wrote in message
ups.com...
Hi,
Thanks for the quick reply :-)

The cable will run about 10m or so from the house's consumer unit to
the shed. I've already wired the shed in a radial circuit with 6mm
cable (a little big maybe, but I had it to hand). That circuit is only
about 4m.

The sockets in the shed will have to cope with a (small) table saw,
bench drill, uv exposure box and a tv. I'm working on some video
electronics stuff (12volts I'm ok with... but 240 _scares_ me!). Not
all of the above will be on at the same time tho

As for the MCB at the house end, the whole story is that I'm going to
change over the consumer unit in the house (before I connect up the
shed) to one with RCD protection because at the moment all my sockets
upstairs and down run off a single fuseway, as do the lights (I'll
change it to a split load one with the lighing in the house running off
2 unprotected MCB's and the remaining of the ciruits (sockets down and
up, shower and shed) off the protected ones.

Am I barking up the right tree? :-D

Nearly :-)

Just to point out, under Part P, you are not permitted to just do this on
your own, unless you get building control involved (because it is a new
circuit, and also because it is also outside) you are also not permitted to
change the CU.
"Am I Bovvered" I hear you all cry :-)

It would be best to run the shed off the non RCD side really, or a problem
in the shed will probably trip both the shed RCD and the house one, this is
not unsafe, but is bloody irritating!

As you are changing the CU, I would also recommend you run a radial to the
kitchen fixed appliances (Freezer, fridge at least), the boiler and alarm if
you have one, from the non RCD side too (So if you are away, and something
trips the RCD you do not loose the freezer's content, or the alarm) again,
Part P says you can't just do this one either, as it is in the Kitchen and
also because it would be a new circuit.

So, back to the shed, sorry, workshop.

I assume the workshop is a wooden construction?

The 6mm cable is a little on the large side, 2.5mm would have been fine
here, however, this is not a problem (except when you try to jam in a load
of cables into one socket/spur etc., then you run out of room faster!)

As it is only 10M of cable, then you can use the house earthing (otherwise
you would need to install a TT system in the workshop, providing local earth
with an earth rod etc.)

It would probably be better to opt for a non RCD overall protected consumer
unit in the workshop, and use RCBO's here (these are a combined RCD and MCB,
but are quite expensive!)- the reason for this is mainly because you mention
potentially dangerous power tools (the table saw). If the lighting and this
saw were both needed (you were sawing something when it was dark outside)
and the RCD tripped, you would be plunged into darkness with a table saw
still spinning down. I have taken the emergency light route in my workshop
(overall protection is an RCD, and the emergency light is wired up to the
same circuit as the main lighting, so in the event of either the lighting
MCB or the RCD tripping, the emergency light illuminates) If cost is a
factor, then the emergency light route will be cheaper (It was when I did
it, and the fact I don't use this type of tool that often, made be choose
it!) It will also illuminate if the workshop MCB in the house trips.

It would probably be best to run SWA cable (Steel Wire Armour) from the
house to the workshop, as it provides very good protection (and can be
directly buried) If the run from the cable entry point in the house to the
CU is a bit tricky, then just terminate the SWA in a box (In the house, or a
box screwed to it) and run the rest in normal T&E (This is what I have done,
as it would have been a major pain in the arse to run SWA through the
house!)
I found the three core SWA (really meant for three phase) was more easily
available than the two core (The earth would have run through the armour on
two core) so I used this, and sleeved the wires each end accordingly. I did
use the third core as the earth, but also earthed the armour)

If you are going to dig up the patio, it may also be a good idea to lay some
conduit under there so you can run additional wires (network, phone,
doorbell etc.) at a later date (It is best to lay a draw string in there
too, but if you forget, or it breaks, a vacuum cleaner will usually suck
one through later!)

I would use 6mm SWA cable, but you could get away with 4mm (6mm will give
you room to expand a bit later)
I would use a 32A MCB in the house
A 20A MCB or RCBO for the sockets (as it is a radial)
A 6A MCB for the lighting
A 6A MCB or RCBO for the water feature (assuming this is enough, and you
don't have an imitation Niagara falls/Trafalgar square in the garden!

Sparks (Who is NOT a qualified electrician by the way!)