A 4-20 mA Signal Goes The Distance

A 4-20 mA control signal is one of the signals of choice in the Building Automation industry as well as in the Industrial Process arena. Most of the various transmitters and output devices available from Kele utilize a 4-20 mA input or output signal. In large commercial buildings, expansive factories, multi-building university campuses and other large projects, it is not unusual to find an application where a transmitter is located a long distance from the nearest building automation controller. We are frequently asked, “How far can 4-20 mA signal wires be run?”. The answer is quite amazing.

A 4-20 mA current loop consists of a number of devices wired in a series loop. These devices include a power supply, a sensor/transmitter and one or more loads such as a building automation input, a chart recorder, an alarm module or a digital display. Depending on the type of transmitter used, the power supply may either power the transmitter external to the loop or the power supply may be one of the devices in the series loop as shown in Figure 1.

  

The transmitter is a 4-20 mA current source that requires a DC voltage supply to operate. The transmitter in Figure 1 is a model PTX1 pressure transmitter. Reviewing the PTX1’s specifications in the Kele catalog shows that it can operate with a power supply voltage from 10 to 30 VDC. However, the actual power supply voltage used will directly affect the output capability of the transmitter. To provide its full output, the transmitter must be capable of supporting the total resistance of all loads that are wired in the loop. Again looking to the specifications of the PTX1, there is a graph (Figure 2) that indicates the output load capabilities of the PTX1 for supply voltages from 10 to 30 VDC.

A quick study of the graph shows that when using a 10 VDC power supply, the PTX1 can support a maximum load of zero ohms! In other words, the PTX1 requires 10 VDC just to power its internal electronics (power supply overhead). For the transmitter to drive its 4-20 mA signal through any load at all, the power supply must be increased above the 10 VDC overhead required by the PTX1. As indicated by the graph, a 30 VDC supply will allow the transmitter to handle over 900 ohms of load in the loop. At 24 VDC the transmitter is rated for 650 ohms.

Looking back to Figure 1 shows that the building automation input is 250 ohms and the chart recorder adds 250 ohms. The total load of 500 ohms is within the 650 ohm capability of the PTX1 with a 24 VDC supply.

So what does all of this have to do with how far 4-20 mA signal wires can be run? There is an additional load on the loop that has not yet been considered. This additional load is the resistance of the wiring connecting the devices in the loop. For shorter signal wiring runs the resistance added by the wire is negligible, but what effect will long wiring distances have on the loop? By considering the wiring resistance as an additional load, the allowable length of wiring in the loop can be determined.

 

In Figure 3 the loop is powered by 24 VDC which means the PTX1 can drive its 4-20 mA output into a maximum of 650 ohms. The distance from the control panel to the PTX1 transmitter is 1000 feet and is to be wired with 18 gauge wire. The resistance of 18 gauge wire is 0.00651 ohms per foot. The total resistance of the wire in the loop, Rw x 2, is 13.02 ohms (1000 ft x 0.00651 ohms/ft x 2 = 13.02 ohms). The total resistance of the loop then is 513.02 ohms, which includes the resistance of the wiring added to the building automation input and chart recorder. In the real world, additional resistance may be added to the actual total loop resistance by wiring connection points such as terminal blocks, splices, etc. The total loop resistance of 513.02 ohms is less than the maximum output rating of 650 ohms so the PTX1 will work in this application.

The amazing part is to do the math to determine the maximum allowable wiring distance for the example in Figure 3.

Even with two loads (500 ohms), a building automation input and a chart recorder, the signal wiring from the control panel to the PTX1 transmitter could be as long as 11,520 feet or more than 2 miles! The characteristics of the PTX1 are not unique. Kele has 4-20 mA transmitters available to handle loads from 300 to over 1000 ohms.

 Of course, there are additional factors to consider when running signal wiring over a long or even short distance. These factors include protecting against the effects of radio frequency interference (RFI) and electromagnetic interference (EMI) by never running signal wires in the same conduit as AC power lines, considering the use of shielded twisted wires, employing surge protection when running between buildings, and always following national and local electrical codes.

Remember, when you face a long signal wiring run, a 4-20 mA transmitter from Kele will go the distance.

Reprinted from Winter 1996 Kele Insights