Can someone help me with their thoughts on shed electrics.
I want to run electric to my shed, and am planning to use 2.5mm SWA
cable so its protected. How deep should I bury it - I can do this
under a hedge so should avoid random spade attack etc.
I want to run 1 internal double socket, I IP55 external socket, 1
internal strip light and an external PIR light.
Real questions are:-
1) What should the MCB rating be at the house consumer unit and should
it be RCD protected?
2) Do I need to add another consumer unit in the shed? If so what
should the maximum RCD rating be on this?
3) Should I add a local earth rod? If so what guage of earth wire
should I use to connect to it?
4) Is there an electrical book that I can read, that can help me with
this kind of question in the future?
Have a Google there are plenty of threads on this.
Assuming that is sufficient.
It's not that easy digging near a hedge...
There are no specific regs, If it is an area that is at all likely to be
cultivated then go for about 450mm. i did the one up the garden like
that, the one from GH to shed runs beneath a path and I put in
16 Amp should do. Personally I would have an RCD at the shed end in the
CU, and instead of an MCB use a switch fuse unit.
You could get away without it I guess, but it would be better with one.
an RCD protected one with say 2-3 ways - 16A MCB for the sockets, 5A for
the lights. you can get pretty cheap small 'garage' CU from Screwfix for
Possibly, there have been discussions on this, unless the shed is near
the house I would go for this option.
The Which book is pretty good, though some bits are not covered so much.
One important question to answer is the length of the cable run. This
affects the cable size required as on longer lengths such in gardens
voltage drop can be an issue. You might need greater than 2.5mm cable.
If you use SWA cable for the supply, then try to go as heavy as you can
without being over the top. If you use a two core SWA with 6mm conductors
from the house and have it protected with a suitable breaker, 45amps will do
it, within the consumer or sub-mains switch, then you can really take the
supply as dealt with and safe for anything, other than welders and massive
heating elements, you want to fit and work with in the shed. Burying the
cable is the one thing people don't usually do properly and the only advice
I give them is to bury it as far down as possible so it doesn't get dug up
again accidentally. And that doesn't mean by you because you know it's
there, it's also by other people who might live in your house after you do.
Having power into the shed is also a great selling point remember.
Within the shed itself you should have a consumer unit with enough ways to
take a small ring mains circuit protected at 30amps for wired fuses, or
32amps for an MCB breaker, and run your sockets from this. A lighting
circuit will be protected by a 5amps wired fuse, or 6amps MCB breaker,, and
this is enough to run even halogen security lights from if you wish. It's
also a good idea to have an all weather socket low down beside the door and
separately fused from the other circuits and can be used for all manner of
things in the garden.
Always make the system separately earthed from the house circuits, but this
doesn't mean separating the bonding on the SWA cable between the two points.
Use the house earth to protect the outer skin of the cable between the two
consumer units, but have a separate earthing rod into the ground locally to
the shed and connected directly to the earthing bar in the shed consumer
unit. It doesn't interfere with any of the systems proper working profiles
and just gives the added local protection needed for this type of
installation. Make sure that the impedance between the rod and the ground
is as low as humanly possible, so this night mean you have to bang in a two
metre length of 15mm copper pipe to have a proper protective local earth.
Regarding the earth.
So, you take the cable from the house csu to the shed and do you
connect the earth to the shed csu or direct to the earthing rod.
The which? book doesn't make this clear and actualy says not to
connect the house>shed cable earth wire to the shed csu, connect the
shed csu to an earth rod, but what would you do with the house>shed
It's probably obvious to you all but not to me.
It seems that some people isolate the cable earth at the shed, whilst others
connect it to the earthing terminal. Obviously, there is always a connection
to the earth rod.
I can see advantages to both. Using the cable introduces a second earth to
the system, in case one earth fails. However, using the cable could result
in current flowing from the house to the shed under earth fault conditions.
However, I suspect any such flow would be shortlived and limited in current,
preventing the small earth SWA earth overheating. Also, the supplier's earth
is probably much better anyway.
