I recently worked on a project that involved a 300 feet cable run from one part of a building to another at our plant. Its a simple 120V circuit that originates in a control panel, travels 150 feet to the NO contacts of a temperature controller, and back to the input card of a PLC inside the control panel...for a total of 300 feet. All the circuit is doing is detecting if the temperature has dropped below the setpoint which in turn turns the heater on.
Now as to why there is no remote I/O rack located near the temp controller is a story for another day. I am fully aware that this is not a well designed system but this is what we have for now.
First thing I encountered in my design was sizing the cable. I have designed power circuits in the past and figuring out voltage drop in power circuits is fairly straight forward as you know the power rating of the load and can easily calculate the current for a 3-phase 480V system.
Not so straight forward for control circuits apparently, because I am not sure what my load is (is it the I/O card?) and how much current it's pulling. Since I don't know the current flowing through the cable, I can't figure out the voltage drop. Even if I were to ignore voltage drop for a second, how could I size the conductor without knowing the current.
Based on an experienced colleague's suggestion, I ended up using 16AWG. But I'd like to know the specifics. Please advise.