It depends on design, environment and maintenance. If it's well protected against oxidation and it's static, it's not really a problem. Heat issues are often at the joints, which are most vulnerable, but can be coated and encapsulated to mitigate. For some uses, aluminium is superior to copper all things considered. Obviously the diameter of the wire/cable depends on which conductor you use, AC/DC, current and service temp - for some applications, that may favour one material over the other.