Max distance from PLC to float

[d0ubleth1nk]

Does anyone know how the maximum distance a 120 VAC digital input loop is allowed to be? I'm having a float switch in a diesel tank wired, but it's 70 meters from the PLC, so 140 meters for the loop. Thanks.

[RussB]

This is a function of the resistance of the wire used, plus you will need to know the on and off thresholds of the monitoring device. I can not imagine 140 meters being a problem with 120 VAC device, so long as your wiring practices are good. Edited April 20, 2011 by RussB

[paulengr]

No "limit" as such. If you have additional wiring running in parallel and run pretty good distances, AC wiring will eventually cause you problems with inductive coupling. DC never has this problem; instead, you trade it off for voltage drop.

[panic mode]

as mentioned by previous post this is simply function of resistance and voltage drop. the only other thing that could give be an issue is capacitance. if the wiring is long enough and PLC input is sensitive enough (detected very small current, i've seen this on some smart relays with AC inputs) you may get false ON signal. if this is the case, simply add additional load in parallel with PLC input.