Nick,

There are a few aspects of taking power outside like this that can get a
bit complex. You may find having a read through this:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

may help.


Nick wrote:

I'm planning to have wiring extended from an existing external socket
into a small garden for the purpose of lighting, water features and
use in a garden room, and would appreciate any advice on cable choice
and routing. I put a rough sketch with some dimensions and layout at
http://64.191.63.245/garden-wiring.gif

So far there is a 20m run of 2.5mm SWA cable from the CU to an
external socket. The electrician put a 32A RCB on the circuit, which

If this run is inside the house, why is it in SWA?

I assume by RCB you mean MCB...

You are going to have to take care here since it sounds like a design
decision has been made that could have implications that may come and
bite you later!

Circuits need two types of protection: overcurrent and fault current.
The former is designed to clear a sustained overload that results in too
much current being drawn, and the latter is designed to operate when you
have an outright fault like a short circuit that results in a massive
fault current flowing. Normally both types of protection are provided by
the MCB at the head of the cable. However this does not appear to be the
case in your existing setup. I would guess the designed was relying on
the 32A MCB to provide only the fault current protection for the SWA
cable, and was relying on the fuse in the plug connected to the external
socket to limit the maximum current since the 32A MCB is of a too high a
rating to do this itself.

is also covered by the RCCD that some other house circuits are on. I

This is generally a poor design choice. A fault in the garden will trip
power in the house, and also make nuisance trips more likely since
garden circuits are more likely to give rise to problems.

See the nuisance trip section he

http://wiki.diyfaq.org.uk/index.php?title=RCD

was thinking of SWA running from the end of the existing circuit,
under a path to one corner of the garden, with a junction box to run
to the room corner (D) and up to the far corner (C). Cable runs for LV
circuits could fan out from the corners and shouldn't be excessively
long. Lighting will be LED's with possibly some LV halogens, computer
controlled (the house already uses DMX), and along the garden
periphery, in trees etc. Pumps will be LV if possible around areas P1
and P2 but we might need a mains feed for those (or for any UV unit).
I'd also considered a short run from either another junction box to
point A or from the planned junction box to point B for LV circuits
back along the path and trees.

I would suggest a SWA cable submain to the outbuilding, and a consumer
unit installed there to supply any other circuits required.

I'm not sure what the max likely current demand would be, but as
there's 20m of 2.5mm already from the CU, when calculating voltage
loss (based on worst case) I figured that 4mm or even 6mm SWA would be
prudent so that there'd be at least a 13 amp circuit for the room. The

You may want to consider running a larger cable through the house and
then converting to SWA for the external run.

house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok

No - absolutely not! This is just the situation you really don't want to
extend a TN-C-S earth outside. If it were just going into the
outbuilding then it may have been a acceptable solution if implemented
correctly. However since you also need feeds into the garden itself for
lights and water features itself, using the TN-C-S earth is technically
not feasible, and could pose a serious safety hazard if you did it.

and that there'd be no need to have any
complications of a separate earth.

This is what you need to do. The outbuilding wants to be a TT install
with its own earth spike and RCD protection.


--
Cheers,

John.

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