4-20mA in long distances


Thread Starter


Hi guys, I wonder if you know how to handle several 4-20mA signals from a long distance (1-3km) Can I use just the normal wiring or do I need to use something else?

Thanks in advance.

James Ingraham

IMHO, put an analog input block next to the source of the signal, then use some form of fieldbus to connect it to your destination. Foundation Fieldbus, Ethernet (probably using Modbus/TCP), and Profibus come to mind if you really need > 1km. I think even these will need repeaters, and possibly fiber optic calbes instead of copper.

Maybe someone else knows a simpler solution, but I promise you a 4-20mA won't go a kilometer unless you've got a kilometer of superconductor.

-James Ingraham
Sage Automation, Inc.

Bill Townsley

Depending on the number of signals, using short-haul or long-haul modems will work just fine. Lots of vendors out there.. Blackbox, BB etc.

Radio transmitter is another way. Works well for indication only type signals - don't know if I would use it for control, though.
Take a look at the Weidmuller Telephone Line RTU Modules. Catalog numbers 991652 and 991653. They simply "move" 4 analog and 4 digital signals from one end of a line to the other. No PLC or DCS or communications convertors required. Switch closes at the remote location and a contact closes at the receiver. Works over a dedicated pair, and distance is not an issue since they work with proven telephone line technology.
I suppose, you can make the normal wiring. I already use a 4-20mA in a cable with 900m. I do not had any problem.
If we think that the cable is a resistance, you have a voltage drop that correspond to the current (max is 20 mA) multiplied by the resistance of the cable.

Best regards
I have seen 4-20 mA signals work reliable over a mile (1.6 km) distance with unshielded 22-gauge solid copper twisted pairs, not that it is a recommended practice. If you have multiple signals coming from one remote location, it might make sense to multiplex them to save wiring cost. If you have individual signals from many locations, I recommend you consider shielded twisted pair wiring as an option. You can calculate the loop resistance and use Ohm's law to figure out the voltage drop. The equipment specifications should tell you whether it can handle the voltage drop through the loop. Compare the cost to run the wires to the cost of the other options and select the most cost efficient approach.
Try a wireless I/O interface...maybe Phoenix Contact's MCR-RAD transmitter/reciever set.
Good for 4 - 5 miles, line of sight, flat terrain. It's only good for 600-1000 feet in-plant with no line of sight.

David Bergeron, P.E.

Look at the maximum resistance that your 4-20mA transmitter can handle. It should be in the specs. Then add up all the resistances around the loop including the resistance of the wire. As long as your total loop resistance is below the maximum resistance of the transmitter, you are OK. If using a 2 wire transmitter, you also need to check that your transmitter is getting enough voltage. Multiply 20mA times the total loop resistance and subtract the product from your power supply voltage. The result should be greater than the minimum transmitter voltage.

David Bergeron, P.E.
Thompson Equipment Co.
125 Industrial Ave
New Orleans, LA 70121
Phone: 504-833-6381
Fax: 504-831-4664
What about Devar plug-and-send wireless 4-20? They are quite a bit cheaper than most of the wireless solutions out there.
Also check that the transmitter does not use an overcurrent to indicate a fault - some older Rosemounts pull 25 mA.
You can use multiplexers from MOORE (Cable Concentrator System, CCS modules), for 16 analog/digital signals (channels are configurable). Maximum distance with 24AWG twisted pair wiring is 2 miles (3.2Km).

So many different options it may very well work using normal practices I have seen long distance runs before but assuming it does work maintenence issues arise down the road if a problem occurs with the cables. To me this sounds like a perfect application for radio either via multiplexers or standalone 4-20mA signals. There are increasingly more and more providers of this technology. I have used spreadspectrum radio's for comms of more than 3KM with very good reliablity. Keep in mind it can get expensive depending on I/O requirements.
I've seen an installation at a water plant where a 4-20mA signal was 1.5 miles, start to finish, and it works fine. I shorted the wires together to test the resistance: 0.9 ohms.
1) What is your power supply voltage?

2) What is the minimum permissible voltage across the transmitter? (typically about 10-14 V)

3) What is the load resistor? (usually 250 ohms but no always)

4) What is the maximum current the transmitter pulls (can be 25 mA to indicate a fault).

5) Find the maximum load voltage by (3) x (4)

6) Find the maximum acceptable voltage drop on the conductors by (1) - (2) - (5)

7) Find the maximum circuit resistance by (6) / (4)

8) Find info on the cable you are using - resistance per 100 m or similar (typically, for 0.5 mm^2, this is about 3.9 ohm/100 m).

9) Maximum circuit length (out and back) is (7) / (8) x 100m. Divide this by 2 to get maximum cable run distance.