Instruments at Long Distance


Thread Starter


Hello everybody,

I am about to make my first own project, the instruments and sensors are far, about 800 meters from the control room (DCS control system).

We are dealing with American system "AWG", usually i use #18 AWG or #16 AWG wire size for the instruments in the range of 100-200 meters, but in case of 800 meters iI think to go for the 14 AWG to decrease the voltage drop. Do you think 14 AWG for 4-20 ma flow meter 800 meters far will be ok?

Thank you
I doubt you'll even need 14g, but what's the arithmetic for the total loop resistance tell you?

a. Wire resistance
800m (2640 ft) of 12g wire resistance is 4.20 ohms *2 = 8.4 ohms
800m (2640 ft) of 14g wire resistance is 6.67 ohms * 2 = 13.34 ohms
800m (2640 ft) of 16g wire resistance is 10.60 ohms * 2 = 21.2 ohms

b. What's the resistance of the analog input?

c. What is the total load resistance that the transmitter can drive at the available power supply voltage? 500 ohms? 600


arithmetic: Is a + b < c ?
At our plant our standard for control wiring is 14 AWG because the current load can be much higher than current loops.

Lately foolish bean counters have been using 16 or 18 ga wire for 24VDC control wiring. Even with low current solenoids there can be problems.

For current loops we used 20 or 22 AWG shielded twisted pairs since the max current at 20 mA results in a very low voltage drop.
At 800m x2 20 AWG is 55.5 ohms for a voltage drop of 1.11 VDC.
At 800m x2 22 AWG is 88 ohms for a voltage drop of 1.76 VDC.
This is why 4-20 mA current loops are used, long runs with low voltage drop.

800m is NOT a long run.

good luck
16 AWG or 18 AWG for the 800m should be alright. Please check the power consumption of the flowmeter. If you are using normal DP flow transmitter, that requires approximate 12 V to operate. So for a 24 V supply, you have approx. 12 V to drop on the loop. For a DCS conditioning resistor 250 ohm in the loop ( voltage drop 250 ohm x 20 mA = 5 V max), you have 7 V to drop on the cable. As long as your total voltage drop on the cable is <7 V, it is absolutely fine.