I think that wiring 'outside' is extremely dangerous. Even with low voltage it
is easy to build up lethal voltages between buildings (voltages which are
unrelated to the wiring proper). It is also very hard to protect against shorts
and fire and against induced external voltages (what happens when lightning
strikes nearby or if a power line falls on some part of your installation).

I think that this is a very large and complex issue.

If I'd have to do this I'd probably research the biggest 'safe' garden lighting
transformer available and see what can be connected to the 'far' end to step the
voltage back up if absolutely necessary (perhaps the next smaller size of garden
lighting transformer). I think that obtaining some 500 W over 50 - 100 meters
can be done using approved parts. The wire will be thick and expensive but since
it is buried a widely overrated aluminium conductor cable purchased surplus from
a power installation company could be used. That could be 3-phase (4-core)
extruded solid aluminium cable with 450mm^2 total conductor area (this is used
to wire buildings sometimes - this is what a backhoe would typically pull out
when the computers die). 

I'd also use an isolation transformer at the far end and local grounding even
for low voltage if it is ever to be touched by humans (e.g. plugs or other
things that can be manipulated).

Peter


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