Electrical cable outside

[Andy Lewis]

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? Thanks......

[Harry Bloomfield]

Andy Lewis expressed precisely :

SWA (steel wire armoured) cable and yes it can be buried. Depth depends upon where it is to be buried or rather what is along the intended route.

[:::Jerry::::]

And just what are you going to use this cable for, a 5w bulb or a sub main for your 300amp stand-by generator ?....

I other words, more info is required.

[Tim S]

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 not necessarily).

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 right.

HTH

Tim

[Andy Lewis]

Many thanks to those who have taken time to respond.. The cable will only be used to supply lighting to the shed and power to a submersible water pump Thanks

[Owain]

"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.

30m you will have to do proper calculations to prove the cable size is suitable taking into accoutn voltage drop and fault impedance etc

Owain

[Tim S]

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 :)

Tim

[John Rumm]

This will get you started:-

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.

[Ian Stirling]

Does it have to be cable warning marking tape? I used yellow painted dexian (steel angle) 5cm over the conduit.

[Rick]

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 .......

Rick

[s--p--o--n--i--x]

Anyone know if a cable supported by a catenary wire is still allowed?

sPoNiX

[Stefek Zaba]

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).

[Bob Eager]

The errata/update to the OSG makes it 5.8m pretty well everywhere - 5.2m only appears in one place now.

[Andy Wade]

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 identified."

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.

[Ian Stirling]

Fairly substatial, you're not going through it with a spade. (buried 15 years stuff was recovered more or less intact, unpainted.)

Good, it's about that down. :)