Maximum cable lengths for analog sensors

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…10 V, 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…10 V, -10…+10 V, 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…20 mA, 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.