armoured cable

Postby ocean » Mon Jan 05, 2009 12:15 pm

I wish to install a exterior socket, primarily to run an electric livestock fence. However would be nice to use for other typical electric domestic power tools, power washer and such like. I intend to use as a permenant extension lead buried in a trench but connected to the mains with a normal 3 pin plug into my barn which is equipped with a mains supply and its own fuse box (modern) I will of course fit with a circuit breaker. Distance from interior socket to exterior socket willbe 30-50 meters. Which armoured cable should i buy - 3 core 2.5 mm good enough i see a number of different ratings and such like - please advise.
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Joined: Mon Jan 05, 2009 12:06 pm


Simply Build It

Postby ericmark » Mon Jan 05, 2009 5:58 pm

Running the distance you propose you need to consider volt drop. If you look at all the volt drops are given.
If I take 2.5mm at 50 meters and 13 amp as an example we have 11.7 volt drop which just misses the allowance for sockets at 11.5 volt. Although the equipment you use may be able to stand that drop as the impedance drops also you reach a point where a short circuit will not work the magnetic part of a normal MCB. Of course on a farm all sockets of 32 amp or less must be RCD protected to 30ma and above 32 amp RCD protected to 100ma but the special section for farms has a few other conditions as well especially on earthing.
I seem to remember at least 4mm for any bonding conductor and I would tend not to fit any SWA under 6mm because at 6mm the combined earth of core and outer steel comply with earthing but again electric fence comes under BS EN 60335-2-76 Amended July 2008 and I think there may be electrolysis issues as well.
When working in industry we would keep 6mm 3 core SWA in stock it was cheaper by the role and it was rare to use anything smaller as it cost more to get a length in special.
It was biggest cable that would fit a 20mm gland.
All best Eric

Postby singer » Tue Jan 06, 2009 1:05 am

In my opinion there are so many issues & regulations to comply with in the job you're describing you would be mad to go ahead and install it yourself, and if the building obtains its supply from the same source as the domestic supply then part P applies and you would be breaking the law to continue.
Call the experts in and pay the going rate. At least you'll still be alive to cry after you've paid the bill!!!
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Postby ericmark » Tue Jan 06, 2009 2:14 am

I have little more time on my hands so I will go a little further.
2.5mm is about the thickest cable that will fit in a plug and connecting the wire armour directly would be impossible so you would need some sort of box to change cable size. A Metal Clad Double Pole Switch & Flex Outlet 20A would allow this and also allow isolation or 13A RCD FCU Metal Clad if the supply needs fusing and RCD protection see ... Metal-Clad for an example. These however have a maximum size of 4mm cable which would change what I had said before.
The same would apply the other end and since 4mm will go directly into holes in metal clad sockets it would seem better to use the smaller cable.
With 11mv/A/m at 13 amp it would be good for 80 meters. Of course there will be resistance feeding the cable and away from cable so will be under 80 meters but you are only looking for 50 meters.

Postby ocean » Tue Jan 06, 2009 2:40 pm

ok, basically this is not a d.i.y. job for someone whos knowledge runs as far as wiring a plug and swapping a socket face over. Call an electrician, I assumed i could just get a length of armoured cable - stick a plug on one end and a socket on the other and bobs your uncle - it appears bob is not my uncle so id better call in a sparks. Of course i was aware my assumption could be wrong - hence the question. Intelligent enough to be aware that i dont know much. To be honest much of what u have told me i dont understand, and when we are talking electricity ive learnt along with plumbing. Ussually cheaper and safer to pay a man, everything else pretty much im happy to try and conquer and put failure down to experiance. But death and deluge ussually not worth the trouble :lol:
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Joined: Mon Jan 05, 2009 12:06 pm

Postby ericmark » Wed Jan 07, 2009 11:49 am

Glad you can see your limitations. Not a normal farmer then, the ones I know are mechanics, plumbers, electricians, builders, and on top of that a little animal husbandry????

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