Sorry guys, voltage drop over a 20mA loop cable is very small!
Typical resistance of copper wiring is 0.0175 Ohms.mm²/m (at 15°C).
To give an example: 50m of 0.25mm² 2-wire cable has a total resistance of only
2(# of conduits) x 50(length) x 0.0175 / 0.25(gauge) = 7 Ohms
A 20mA signal would only result in a 140mV voltage drop! This would have no effect whatsoever since the typical resistance of an analog input is about 250 Ohms.
But voltage drop is not the real reason why you should avoid running any analog signal over extended cables: Noise pickup is a lot worse! Best results can be achieved by using cables with a full copper shielding, like Profibus cable, in stead of weaved shielding cable, like LIYCY. No need to tell you to stay away from power lines!