4-20mA signal cable


Thread Starter


I have a mass flowmeter which sends an output of 4-20mA signal and a proportional valve which accepts 4-20mA signal. Purpose is to drive the
proportional valve based on the feedback from the transmitter of the flowmeter. What is the standard cable available for 4-20mA signal?
I believe you'll need a controller in the loop as well. Simply feeding the flow signal to a control valve is not going to do much for you.

Anyway, to answer your question, 4-20 ma cable should be twisted, shielded pairs. The wire gauge is dependent on the distance involved (total loop impedance figures in here) and the insulation rating is dependent on the equipment this signal interfaces with. Belden 8719 (#16 with 600 volt insulation) and Belden 8760 (#18 with 300 volt insulation) are very common cables for analog signals.

Steve Myres, PE

It would depend somewhat upon transmission distance, signal environment, etc. A typical cable would be (2)#18-22AWG twisted pair, PVC insulation, braid (preferable) or foil shield, with drain wire, PVC jacket. Make sure to connect
the shield drain wire only at one end of the cable, choosing the end with the lowest impedence ground source.

Try to separate this control wiring from any high current power conductors. Three feet separation between parallel runs should be sufficient. Be
particularly careful with runs of power wires for pumps or other loads driven with a VFD (variable frequency AC drive), as the motor leads can radiate a lot of noise.

Al Pawlowski

I would like to add a couple of my opinions to the other good comments on 4-20mA cable:

1) Any two-wire cable, can work if it has sufficient insulation, physical protection and low enough resistance so that the total loop load does exceed the drive capability of the loop driver. I have used "zip cord" cable (often used for power cords and speaker connections) for instance with satisfactory results. Twisted pairs are good because keeping the loop conductors physically close to each other helps reduce coupled noise. But keep in mind that most control devices will not respond to very high frequencies and 4-20mA is a pretty high level signal. This means that cable with normal shielding is often not worth the extra expense. Normal cable shielding is good for keeping high frequency noise, such as radio communications and spurious VFD output emissions, out of devices which may be more sensitive. Microprocessor based devices, for instance, are not always as well protected (IMO) as they should be for industrial use and "chip" internal signals are usually very low level. Also, you need ferrous (say steel conduit) shielding, or distance, to keep out magnetically coupled interference like from nearby high current (say motor power leads) signals; normal shielding is non-ferrous and doesn't do anything to reduce magnetically coupled noise.

2) Normally, the best place to ground a (cable) shield is at the end connected to the device powering the loop. However, if you have a lot of
cables going from one device/panel to other devices with grounds that may be at other potentials, it is better to connect at the individual devices. This helps minimize ground loop currents through the cable shields. Long, or field, cable runs often get insulation nicks which result in multiple grounds along the shield and I have found it useful not to connect these at a central point.

***previously posted*********
>........ Purpose is to drive the proportional valve based on the feedback from the transmitter of the
>flowmeter. What is the standard cable available for 4-20mA signal...

Al Pawlowski, PE
dba ALMONT Engineering
Baton Rouge, LA USA