In any case, there should be an RCD in the shed covering the entire
installation. The "best" solution is a time delayed 100mA RCD with 30mA RCBO
for sockets and 6A MCB for lights. However, unless you are using dangerous
machinery in the shed, this is overkill and you could get away with a 30mA
"whole shed" installation. The difference is (apart from being around 40
quid cheaper) that when you chop the power cord, the lights go out (which
isn't a good idea if you are holding a still rotating dangerous piece of
Do you think having a 2nd earthing source is good practice? My query was do
you feed house supply cable to earth rod or to the shed csu first and then
hook the earth rod into the shed csu.
"The "best" solution is a time delayed 100mA RCD with 30mA RCBO"
Isn't a RCBO a MCB and RCD combined? Which just plugs into the CSU (v.
expensive though). Is it possible to get different mA's for the RCBO?
The "best" solution would have the following
+---> 100mA -------+---> 6A MCB ---------> lights
time delay |
RCD +---> 20A/30mA RCBO --> sockets
The RCBO could be a different size (i.e. 16A/32A) depending on the size of
your main feed cable. The above is total overkill for a shed that isn't
really a workshop with dangerous machinery. The solution above is better,
because in the event of an earth fault on the sockets, the RCBO is
guaranteed to blow before the 100mA time delay unit, keeping your lights on.
For normal shed purposes, the following is cheaper:
+---> 30mA -----+----> 6A MCB ---> lights
+----> 20A MCB --> sockets
You may be able to use the RCDs as replacements for the switch incomer on
the consumer unit. You can almost certainly buy the second solution off the
shelf as a preloaded consumer unit under the "garage" consumer unit section.
It will be much cheaper.
Has a 16A MCB, instead of a 20A. However, at 25 quid, it is cheaper than
just a single RCBO.
I though it was good practice to have the shed earth separate from the
house earth (if there was an earth at the shed end) The supply goes to
the CU not the earth rod.
My shed/GH supply is fed with 2 core SWA. The steel sheath of the SWA is
earthed at the house end. It is not connected to earth at all at the
shed end. The same would apply if the cable was had an earth as well.
The SWA cable terminates at the shed end in a small plastic CU (if you
used a metal CU it would need an isolating gland). The CU earth terminal
is then connected to the earth rod.
Shed CU has a 30mA RCD protecting all the circuits.
At the house end the supply is connected to a Switch fuse unit (Wylex
make them) into the supply via a Henley block.
Did you work out the cable size needed?
This thread seems to be generating more heat than light and not much in the
way of signal-to-noise ratio.
Briefly, there are two main earthing options:
1. Exported house earth
Simply 'export' the house earth via the armour of the SWA cable (the
'submain'). This will be fine for a dry timber shed with its floor raised
well off the ground, so that it remains dry. With this option the shed
installation has TN-S earthing and RCD protection is only essential for
sockets likely to be used for feeding portable equipment outdoors. If
non-RCD protected circuits are used then you need to be sure that the earth
fault loop impedance (Zs) is low enough to operate the fuse or MCB at the
house end within 5 seconds in the event of an earth fault at the far end of
the submain cable, and to operate the protection for any socket sub-circuits
in the shed within 0.4 s.
If Zs is too high then the options are: (a) use a larger size SWA cable,
and/or; (b) use a 3-core SWA cable with one core as a CPC (earth) in
parallel with the armour, or; (c) use an RCD at the house end (a 100mA
Type-S RCD if you want discrimination with any RCDs in the shed itself).
With this option there is absolutely no point in using a local earth
electrode; its resistance simply will not be low enough to do any good.
Do NOT use this option if the house earthing is TN-C-S (PME) and either of
the following apply:
- the shed floor is damp, or the 'shed' is in fact a greenhouse;
- you plan to use Class I (earthed) appliances outdoors on a regular
basis (most portable tools etc. are Class II ('double insulated).
If there are any metal service pipes entering the shed (water, etc.) then
they should be bonded to the incoming earth near the point of entry.
