Exterior wiring

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Many thanks to all who have replied to this thread. I realise from some of the replies that my description of what I want to achieve could have been clearer so I'm replying to my own original message.
First of all, these are not security lights and there was never any requirement for the high wattage halogen type things - I just want a bit of light so that I can take something out the back to the bin at night without stepping in dog poo (our dog has access to the outside via a flap on the back door).
Secondly, I had never really considered using PIRs - I imagined having a switch next to the back door, though the replies I have received have given me cause to reconsider this. Even so, I want it to be possible to switch the light on or off (overriding any PIR) from somewhere in the kitchen, even if it is not as convenient as being a switch next to the door. This, together with the requirement that there be minimal internal disruption is what led me to the conclusion that most of the runs of wiring need to be on the outside and why the approach that was suggested of dropping the wiring down from the eaves would not work - there is no easy way internally to get cable from the loft to the kitchen.
Perhaps I should explain my original intention a bit better. The door is on the side of the house. The understairs cupboard ('pantry') is accessed from the kitchen and is where the consumer unit is located. One of the walls of this cupboard is part of the side wall of the house. What I had therefore imagined doing was to effectively add a couple of lights to the downstairs light circuit, with the cables on the outside of the house, only entering for the switch next to the back door and into the pantry to connect into the circuit.
This would have entailed a couple of vertical runs of cable that would enter the house at a relatively low level. It is these that I was particularly concerned might get knocked occasionally. (The sort of situation I have in mind is where I am carrying an armful of prunings from the back garden down the side path to the front of the house. My experience is that they tend to scrape along the wall, and catch on obstacles, though the force they exert is not immense.)
If I go with the PIR idea and forget about the need for a convenient switch next to the back door, I can get this down to one vertical run that enters at the top of the pantry (about 5 feet off the ground). I can put a slightly less convenient switch in the pantry for overriding when I need it, but rely on the PIRs for day-to-day convenience. Also, I can run this vertical cable behind the soil pipe, so it should be pretty well protected.
In the light of the helpful replies (thanks to John in particular for the screwfix details), I will use heavy duty PVC trunking. Is the screwfix stuff definitely suitable for exterior use? Also, it was suggested to use standard cable inside trunking - is this OK? As far as I can see, only the temperature aspect of the weather will have a bearing.
I'd like to take on board the comments about light pollution from the bulkhead style lights. However, I would re-iterate that these lights are to go on the side of the house. Also, next door is about 7 ft away, so the light is not going to get much chance to 'escape', and next door already have a light of this type. They will only be switched on for short periods anyway. Having said this, I'll look into the "light in a box" that Jon recommended, and the units that Andy recommended.
Thanks once again for all the help - brilliant response as usual.
- Ian

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Ian wrote:

That was my reason for fitting them - setup so that you get light should you approach the bin from either front or back doors.

Interesting idea - ours still has a conventional sphincter.... Oh I see what you mean! ;-)

Pretty much the setup we have - I selected the consumer unit cupboard (to call it a pantry in our case would be over egging it a bit!) since it was an easy place to find power and was on the right wall.

My switch is in the "pantry" as such... must admit I have never found the need to switch the light off manually as yet, the PIRs work well. Might be handy at bulb changing time though.

In our case the power comes in overhead and runs down the wall in a vertical conduit before going through the wall to the power company fuse etc. Hence to avoid having to make an ugly crossover of conduits, I brought my own connection out about a foot below this and wired out to each lamp from there:-
# # PIR-------LAMP-------- # ----------LAMP--------PIR | # | | # | | # | | # | | # | | # | | # | | | | | ----O-----
Where '#' indicates the incoming feed from the overhead wire, and 'O' is the conduit 2 way junction box.
(The second PIR on the right I actually took about a foot round the corner of the house, so that it is on the back wall of the house near the back door - this was to avoid it "looking" anywhere near the boilers flue)

The conduit is pretty thick walled stuff. If in doubt you can give it a coat of paint at some point to protect it from UV light which will tend to make it brittle over prolonged exposure.
The temperature is not too much of a problem. The PVC insulation on the cable is good for at least 70 deg C. The lower limit is less important since its the flexibility that goes in the cold - and this setup won't (with luck) be moving anywhere.
It is usually simpler to get individual wires through conduit rather than complete PVC sheathed flat cable. Depends much on how many wires and how long the run of conduit is. In my case I fed a bit of gardening wire through first and pulled the cable with that. With hindsight I should have bought an electricians fish tape:-
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts 033&id072
Rather than use three core and earth, I took the feed cable through the bulkhead lamp and out to the PIR and then had a second cable back to the lamp. Hence the short straight 1.5m run between the PIR and bulkhead has two cables in it.


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