Voltage Drop on 4 to 20 mA loop

Voltage drop on 4 – 20 mA loop:

Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. Designing 4-20 mA current loops is just managing the voltage drops around the loop. The voltage drops occur in the wire, the transmitter and load resistor.

Formula for calculating the DC voltage-drop is V = 2xIxRxL, where:
V = Voltage-Drop, in volts.
I = Current, in amperes.
R = Resistance, in ohms/km.
L = Distance, in km.

Table 1 Copper Wire Resistance @ 20°C (68°F)

Ohms per American 1,000 feet (305 meters)

Wire Gauge         Solid Wire           Stranded Wire

14                           2.53Ω                                     2.73Ω

16                           4.02Ω                                    4.35Ω

18                           6.39Ω                                    6.92Ω

20                           10.15Ω                                  10.90Ω

22                           16.14Ω                                  16.50Ω

24                           25.67Ω                                  27.70Ω

The voltage drop in wire is directly proportional to the current flowing through it by Ohms law, current x resistance equals voltage. The transmitter consumes 7 to 15 VDC of loop voltage, depending on model, to power itself.

Finally, the controller’s load resistor converts the loop current to a voltage for easy signal processing. Most commonly a 250 Ω resistor is used to convert the loop current back to a voltage. At 4 mA, a 250 Ω resistor drops 1 volt; at 12 mA, a 250 resistor drops 3 volts and at 20 mA, a 250-Ohm resistor drops 5 volts. The load resistor can be internal to the controller or external.