Anybody kindly furnish me the standard where I can justify
that equivalent sizes for wire size 4mm2,6mm2, and 10mm2 are respectively AWG#12,AWG#10,AWG#8.
Look in the National Electrical Code (NEC), on page 70-625, Table 8, Conductor Properties. For wire sizes from 18 AWG to 4/0 AWG, the area in mm2 is listed.