My place is fed by an underground service that climbs up a power pole, 2ga in conduit (for a 100A service). This pole is shared by a neighboring house, which also has an underground service using 4ga. The POCO has an overhead connection that looks fairly heavy crossing the street to get to the line that in turn goes to a 3rd pole that has the transformer. It looks like the power company gave us the heavy line to our pole because it serves 2 houses. Which makes for less voltage drop when we turn on heavy loads in our house. If we had an overhead service, that wire would likely be much thinner.
