Single Cable to Junction Boxes

Depends on what the 'protocol' is. 4-20mA? Profibus? Foundation Fieldbus? Modbus?

For a 4-20mA current loop, the maximum copper wire distance is only limited by Ohms Law considering

- the power supply voltage

- what ever the application uses as a maximum current; the upper failsafe value is typically above 21.0+ mA

- the total loop resistance that the device can regulate to the maximum current which is

* load resistance
* copper wire resistance out and back
* I/S barrier resistance (if applicable)

Transmitter documentation typically has a load line chart showing resistance vs loop voltage

The Fieldbuses each have their own distance limitations.