hey guys,

there's some instrument in the field use 4-20mA signal looped powered by 24vDC. how far distance can the signal transfer from the field to substation. what cable cross section size should be use? is there a cable spec of this situation? because some special instrument are 1km away the substation, should i choose a bigger size of the cable cross section??

 

Kefi_netc... What is the receiving-end load?

Regards, Phil Corso

 

Every loop powered device has its own requirements for the minimum voltage needed to run. Somewhere in the instrument's documentation, there's a statement of minimum voltage or a graph showing the amount of loop resistance the transmitter can drive with a given loop power supply voltage.

Calculate the wire resistance for the length of the run (remember that it's twice the run distance, out and back), add the analog input resistance to find the loop resistance and check the graph to see how much supply voltage is needed to drive the loop resistance.

Or use Ohms Law to figure out how voltage the loop resistance drops at 20mA to see whether your power supply voltage is adequate to drive the minimum transmitter voltage requirement and the loop resistance.

 

Analog input/output modules specifies max. Loop resistance approx. 600 ohms, so u check cable resistance/m or KM in datasheet, better u use 2.5mm x'section cable.