Hi I am looking for some advice in installing an outside plug socket
and security light in the garden. This is what I though I would do:

1.Take a spur from the downstairs main ring.
2.Connect the spur to a RCD.
3.Connect a cable from the RCD to a junction box
4.Connect 2.5mm armoured cable (about 12 meters) to the junction box
and lay under the lawn
5.Connect to a waterproof socket
6.From the waterproof socket spur to a 150W security light.

Does this sound expectable? Also I'm not to sure what size RCD I
should install. The only things that would be connected to the outside
sockets would be garden tools, or maybe a radio. (not forgetting the
150W light)

Could I use 1.5mm cable? or should I stick with 2.5 to reduce the
voltage drop?

Also I read somewhere that I may need to install an extra earth in the
garden near the socket. Is this correct. My supply is a TN-S.



Thanks

John