2. Separate 'TT' installation
This option uses independent local earthing and is ultimately safer, but
only if the RCD(s) are correctly selected and regularly tested, and the
earth electrode is properly installed and maintained. The rationale is that
by limiting the current dumped into the local earth, its voltage will not
rise to a dangerous voltage relative to the ground.
The armour of the SWA cable is earthed at the house end, but MUST be
isolated at the shed end. This can be achieved by using a special isolating
gland (expensive) or by terminating the cable into a plastic housing via a
plastic cable gland. The armour should be treated as being live, so should
not accessible or connected to accessible metalwork.
ALL circuits in the shed must have RCD protection. For a comprehensive
workshop installation follow the advice in the OSG and use at least two
RCDs, or separate RCBOs, but for a simple garden shed a single 30mA RCD will
be quite OK. This can be at either the house or shed end, but it migh be
inconvenient to have to walk back to the house if it trips. If the RCD is
at the shed end then you still have to make sure that the submain cable is
fault and earth fault protected - see remarks about Zs, above.
An earthing system is required. In most soils a single 8 ft. earth rod (two
4 ft. sections screwed together) will be OK, but the earthing resistance
should always be measured. The BS 7671 requirement is that the product of
the earth electrode's resistance to earth and the highest rated RCD must not
exceed 50 volts - or 25 V if it's a 'horticultural installation'
(greenhouse). This allows quite high earth resistances to comply with
regs - 1,600 ohms with a 30 mA RCD, but note that the OSG recommends a
maximum value of 200 ohms. The connection to the earth rod should be in
16mm^2 wire (unless protected in conduit, etc.) and the joint should be
accessible, unless welded. Using a proper 'earth pit' from the electical
wholesaler is the professional way to do this.
~ ~ ~ ~ ~ ~ ~
Going back to the original enquiry: this is only two lights and 'small
power' in a shed, probably just for running portable power tools and maybe a
fan heater, with the outside socket being for a lawnmower or hedge trimmer,
etc. It's hardly a comprehensive workshop installation.
The first step, as always, is to establish the maximum demand and I'd guess
that about 4kW would be adequate, or 5kW at the most. If that's the case
then a 16A (3.7kW max.) or 20A (4.6kW max.) circuit from the house is all
that's required -- unless the OP wants to provide for future expansion.
There's no real need for a separate consumer unit here. To keep things
simple the sockets could be connected directly to the circuit cable and the
lights fed via switched fused connection units, fitted with a 5A fuses and
used as the lightswitches. You could also add a 20A control switch to act
as a main isolator if desired. Something like this, perhaps (or re-arranged
to suit the desired physical layout):
DP SW FCU DBL SKT FCU
20A 5A 13A 5A
__ __ __ __
From house >----------<|__|-----|__|-----|__|------|__|
+ | ^ *| |
| | |
| | |
Shed <-- | --> PIR
light | light
RCD options (if no RCD at house end):
+ use RCD as main isolator (essential if using local TT earth option)
^ insert RCD here (for 'exported' TN earthing only), or
* use RCD sockets
For a tidy looking job use metalclad wiring accessories mounted side-by-side
and coupled together using 20mm conduit bushes and lock-rings.
Terminate the incoming SWA onto the first accessory (the DP switch in my
sketch). The only additional wiring is then that to the lights and outside
The max. circuit lengths in 2.5mm^2 cable are 33m for a 16A circuit and 27m
for 20A (voltage drop limited). This makes the use of 2.5mm^2 SWA rather
marginal and I'd strongly recommend using 4mm^2 to keep the voltage drop
down. The difference in cost is tiny.
It is the "MUST" part above that needs explaining. We established in a
previous thread that in a TN supply/TT generator switchover system, that the
TT earthing rod required for generator TT operation may be connected to the
TN supplier's earth. Why does the same not apply here, provided that the
earth rod is tested to below 200 ohms and the RCD requirements are met? Is
it some sort of earth loop current issue?
