I intend to provide a feed to my shed 20mts away from the house.

I need to know which armored cable size would be needed to run an internal shed light, also a socket for typical garden tools and maybe another feed for some garden lights.
Would 2.5mm be sufficient or would I need 4mm for the main cable run fro the house.

I intend to add a small shed type consumer unit with 2 x RCB of 16a & 6a for the internal light, would this be sufficient?

Also as a pickup point in the house I intend to join the armored cable to
internal house cable via an isolator switch fitted internally.
I have a 30Amp cooker feed near the cable entry point which is not currently used and runs back the the main house consumer unit, would that be okay to pickup from that point?

I appreciate any assistance provided.

Well assuming 6A lighting and 16A for socket(s) and using a 'garage cu' and 20 metres clipped then -

3 core xple swa works out to be 2.5 mm csa gives 6.96 volt drop (3%) so that will do. May wish to provide a bit of expansion so you may choose to go fro 4mm up to you and budget but 2.5 is OK.

As for protection the most important side of things;

Depends on what you have in the house- is the earthing arrangement in your house TN-S or PME or TT? (if you don't know say so and someone here will explain)

Do you have an RCD in you house consume unit (fuse box)?

I asked about an RCD on the basis of wanting to avoid the installation of a double RCD (same trip current) which is a no-no.

Concur about the cable csa - but calc's do indicate that 2.5 xple (not ordinary swa and certainly NOT t/e) will meet the volt drop ratings. As always go for the largest swa, as TT says go for 10mm that will meet just about every requirement for now and the future - but cost?

As Lec says or implies don't use anything other than swa or xple (another type of SWA)

Agreed TT, I also always install a rod at far end regardless - the cost of a rod and pit box is nothing and it does not take long to install.

Ah, yes.  He does mention armoured.  Sorry.


As for the earthing type.  TN-S and TT can be exported with no worry, but PME requires special consideration....think lost neutral in supply

Maybe it's a personal thing but I would NOT export PME. I would far rather make to remote site TT with a rod, I think the risk aint worth the cost.

First of all - Google for 'PME' - Protective Multiple Earthing'.

Sod it!

Here you are - Scroll down for the diagram.

http://www.tlc-direct.co.uk/Book/5.6.3.htm

Now, some pundits would have you believe that the number of instances where a  lost neutral has seriously caused death or injury are high enough for you not to ever 'export pme' outside the equipotential zone.
In certain instances, BS7671 says you shouldn't do it.
Indeed, the supply companies can be funny about it too.

I'll refer you to the following IET links

http://www.theiet.org/forums/forum/searchresults.cfm

And a well-informed and pragmatic view, although not endorsed by the IET,

http://www2.theiet.org/Publish/WireRegs/WiringMatters/Documents/Issue16/2005_16_...


Now, there are also some views which suggest that you shouldn't export PME at all.
My view is that it is OK to export PME AND connect the original earth to a rod - again, it's belt and braces, but if the supply neutral is lost, then at least you still have a path to earth via the rod.
Again, this is my view, but some would suggest that this is bad practise and that only one type of earthing system should be installed, and not a mixture of both.
The reason for this is that there may well be a potential difference in the true earth potential at the rod, and the exported PME earth.
So, theoretically, you could connect a voltmeter between the PME earth, and the rod, and measure a voltage.
Personally, I think this is a load of bollerks in most cases, but may be valid in others.

Sadly, no records kept of instances where such set-ups have caused a fatality.

Nor are there any records kept which record the number of instances where a 'lost supply neutral' has resulted in the death of someone at the 'wrong end' at the time.

However, the supply companies are strictly regulated and should ensure that the neutral is not 'lost' through physical damage to their cables.

So in the end, it's your call.
Make sure you understand exaclty what you are doing, then get it exported, and tie it down to earth via a rod at it's ultimate destination

Make sure that everything is bonded to ferk and all will be well.

TT

What's a "pit box" ?

Used most the them in the past Lec. I was under the impression that a rod should always be sunk below the surface level on the basis that under a fault condition, if the rod is above the local surface, the fault current can so high as to be fatal to any animal that is with 3 metres of the rod