I see that B&Q do a 50m armoured cable for about £78. It is 2.5mm csa.
1. Would this be suitable if I use the whole length and know that I will never run more than 1KW from it?
I will be laying it directly in the ground (say about a 30cm down next to a wall).
2. Given that I will take it off the main supply unit with it's own RCD what would be the rating and what should I use at the garage end to provide power and lighting?
It would give a volt drop of 3.9 volts and for lights you are allowed 6.9 however since one can't by a 4.3A MCB one need to work it out for a 6A MCB which gives 5.4 V still within the allowed drop.
Since using a 6A MCB for whole supply there is no need to reduce the supply further for the lights so no fuse or consumer unit is required in the garage.
If you use 38 meters or less then one could use a 10 A MCB in which case because the standard ceiling rose is rated 6A you would need to use a switched FCU (fused connection unit) with 3A fuse as light switch.
Do remember to inform the LABC first likely cost £100 plus and budget to hire the earth loop impedance meter at around £70.
I real terms best option is dig trench then ask a registered electrician to install telling him you will back fill trench likely it will cost less than to DIY whole job.
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