Running Analog Signal


Thread Starter


I'm trying to run some 4-20ma signal about 15 feet through some conduit. I have basically two options. There is an existing 1/2" stainless steel conduit that is already running in the path that I need it to go, but there are two 110v controls lines running to a vfd. Would it be safe to run the 4-20 if I used some properly shielded cable? If so, would some 2-conductor twisted pair cable work? If running it through this same conduit is not a reality, then would it be safe to run a separate stainless steel conduit parallel to the existing conduit? Because of spacial constraints, this conduit would need to be about an inch or two away from the existing conduit line. Thanks in advance for any help!

William Sturm

It is not considered a good practice to run low voltage signal wires in the same conduit as high (> 32 Volts) power wiring.

Curt Wuollet

For 15 feet the first idea would probably _work_ fine with proper cable. A shield within a shield can do some weird things and it's usually not good, or in many cases, permitted, to run high and low voltage together. I see no problem at all with the second.

Basically, anybody can do anything they like, and the deity knows how often I¹ve seen what you¹re suggesting and worse. I wouldn¹t do it. I would run a completely separate conduit to carry the signal wires. Twisted pair shielded, floating at one end.


Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Automation and Process Control INSIDER (**
The shielded, twisted pair cable is a must since that will help prevent any electrical induction. The separate conduit will also help to reduce interference so your second option is the best way to go. Also for very short distances electrical induction is not much of a problem. You should never try this over a long distance. General rule is minimum of 0.5 meters away from electrical cables on long runs, preferably 1 meter.

Garry Shortt

I agree with allot of the responses so far. If at all possible, try to keep the 4-20ma signal separated in a separate conduit. If you cannot and you are experiencing noise on the line then this can be filtered by a capacitor to ground at the signal input device.
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