Many people run their own electrical and are very successful and a lot seem to have problems understanding voltage drop. Say you have a run about 400 feet long and want 120 volts at 20 amps at the outermost end. Your wire size you figure will be 12/2 because naturally 12/2 carries 20 amps. But because of the length and resistance heating you won't get 20 amps at the end unless you increase your wire size accordingly. The wire size will increase with length for the load.
I have found a very helpful site that deals with voltage drop, wire size and other things.
http://www.ifigure.com/engineer/electric/electric.htm
is the site url.
There are three voltage drop calculators with the last one not working. It also applies ohms law near the top of the page.
Check it out[
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