Why are stranded wires commonly used outside the US?
In nations where the supply frequency is 60 Hz (HIGH), solid conductors are utilized. The reason for this is that the skin effect is typically greater in higher frequency power systems and less in solid conductors. However, in nations where the supply frequency is 50Hz (LOW), stranded cables are utilized. A 60Hz electricity grid is to use all over the USA. Thus, solid cables are utilized in the United States whereas stranded cables or wires used in other countries.