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Shed Electrics |
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| message from Paul on 21 May 2004 |
Hi,
I've been reading many of the threads regarding shed or greenhouse
electrics, but I'm still not 100% on the way to go, so I thought it
best to clarify some things before I proceed.
I have a split-load consumer unit. From the CU to the outside of the
house it approx 8m. From the outside of the house to the brick built
shed it is approx another 8m. I am planning to have a double socket
and a light. The double socket will be used for the occassional use of
a power tool such as a jigsaw or a lawnmower (about 2000w max). The
light will be a standard 100w. Also I plan to power a water feature (2
metres from shed, 25w), a pond pump (15m from shed, 25w), a security
light (500w) attached to the outside of the shed and also have a
single waterproof socket (15m from shed, by pond) just incase I need
to plug something in at the top of my garden.
My questions are (sorry there are a lot of them) :
1. I'm going to use a spare 20Amp 'B' type MCB on the non-RCD
protected side of the CU using 4mm T+E cable. Can I use normal T+E
cable for the first 8m (up to the outside of the house) and then
connect it using a junction box, to 8m of SWA 4mm cable to the shed?
2. I was planning to place the RCD spur unit inside the shed at the
end of the SWA cable. Can I then continue the radial circuit using
normal 4mm T+E or could I use a Fused Connection Unit after the RCD
and continue with 2.5mm T+E?
3. Can I use a plastic back box for the double socket or are there
special 'shed' style sockets?
4. I have estimated a total of 10m of cable being used inside the
shed. Does this have any impact of whether I should use a separate
earth rod or can I just connect the power to the house earth?
Just so you know, I was going to spur to use a 5amp FCU for the shed
light and the security light. Also, all the outside cabling is SWA and
will be in that 20mm blue plastic piping that you can buy from Wickes,
buried deep deep underground.
Thank you in advance!
Paul
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| BigWallop replied to Paul on 21 May 2004 |
<<<snipped>>>
Any external wiring should be protected as much as possible, so I'd personally
advise you to use the RCD side of the CU.
It's also best and safest if you can make the SWA in a single run all the way
between the two points. Joints in anything, by nature, make them less
efficient. And I'd also go for 6 mm csa' SWA on that full length to make sure
that supply is adequate enough to take any fault currents.
The circuits you intend to install from the shed supply are not going to be huge
loads, so 2.5 mm csa' should be enough for radial connection to them.
You might also find that you need to install quite a large, not huge, junction
box inside the shed to take all the seperate circuit connections you want to
make. Not just now, but in the future.
Try to go for industrial socket plates and light fittings because they are
slightly safer for outdoor use. They also take more of a bashing in extreme
weather conditions. Weather proof is even better for building that are not
going to be at constant temperature and humidities.
Relying on the Wire Armour is never a good thing really. So I'd advise that a
seperate local spike be driven. You should then be using a separate CU in the
shed and make your individual circuits from that.
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| Lurch replied to BigWallop on 21 May 2004 |
Ignore this bit, it is unneccesary and adds to the nuisance tripping.
Somewhat excessive that is. 4mm is plenty adequate, and as for the
joints, 1 mid point connection isn't going to have any influence on
the current carrying capacity of the circuit or its reliability.
He never asked if he could use the SWA as an earth, just whether he
can use an external earth rod. If he uses 3 core SWA cable then the
third core is the earth.
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| BigWallop replied to Lurch on 21 May 2004 |
But you know what like these DIYers are. They say they'll only put a certain
load on things, and then, before you know it, they're running disco's in the
garden for the kids. :-))
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| Lurch replied to BigWallop on 21 May 2004 |
Well, quite! I've just re-read my post and it sounded a bit angry,
sorry about that!
But, I've run complete DJ rigs, including most if not all of the
lighting, from 13A extension leads before now. I don't think a kids
disco is going to pull more than a couple of amps.
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| John Rumm replied to Lurch on 21 May 2004 |
Not only that - if the OP selects the correct MCB rating at the house
end then one of its jobs will be to protect the SWA from overloading.
