I am trying to find the differences between electric cables designed for High voltage, medium voltage and low voltage electric transport, but I am failing to find an answer. I was wondering if you could help me with this. I know that aluminum is almost always the metal of choice, thicker is good but "Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire." (from http://en.wikipedia.org/wiki/Electric_power_transmission). So what are the differences? Also are the high voltage cables always insulated? I am referring to what usually the power distribution companies use.