Sorry for a rather delayed follow-up. If you don't break the earth then you
have effectively exported the house earth and your shed is a TN system, not
TT. And, unless you've sunk a massive earthing system, your local earth
electrode will be bugger-all use in preventing a rise in voltage above the
local ground potential if a supply network fault is trying to pull it up
The need to get away from the supplier's TN earth is relevant where the
equipotential zone concept is difficult or impossible to apply - hence the
reference to greenhouses and Class I equipment used outdoors. Remember that
TN earths can rise to potentially dangerous voltages due to (for example)
cable faults on TN-S systems, or the classic broken service neutral on a PME
supply. OK, these things are pretty rare, but they do happen, which is one
reason why equipotential bonding is required. The big advantage of TT
earthing, properly implemented and maintained, is that your earth really is
at the local ground potential, and will stay there. Even if the earth
electrode has the maximum recommended resistance value of 200 ohms (see
OSG), then to trip a 100mA RCD shouldn't lift the earth by more than 20V.
The downside of TT is the reliance on RCDs, which don't always fail safe,
and the need to ensure that the earth system is maintained. Hence in dry
conditions it's probably best to stick with TN.
An alternative protective measure for PME installations now being
recommended by the IEE  is to provide your own earth electrode and
connect it to the main earth teminal in the _house_ installation - i.e. in
parallel with the supplier's multiple earthing. The recommended maximum
earth resistance in this case is, for domestic installations, 20 ohms - not
necessarily easy to achieve with DIY efforts in some soils. Then, in the
event of a break in the supply neutral, there is a path to earth which will
tend to reduce the touch voltage on your earthing. If you only have a few
lights switched on this will be a quite effective safety measure, but it
won't help a great deal if an electic shower is in use at the time of the
fault - that's when you really rely on the bathroom's supplementary bonding
to save your life.
I hope that's clearer now from the above. A standby genny on your own
premises can always be operated as a TN system - why would you want use TT
for that? You do have to provide your own independent earthing of course -
and the parallel earth electrode approach referred to above would kill two
birds with one stone.
 See, for example, section 12.5.4 of the current /commentary/ on
BS 7671, ISBN 0852962371. (Recommended reading for all wiring
Would you care to explain that a bit more?
I am considering a small genny to power my central heating plus a couple of
freezers and some lighting in the event of a power cut. All devices fed by
the genny would be unplugged from their usual mains sockets and plugged into
the genny instead - and would thus be totally isolated from the house
supply. [I have wired my CH through a 13A plug rather than a FCU to
facilitate this.] What do I need to do about earthing these appliances when
they are being powered by the genny?
<snip a whole load of "signal">
I would vote for the inclusion of that in the electrical section of the
uk.d-i-y FAQ, if it is sufficiently generic to deal with a lot of the
export/don't export earth questions.
email me at
richard at olifant d-ot co do-t uk
Nice write up. As this comes up a lot, how about sending to Phil to
go into the FAQ....
I've used both of the described methods for different applications.
Do you have any particular suggestions where the provisioning is for a
workshop and heavier equipment is in use - say 30A single phase, where
the workshop is dry and the house supply is TN-C-S?
To email, substitute .nospam with .gl
"Andy Wade" wrote
| Briefly, there are two main earthing options:
| 1. Exported house earth
| Simply 'export' the house earth via the armour of the SWA cable (the
| 'submain'). This will be fine for a dry timber shed with its floor
| raised well off the ground, so that it remains dry. ...
| Do NOT use this option if the house earthing is TN-C-S (PME) and
Should that 'and' be an 'or' (PME earths should never be exported)?
| For a tidy looking job use metalclad wiring accessories mounted
| side-by-side and coupled together using 20mm conduit bushes and
And I'd suggest running a green/yellow earth wire from the SWA gland through
each box and connecting to the earth terminal in each box and on each
faceplate, rather than relying on conduit-box-faceplate touching contacts.
Maybe you assumed this would be done, but better to be explicit.
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