1-In case of long distance approx.3 KM.Can we use 14 AWG for transmission of Analog signals & digital signals?

2-What will be load or rating of relay.Which is used in Local Control Panel for the Start of Fire water Pump from DCS? (length from DCS to LCP approx.2.5 KM)

3-Can any body guide me how can we calculate Voltage Drop in Case of digital output from Dcs?

 

For No. 1 it depends on the load and voltage. Go to this site for a calculator: http://www.stealth316.com/2-wire-resistance.htm
You need to know what a 4-20mA device (24 VDC) actually draws (usually ~60mA or .06 Amps) and once you calculate it, what voltage will the device stop working at (~21.5 volts). The principle is the same for AC digital circuits (solenoids). A solenoid (ASCO Red Hat 2) draws a lot more current (~750mA @ 120VAC) unless you get a special low amp device. You have to calculate it.

Again, the calculator will solve most of your problems but you have to do the work. Here is another. http://genuinedealz.com/voltage-drop.html Is it AC or DC, 120V or 24V?? You need to know this stuff. Look at the name plate of the device or a cut-sheet.

For No. 2 you probably want an interposing relay for the pump control circuit. The DO for the pump will pull in a small coil relay in the DCS panel and permit a larger control current/voltage to go to the starter in the bucket, which inturn allow an even larger voltage/current (480V/3ph/60Hz 50A) to go to the motor to start it. Again, calculate the drop from the DCS to the LCP relay.