On Mon, 07 Feb 2005 19:16:34 +0000, Andy Lewis wrote:
Armoured (SWA) is an option (there are others). I'm not qualified BTW -
but there are a few things that need to be considered. It's not
necessarily as simple as lob cable in ground and connect (it might be, but
Cable needs to be routed suitably - including burying in correctly
prepared trench of certain minimum depth with warning tape above by a
specified amount (so far, still cheap, but much hard work involved).
Surround cable with sieved soil as stones can damage it. Cable can be
clipped to solid structures for some of its run - but not to a fence,
deemed too likely to fall over.
Also, there is a serious need to consider the earthing arrangements - I'm
studying this myself and am so far not totally clear - but it is *not*
necessarily the case that you export the supply earth from the house. It's
worth first finding out what type of earth the supply presents to your
house as this makes a difference to the design. You can end up in certain
cases with having to supply a local earth via rods and these should be
tested. That's going to cost but I can't stress enough that it's important
and is money well spent.
SWA is not particularly difficult to work with, but you need to have the
correct brass termination glands and know how to deal with the earths at
each end. It's best (to the best of my knowledge) to use 3 core for this
type of job, using one core as an earth rather than rely on the steel wire
armouring (which *must* also be earthed solidly or the cable is
potentially dangerous if you stick a fork through it).
I'm being cautious here as regulations applying to outside wiring are
quite onerous. I would recommend seeking an engineer's advice (or a
seasoned sparky) on the earthing requirements in particular,
which is what I am expecting to do myself.
Sorry to make it sound painful, but outside power really needs to be done
On Mon, 07 Feb 2005 21:08:15 +0000, Andy Lewis wrote:
I was wondering myself about a lights only supply (sealed lights on gate
posts specifically). Does anyone know definitively if exporting the house
earth is generically OK in this case?
I think in your case, as a pond pump is involved, you should go the whole
hog and seek professional advice. Running and connecting
the cable is quite within the abilities of a competent DIYer but the
planning is not cut and dried.
Don't forget the RCD(s) too :)
"Andy Lewis" wrote
| I wish to lay an outside electrical cable to my garden shed 30 metres
| from house, what is the best sort of cable? Cheap but safe and can I
| bury the cable?
Steel wire armoured (SWA) cable is suitable for burial. It must be buried
deep enough it will not be disturbed/damaged, which usually means two forks
deep on lawn and deeper on beds with will be double-dug. You can get buried
cable warning tape to bury in the trench just below the surface.
At 30m you will have to do proper calculations to prove the cable size is
suitable taking into accoutn voltage drop and fault impedance etc
This will get you started:-
(SWA is typically XLPE these days)
At thirty meters I would be more inclined to not export the house earth
(i.e. use it to earth the screen of a two core cable, but terminate that
in a insulated consumer unit at the far end. Then use a local earth rod
with local 30mA trip RCD to provide a TT setup. You will also need to
consider protection for the cable at the source end in the event of a
catastrophic L to N or L to E fault in the cable.
The idea of the warning tape, is so that somebody who digs, finds the
tape long before the cable. For the reason the tapes is put somewhere
neer the surface. An excavator bucket (small machine) can take out 2
foot deep in one bite .......
Yes; but to be Regs-compliant, the catenary wire needs to be earthed,
and the suspended cable needs to be flown pretty damn high: 3.5m above
ground when "inaccessible to vehicular traffic", or 5.2m otherwise (and
5.8m at road crossings - presumably because double-deckers mount Minis
preferentially at road crossings ;-). There are even prescribed - and
quite tight - intervals at which the cables are to be supported, if not
"continually bound up with" the catenary - roughly every foot (300 of
your earth millimiters) for non-armoured, increasing a little for
thicker cables and for armoured - but not by much, with SWA-over-20mm
needing support every 450mm.
It's also Permitted to fly a short (no more than 3m) span of T&E, HiTuf,
or similar - at the same heights, without a catenary, and with sensible
supports (well-rounded, strain-relieving, non-chafing).
Reg. 522-06-03 of BS 7671 allows cable covers or marking tape. Cable
covers are usually pre-cast concrete things, not bits of old scrap iron.
Oh, hang on, you said conduit. The above is for a directly buried
cable. All it says for buried conduits is that they shall be "suitably
Also: "buried cables, conduits and ducts shall be at a sufficient depth
to avoid being damaged by any reasonably foreseeable disturbance of the
ground." This is usually considered to mean a minimum depth of 450 mm.
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