Have an existing underground circuit that goes from the house to the end of the back yard. It's about 40 years old, and I have no idea what shape the wire is in, but it still works. I'm thinking about build a shed at the end of the yard to use as a woodshop. I don't want to actually wire the building up, but I was thinking of running a new line underground. What would I need to run a 20 amp line around 100 feet underground.
You can use a calculator like http://wiresizecalculator.net/ to figure this out.
Keep in mind that the distance is not just the distance underground, but the entire wire distance from the panel to the receptacle/switch/fixture.
For 140', you'll need #6 copper or #4 aluminum.
For a 120V, 20A circuit, protected by GFCI, in residential use, using direct-burial rated wire (UF), the minimum depth requirement is 12".