Running a line that carries a 40 amp load 500 feet would require a No.
2 wire.

This is not realistic. You need to figure the real steady load
current. For an observatory, even with music, :-), 10 amps total
would be a lot. Then you can get by with a No. 8 wire. Even then it
will be costly for the outdoor quality wire you will need. You need
to make a very realistic estimate of your power needs and limit them
to reasonable levels.

Every three wire sizes gives twice the resistance. Do not forget to
calculate for both directions.

Doc G


"Anthony J. Kroes" wrote:
>
> I am in the process of building a small (10'x12') roll-off roof obs for my
> new 12" LX200 and was hoping someone could help me with power. The building
> is located a fair distance away from the house, out behind a barn (no power
> there) and some silos. How do I figure the proper size of wiring to run
> electricity out there? I know the farther I go, the bigger the wires need
> to be, but don't know the calc's.
>
> I plan on running two 15amp or 20amp circuits to the obs (one for scope and
> computers, the other for lights and misc. Distance from the main breaker
> panel back at the house is probably between 350'-500' (haven't measured
> exactly yet - depends on whether I run the wiring through the old barn, or
> around it). Will be buried (probably) or hung (from existing poles around
> the barn) so I know there will be a difference in the type of wire I get,
> but that shouldn't affect the gauge I use (or does it?).
>
> I am completely familiar with standard house wiring, but this long-distance
> stuff makes a few questions pop up...
>
> Anthony J. Kroes
> Green Bay, WI

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