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wiring to the shed via 1.5mm armourd cable

Postby nick323f » Sun May 24, 2009 12:32 am

Hi All,

could anyone help me with this one?

I've recently had a bit of building work done and while the builder had a channel open (for the drainage) we dropped in some armourd cable between the house and the shed (this is a brick building) distance of about 3m. This is now burried under the concreat.
Unfortunatly we used only 1.5mm armourd cable. :roll:

So my question is what is the maximum current I can pass through this cable? i.e. what RCD should I use at the CU in the house?

I also have to complete the run back to the CU. I could do this in 2,5mm cable or 6mm cable?

I've looked on the armoured cable manufactures website and they clame 25A max. But this seems very high to me

At the CU in the house I will be connecting the run to the RCD side of things.
At the shed end I would like to connect a small CU for light and socket arrangments. But again I'm not sure what size MCB I could use for the sokets?

any help greatly appreciated
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Postby bd3cc » Sun May 24, 2009 9:33 pm

Cant see the point of connecting 2.5/6.00 mm to 1.5.
At the CU i would put a max of 20A mcb, and from the shed cu rate the sockets at 16A.
As this is a special location I am sure it is noifiable under PartP
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Postby kbrownie » Mon May 25, 2009 8:06 pm

A few issues here and bads news too!
It is notifiable to building controls as all new circuits are and the fact it is running outside makes it a special location too.
1.5 cable not suitable for the work your wanting it to do, don't understand what you mean by the run back have you not completed the ring, if not how do you intend to do it?
If you was going to and don't see the reason why you are joining different size cables up, different size cables should be discrimenated and down fused at the connection point of those two cables. You have 1.5mm layed so can't see point of adding 2.5 or 6mm because you can only fuse rate it for 1.5mm as that is the highest current value you are safe with.
Building controls would want to inspect the first fix of all new cables to check they comply to both BS7671 and part P, so you would need to dig the concrete back up anyway and start again.
Sorry to say you've jumped the gun a little on this project.
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Postby sparx » Tue May 26, 2009 7:41 pm

Hi all,
hold on a minute fellas....
a buried 1.5mm SWA has a rating of 25A from table 4E4A, so no reason to fuse at less than 25A at origin. To run from consumer unit to SWA in twin/cpc would require it to be a 2.5mm if clipped surface or a 4mm using any other route.
As long as smallest conductor is protected at its max rating it could be run in 25mm to the SWA to limit volt drop!
Absolutely no need to fuse down when going down a cable size if above applies, another 'old wives tale'.
However comments re. Part 'p1ss' do apply
regards Sparx
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Postby kbrownie » Wed May 27, 2009 6:26 pm

May have over elaborated, so not made my point well.
I was more concerned with the run back to the CU as not clear if this was part of a ring or a radial? Can assume it is not but don't like to, as may give unsafe info.
I think I confused myself be not reading original post better!

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