A question came in recently concerning the maximum recommended cable length for analog sensors. Even as digital interfaces gain popularity, sensors with analog interfaces (0-10V, 4-20 mA, etc.) still represent the overwhelming majority of continuous position sensors used in industrial applications.
The question about maximum cable length for analog sensors comes up pretty frequently. Generally speaking, the issue is that electrical conductors, even good ones, have some resistance to the flow of current (signals). If the resistance of the conductor (the cable) gets high enough, the sensor’s signal can be degraded to the point where accuracy suffers, or even to the point where it becomes unusable. Unfortunately, there is no hard and fast answer to the question. Variables such as wire gauge, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, and other factors come into play, and need to be considered. A discussion about all of these variables could fill a book, but we can make some general recommendations:
- Analog Voltage Signals (0…10V, -10…+10V, etc): Generally, we recommend that cable length for analog voltage signals be limited to 50 feet*. Cable lengths longer than 50 feet can (and often are) used provided the factors mentioned above are considered and addressed, but the safe bet is stay under 50 feet.
- Analog Current Signals (0-20mA, 4-20 mA): Typical industrial position sensors with an analog current interface can tolerate significantly longer cable lengths compared to comparable voltage interfaces. Again making a somewhat broad generalization, analog current signals can tolerate cable lengths up to 500 feet*.
*Both of the recommendations above assume the use of properly terminated, shielded cable. It’s also assumed that the cable is not routed near high-energy AC sources, against which even a shielded cable offers little protection.
If longer cables lengths are necessary, or if the application is electrically “noisy”, your best bet is to consider sensors with a digital interface. Fortunately, there is an increasingly wide variety of continuous position sensors that use highly noise immune digital interfaces.
Dear, I have 3 wire configuration Fire and gas Detector installed at Cooling tower, the Distance from control room to Cooling tower sensor installation is 1300 meter, and the cable size is 2.5 square mm.
and the device mA and 24 V dc is fluctuating when we connected the Sensor to the transmitter. 3.8mA to 4.1 mA and 20.1 VDC to 21.2 Vdc. we also have checked the Device to the control room, it is working fine and no variation in power supply and mA.
is there any suggestion from your Side? how to stable mA or 24Vdc power supply. the sensor is basically IR type, During the normal operation IR source will consume the power.
Nice post.
I was left wanting to learn more! What about a digital signal? isn’t it an electrical signal after all? Would not be expected that it also suffers from these problems? I have read somewhere that a connection using RS485 can be km-long, is that true and how does it not suffer from those problems in your post? Thanks again, we learn a lot from your posts.
Hello
I have temperature monitoring application in which I have to monitor 15 temperatures from 120-metre distance approx. with sensor type RTD with 4 to 20 mA temperature transmitters, so can you suggest which cable size suits for this application. I have to read all this signals in plc analog input card of Micrologix 1400 Allen Bradley
I cannot make specific recommendations in this case. I would recommend that you check with the supplier of the RTD’s.
Thanks a lot. Helpful
Hi, is there any standard regarding this cable length for analog signals i.e.., for 0-10V and 4-20mA?
As addressed in the article, the general recommendation is that cable lengths for 0…10V signals be limited to 50 feet. For 4…20 mA signals, the recommended maximum cable length is 500 feet. These recommendations assume the use of shielded cable.
Thanks.
Good Suggestion!!
Can u tell me the size of the cable (dia) considered while calculation.
Thanks for your question. We are not really specifying any particular wire size, as these are just general recommendations. Most typically, wire gauges used for industrial sensors range from 26 AWG to 20 AWG (0.29 mm^2 to 0.518 mm^2).
Thanks , it’s a good information.
Hongwei, Wang.
From Canaan.