Electrical trunking query

If it's under the floorboards, why do you want to use trunking at all? Can't you just lay the cable under the floor, with both ends suitably isolated and marked up clearly as to what they are, so that you or somebody else in X years' time will know what the cable is?

David

Good point. As I don't know enough about the requirements for outside wiring I just want to make sure that there is some way to install the cables without having to lift the floor. If I did run T&E and leave it unconnected what size would be appropriate - the run down the garden could be up to 60 metres?

Mark

Ah - running power "down" the garden, as opposed to "to" the garden as you said in your original post - that's a bit different.

Power "to" the garden suggests a waterproof socket on the outside wall of your house, wired directly using standard cable used for an ordinary ringmain inside. No exposed cable.

Power "down" the garden is a whole different ball game, as you suspected, for which you need special armoured cable, and there are alls sorts of other consideration. However, at this stage, I wouldn't have thought that's a problem, because you wouldn't be running armoured cable all the way back to the CU anyway. Somebody else will along soon to advise you better, this is now out of my comfort zone! but I would have thought what you should be aiming at now would just be to get ordinary T&E as far as a special junction box (a metal adaptable box?) inside the house close to where the armoured cable will exit through the wall for the garden.

Either way, for 'future-proofing' you want to install T&E under your floor, to the same position.

David

As Mr Lobster says, "There is a lot more to running electricity to an outside point" so listen up. What are you proposing to run at the other end of the cable? If it's an outhouse or garden office, then you really need to do a full sub-mains installations for it to be safe and to pass your local council testing certification.

If you want a double socket for the pond pump and a couple of small lights, then you need to place armoured cable to a weather tight box that is removed from the ground on proper non-corrosive supports, and lifted to a minimum height of 300 millimetres. A length of 60 Metres will mean you'd need in the region of a 4 mm *csa' to 6 mm *csa' cable to properly supply a double socket without losses, safely.

For an outbuilding or garden office, then this will all depend on what you are placing in and around the build. For a proper outbuilding workshop area, then you need to run a totally separate sub-mains supply from a totally separate isolator switch at your present electrical supply head end. The cable itself will probably need to be in region of a 16 mm *csa' to 25 mm *csa', dependant on the total power you need at the outhouse. This is not a job for a DIYer, as there is much more to it than just getting power to the building. It also involves safety earth loops and safety drop-out gear to be properly and safely used. These all need to pass strict testing before they are passed on to you by any contractor.

Get back with your needs and I'll try to help further, if you wish. I am quite busy at the moment, so please allow me at least a day to reply.

I don't expect that the requirement at the far end will be for any more than;

• a couple of sockets for domestic garden kit such as lawn mower, hedge trimmer
• lighting for shed/greenhouse - single fluorescent tube or similar
• automated vents for small greenhouse

• small number of garden lights

That's the tye of setup was envisaging, with the armoured cable terminating in the shed.

Nothing that significant.

Thanks for the offer. I don't intend to attempt the outside work myself as it's well beyond anything I could do safely. What I do want to do is to make sure that I have the necessary bits in place internally to enable the above without having to lift the floor to run cable to the CU.

Mark

How far from the outside wall is the current mains head end? Wire-Armoured cable is quite rigid, and it can be pushed quite easily for long distances under floor voids and things. You do need a tight fitting cap on the end, two layers of insulating tape wrapped tight, to stop it catching debris, but it is possible to get a 10 mm csa' SWA(steel wire armoured) Cable to travel long distances in gaps under floor voids. Easily 3 to 4 metres or more.

So, if you only have a short distance to travel under the floor, then you'd probably only need to lift a small bit of floor in the cupboard where the head end is, and push the cable through the wall toward it. Enough hole in the floor to get an arm through to feel for the cable should be enough. Or get your head in with a big torch to make life really easy.