Same Cable size for same circuit

[helpyou]

Hi ,
I have seen in some cases cable size is difference for the same circuit, for example from transmitter to junction box is 1.5 mm and from junction box to control room it is 0.5 mm .. is this acceptable and why is that ?

Thank you

 

[danw2]

If it's for 4-20mA signals, 0.5mm (20g) conductor is more than big enough. There would have to be more than a couple kilometers of cable before cable resistance would be a factor for typical instruments with a lift off voltage of 12-13Vdc, leaving up to 5V (for a typical 24Vdc loop power supply) for copper wire IR drop after accounting for a max 5V drop across the receiver input.

As to why there's two different cable sizes, maybe the instruments get wired by a different crew than those running the home run cables.

I know of a couple plants that don't use sizes smaller than 18g (0.75mm) because that's the minimum size for 'robustness'.