(1) The conduit looks OK. It should be buried at least 750 mm deep and for extra safety you could put timber - say a 6x1 rough board - over the conduit for extra protection. Warning tape laid in the trench, 300 mm above the conduit would also be a good idea.
(2) You need to specify the load. You also need to specify how far away the garage is from the point of supply (the house). The longer the cable, the greater it's resistance and you are supposed to ensure that the voltage drop is no more than 4% of the nominal supply voltage. A very long cable might have to be increased to the next higher cross-sectional area to prevent excessive voltage drop.
Assume 5 outside lights at 500 W each = 2500 W at 240 V is just under 13A. Your sockets will be protected (against overcurrent) by a 20 A mcb. Don't know what your electric gate is rated at so you would need a cable for 33A anyway. Assuming a PVC insulated, copper conductor in a conduit buried undergorund, a 4mm2 cable is the minimum required.
To allow for future expansion, a 6mm2 cable would be a good idea. Items such as electric welders which some people might have, would increase these figures considerably.
You would probably have a sub- consumer unit in the garage with a 35A switch fuse unit. The sockets in the garage need to be protected by their own 30mA RCD.
J, March 2008
