Usually the ampacity of an insulated wire in IEC World is standardized in IEC 60364-5-52 /2001
Part 5-52: Selection and erection of electrical equipment ? Wiring systems
concealed in wall directly see Table 52-3 (52H) ? Examples of methods of installation providing instructions for obtaining current-carrying capacity pos.57.
Reference method of installation= C [Annex A]
Table A.52-4 (52-C3) ? Current-carrying capacities in amperes
for methods of installation in table A.52-1 (52-B1) ?
PVC insulation/three loaded conductors/copper or aluminum ?
Conductor temperature: 70 ?C/Ambient temperature: 30 ?C in air, 20 ?C in ground
For method C copper 1.5 mm^2 17.5 A;2.5mm^2 24 A.
NEC for 75 conductor 16AWG [1.3 mm^2 [recalc. for 70 from 90] 16.9 A 14AWG [2.08] 20A
BS7671 Table 6C Single layer on wall or floor recommends method C
For method C copper 1.5 mm^2 18 A;2.5mm^2 25 A. [Table 6D1]