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.gifSo 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 is also covered by the RCCD that some other house circuits are on. I 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'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 house supply is TN-C-S and I'd assumed that extending the earth from the house would be ok and that there'd be no need to have any complications of a separate earth.
There's a debate about how to feed the room. One suggestion is first to take the feed to point D directly into the room through the concrete base (not yet laid), to the circuit in the room and then out from the room to an external socket at the back or side of the room. That socket would be used for any garden power tools. The LV control gear could be in the room and feed out separately to circuits on that part of the garden. Another idea was to take the feed to point D first to an external junction box, and then to feed the room as a spur from the junction box in that corner; basically just a decision of where to first take that mains spur.
As the garden is small, I'd also wondered whether it would make sense simply to extend the mains feed only into the room, with an external socket as spur from the room, and then house the LV control gear for the whole garden either in the room or outside at corner D and fan out all LV wiring from that point. But as LV cable runs could then be up to 15 to 20 meters or so, I'd thought taking mains to at least two opposite corners would be prudent.
So to sum up, I'm not sure about cable choice although am leaning towards 6mm, and also any suggestions and thoughts on routing the mains would be welcome as we've ripping up the garden with diggers now :)
Nick