4-20 mA maximum loop distance
Really long, you can do more than 10km. However, at this distance, other problems may start to creep in (ground loop if not isolated, EMI, so on).
The distance depends on your voltage source and your actual instrumentation voltage span, and cable diameter of course.
Maximum voltage drop is when the instrument consumes 20mA. However, you want it to work up to 24mA, so that your PLC system can detect ‘over-range’ (maybe due to short circuit, or actual over-range on the instrument side).
Let say, for our example:
- Loop powered, with 24VDC at source,
- Our particular instrument only works down to 10 Volts,
- No HART interface.
Due to the above, you only have 14VDC leeway for transmission loss, or 580 ohms. That equates to 8-ish km for 20AWG cable (66 ohm/km, accounting both cores).