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| Joe Lee replied to Lurch on 22 May 2004 |
Wrong. also please explain how it adds to nuisance tripping.
<snip>
Joe Lee
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| John Rumm replied to Joe Lee on 22 May 2004 |
Good advice I thought. "Wrong" seems rather too strong even if the
argument were balanced with pro's and con's
If you add a circuit (any circuit for that matter) to the RCD protected
side of your house CU, then you are by definition adding a new source of
earth leakage that can eat into your total combined "budget" of 30mA
allowed by the RCD. Even perfectly wired circuits may have some small
leakage due to inductive or capacitive coupling in either the circuit
itself, or more likely, the appliances used on them. Electrical
instalations sited in outbuildings are also more likely to experience
dampness from time to time which will also raise the likelihood of them
generating a leakage related trip. Every trip of this type is going to
take out all the RCD protected circuits on your split load CU - sounds
like a nuisance to me.
Since there is very little likelihood that you will be accidentally
chopping through your SWA cable with the hedge trimmers, there is no
great need for it to be RCD protected.
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| Lurch replied to Joe Lee on 22 May 2004 |
together this:
Correct, sort of.
Well, I didn't mean it simply trips the RCD all the time, what I meant was as
it's power equipment in the shed and garden it's more likely to be the cause of
tripping the RCD. That's why it's better with it's own RCD at the shed end, you
don't lose power to half the house when the pond pump goes u\s.
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| Lurch replied to Paul on 21 May 2004 |
Yep, all looking good so far.
Why not use a 'garage unit'. A 25A RCD with a 16A and 6A MCB. Run the
lighting in 1mm from the 6A and the power in 2.5mm from the 16A.
(screwfix.com Cat No. 63138).
You can use plastic sockets, if they're not likely to get battered. I
usually fit metalclad accessories in out buildings and the like.
As it's only over a relatively short distance I would export the house
earth to the shed.
Yep, that or the way I suggested above.
You don't need to enclose SWA in anything, if you do make sure it
isn't in a water pipe! It only confuses people in the future. I would
bury the SWA as it is.
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| steve replied to Lurch on 21 May 2004 |
With ELECTRIC CABLE BELOW tape
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=10422&ts=78647 over the
top of the cable not directly on top a good few inches above it for
obv reasons and might be a good idea to put an accurate map of garden
with the cable run marked on it near the CU for future referance
Steve
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| Lurch replied to steve on 21 May 2004 |
Erm, yes. I thought that at the time but the thought never travelled as far as
my fingers! Good advice.
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| Paul replied to Lurch on 22 May 2004 |
SNIP
I like the idea of using a garage unit - so much tidier!
Could I still use the 4mm SWA (3 core) to this consumer unit? Also,
taking into account that I need to run another 15m of cable from the
shed to the top of my garden for the pond pump and weatheproof socket,
do I need to provide the shed CU with it's own earth rod? If so what
is the correct proceedure to install one?
Paul
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| Lurch replied to Paul on 22 May 2004 |
That's the idea!
Yes, that's fine.
You could do, I personally wouldn't for what it's running. If you did want to
the correct procedure is,
Hammer 4' rod into ground without going through any pipes, it's easier if you
fit the bottom half of the PVC box and the clamp to the rod first.
Test the rod with an earth loop impedance tester. If the reading is too low
install anothe rod somewhere near the first, although generally 1 rod does it.
When you're making the armoured of into the CU in the shed you must ensure that
the SWA armouring isn't connected to anything earth related in the shed, nor
should the third core be used for an earth. The earth for the outbuilding end
should be solely provided by the rod.
Now, using a miniimum of 4mm earth cable connect the rod to the earth bar on the
shed CU.
In basic terms, that's it. In practice it's not as simple as it sounds, but not
overly taxing!
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| Joe Lee replied to Lurch on 22 May 2004 |
No. It must be RCD protected.
<snip>
Joe Lee
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| Lurch replied to Joe Lee on 22 May 2004 |
together this:
Nope, you're talking bollocks. The sockets at the end of it needs RCD
protecting, the cable doesn't. (Unless we were on about a TT system).
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