Bare copper or Copperweld has served me well everywhere I've lived. The
only time I've had a problem with it was at the coast, about a block
from the ocean where the air nearly always contained a fog of salt water
droplets. There, a copper wire turned into blue powder in less than a
year. I suppose you could have similar problems if you're downwind from
a coal-fired plant or something. So if you have a very severe
atmosphere, get the coated Wireman stuff. Otherwise, bare Copperweld
will do as well.

Roy Lewallen, W7EL