He has to check cable catalog in most of cable catalog X is already been calculated and given.Data cable catalog based on IEC are all 50Hz and NEC cables are for 60Hz.
Right answer for the wrong reason - cable construction, termination rules, and ampacities in 50hz countries are all different from the stuff you are used to in the USA. Theres no reason that has to be the case, but it just happens that thats the way it is.
Thus when in Rome (or Southwest Asia) you need to do as the Romans do, and use the local tables for calculating what you may do and not do.
Also note worthy is that the NEC does not rule on permissible voltage drop, but many countries regs do.
You need a copy of the local regs, of if the countries regs are rubbish or non-existant, get a copy of the British wiring regs, as many 50Hz countries use those as a basis.