Conductor size of Instrument analog signals


What is section criteria for Conductor size of Instrument analog signals ? Why mostly1.5 mm2 is selected ?


there are Industry standards which will guide the choice of cable conductor diameter for both Analog and Digital signals. ( These can be ISA or ISO, depending on where you look)
1.5mm for Analog is the generally accepted standard, but each installation requires independent assessment. A normal transmitter or control card will generate 4 to 20mA into a total loop impedance of 750 ohms. Any really long cable runs could push the total over the limit ( perhaps in the case of receivers in series). .
This is not very common with distributed I/O, but it is worth considering when doing the overall design.
Digital signals I have encountered have been 1mm or even 0.5mm. Again it depends on the length of the run, the total impedance and the actual measured voltage of the source or sender of the signal. Normally, 24v for D.I/O.

Standards BS EN 50288-7
PAS 5308