another
task on the tuit list. I have a greenhouse with an overhead T&E feed
(without local earch rod) that was signed-off by someone before I bought
Is there really a difference between a soil floor and a concrete floor
that doesn't have a DPM? Where do I find the details?
Yes, and I would also need to add supplemental cable between the main
earth and the SWA termination.
It's starting to look like I'm better to keep the shed earth rod and not
to export the house earth.
Thanks for your comments.
On 30/09/2017 14:23, email@example.com wrote:
Personally I would still design for at least 13A capacity at the socket,
then you know you can plug it most loads without any difficulty.
(things like a 110V 3KV site transformers may still cause problems
though - they will often trip 20A MCBs with inrush)
Well your call then ;-)
The head end only really needs fault protection in this case - so the
protective device could be sized well above those in the shed. Even 32A
would probably still protect it...
Yup, that's a harder one with a PME supply. You can't realistically make
the greenhouse part of the equipotential zone, so in the event of a loss
of combined earth / neutral on the supply side, you would be at risk in
the greenhouse with your independent earth connections at very different
You would probably have to find a way of measuring the earth impedance
of the concrete to be sure. You could argue that if its 23K Ohms or
higher, then its not an extraneous conductive part that would otherwise
need to be included in the main bonding.
A variation on the test procedure here might work:
(personally though, if there was any doubt, I would make the outbuilding
TT and be done with it)
Yup, this is true if you are not running the SWA right to the CU.
Its what I normally do in most cases unless its a simple run close to
the main building.
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