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External power to be distributed inside.SWA sizing.

Postby Sparkydelux » Wed Jan 02, 2008 9:47 pm


I am to supply a house with a new power feed but having difficulty with rating the armoured cable size.I also have a few more question if anyone can help.

1.The Scenario:A main meter head is being moved and fitted to the external garage (disconnected from the house).I am to take a supply from there back around the house and stab in through the wall and provide power to the basement at the DB. I was under the impression that I had to run armoured of 25mm cores but I have just checked on a web page (TLC) and it is saying that 16mm (2 core) armoured cable carried 110A. The diversity of the house has been worked out to under 100A and the main fuse will be 100A.I am just curious if this would be sufficient?Its plain obvious it WILL but I was looking for confirmation from the experts.

2.Also with the earthing- would it be sufficient if i took the main earth off the armoured sheath from the meter head, through my armoured and into my DB?
Or should i opt for a 3 core SWA instead and use the 3rd as my earth?

3.Once the meter is moved to the external garage wall,it is to be housed in an enclosure. I was wondering if I put an exact housing next to it and used that as my connections feeding the garage and the house?So I would use that as a sort of junction box.Is this allowed?and do i have that option?if not what else can be done?

Many thanks for your time and patience,

Eternally Grateful.

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Postby BLAKEY1963 » Wed Jan 02, 2008 11:22 pm

without going into deatail , i would use 25mm2 , 3 core
you have the 3 rd core as an earth.

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Postby ericmark » Thu Jan 03, 2008 8:04 pm

The supply authority does use 16mm but this is a special cable that only they are allowed to use. As with many questions there is a difference in can and should. And maybe you can use 16mm two core if all other bits comply but you should use 25mm 3 core. One problems is volt drop, another earth loop impedance. Then the chance of corrosion damage. Without being on the job I can't say if these are problems but it is likely plus you need to terminate I would think a metal consumer unit may be best bet? Not sure what you are thinking about at garage side but again remember the meter reader must be able to do that safely he is not an electrician. Since you will need some supply to garage again a metal consumer unit should be considered. 25mm SWA is not easy to bend and to attempt to terminate it into any plastic box would be asking for problems in theory one could use a brass earth ring on plastic but in practice it is likely to rip out. As to the earths one can use the steel wire armor if certain criteria is met. but it is hard to ensure all the requirements are met there are formula in the 16th edition showing one how to be sure you comply but it would be better to use a copper earth as well as the SWA and not rely on the steel. If one does rely on steel you need to test it after fitting to prove the outer sheave has not been damaged should it fail then it's a lot of work to re-lay. The 16th edition 542-02 only states it must be protected against corrosion but the way to ensure this is done is to "pressure test" with high voltage steel wire armor to earth much depends on the council I know Birmingham test all their street lights after any road works this way and have billed many a contractor for cable change when their diggers have damaged the outer PVC covering but other areas never bother testing. As with most things a lot of luck is involved and you may never be asked for results of tests and you could get away with wiring in wet string but this does not make it right.

Postby Sparkydelux » Thu Jan 03, 2008 10:19 pm

Hi All,
Thanks very much for the information.I am going to source 25mm2 (3 core) SWA and run the feed in that.

The SWA will be run in a duct burried the ground so there wont be any excessive corrosion or risk of damage.

I was had probably over estimated the length of the run origionally.Its probably closer to 20m.

I fully understand the reason why to run in 25mm rather than 16mm and,honestly,that was my origional 'gut feeling'.I was just questioning myself as I always do.I just needed the confirmation my instinct was correct.

As for the meter housing: It will be accessible from the exterior of the garage BUT,I want to put another housing of the exact same size butted up next to it and use it to connect my 'junction box' to the meter.
So the one box will be the housing for the meter and intake.The other will be for my 25mm armoured and garage feed.The 2 boxes will be connected by 25mm tails.I was thinking of putting a 3 way DP connector box in the housing so my feed from the meter will go into that and from there take the feed to the house via the 25mm armoured and also feed the garage. I will also provide something for the earth along the same line.

It was the most simple solution rather than running armoureds back and forth. Would this be sensible? Any recommendations?

Thanks again for all the information.I'll definitely use it.

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