What parameter is used for grading wires and know the resistance by gaze ?

The kind of installation, ambient operating temperature, service circumstances, type of load served, and any other related factors are often taken into consideration when choosing wire for any electrical equipment. Since the wire size relies on the current rating of various devices, conductor size is crucial when choosing the wire for any device. The wire is color-coded to make it easier for us to tell if it is a phase or neutral wire, but other than that, it has no bearing on how the wire is installed in any electrical equipment. Because subterranean wiring requires more frequent maintenance than above wire, the wiring insulation used in each is different.
Colour coding is also used as a common nomenclature for understanding the setup and usage things at appropriate level.