How the sensor supply voltage drop calculator works
Three- and four-wire sensors, such as proximity switches, voltage-output pressure sensors, flow meters and IO-Link devices, take their supply over a pair of conductors in the cable. The current they draw drops a voltage along both conductors, and what reaches the sensor is the supply less that drop. A sensor below its minimum supply voltage may still work at room temperature and fail when it is cold or when an output switches.
The calculator gives the voltage at the sensor, the margin above its minimum, the longest cable and the smallest cross-section that still work, the power lost in the cable, and the shift of a voltage output caused by the shared 0 V conductor.
Voltage at the sensor
The drop is I × 2 × ρ × L / A, with copper at 0.0172 Ω·mm²/m at 20 °C and 0.393 % more per kelvin. A sensor drawing 50 mA over 200 m of 0.5 mm² sees 0.69 V less than the 24 V supply. The minimum supply voltage is on the datasheet; leave margin for the supply’s tolerance, typically ±10 % on 24 V systems, and for inrush current.
Enter the current the sensor draws with its outputs active. A switching output that drives a load through the same cable adds that load’s current to the drop.
The shared 0 V conductor
A three-wire sensor with a 0 … 10 V output uses one conductor for supply, one for the signal and one, 0 V, for both the supply return and the signal reference. The receiver measures the signal against its own 0 V, so the voltage that the supply current drops along the 0 V conductor adds to the reading. In the example, 50 mA through 6.9 Ω is 0.34 V, 3.4 % of a 10 V span: a large error that does not show on the bench with a short cable.
A four-wire connection with a separate signal return, a sensor with a current output, or a short cable avoids it. Current loops are not affected because the receiver measures the current, not a voltage; for two-wire 4–20 mA transmitters use the loop power calculator.
Cable length and cross-section
The longest cable is where the voltage at the sensor reaches its minimum. The calculator also gives the smallest cross-section for your length and the next standard size. Larger conductors fix the supply but not the 0 V shift of a voltage output, which falls with resistance and remains proportional to the current.
Frequently asked questions
Does the cable temperature matter?
Copper’s resistance rises by 0.393 % per kelvin, so a cable in a 60 °C tray has 16 % more resistance than at 20 °C. Enter the conductor temperature to include it.
What about several sensors on one cable?
Sensors connected at the end of the same pair add their currents. Sensors spread along a bus see different drops; calculate the one at the far end with the total current as a cautious estimate.
Is AWG supported?
Yes. The conductor list includes AWG sizes alongside mm²; select the size of your cable.