# FPLR/CL3R/CMR cable for 4-20mA circuits



## mburtis (Sep 1, 2018)

So being a new apprentice I'm trying to use every opportunity to learn and research. We have a project coming up where we will be installing a new instrument (hach sc200 controller) to be specific plus rewiring the 4-20mA signals from two old bailey pressure transmitters. There will be two 4-20mA outputs that will run upstairs to a plc. It will be 1 inch ridgid between the field and the plc cabinet. 

As a way to learn I started researching what type of wire we should run, seems most common is PLTC or ITC, but there is so many different types of shielded twisted pair wire its a lot to understand. Well today I ran across an entire 1000ft spool of 16 gauge stranded FPLR/CL3R/CMR in the storeroom (no iea why we have it). Been looking at the specs and i can't see any real difference. Cable runs will be less than 50 ft. 

Any reason functionally or code wise not to use this cable?


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## splatz (May 23, 2015)

I've had good luck with 4-20ma, since the signal is dependent on current rather than voltage you don't have trouble with interference, it isn't fussy about cable type, twisting, shielding, etc.


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## John Valdes (May 17, 2007)

Call the folks you bought the equipment from. They can save you a lot of hassle.
They can tell you exactly what you need and what to do. Then next time you already know.

Welcome to the forum.


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## The Sword (Jul 2, 2019)

Hi, if this is any use to you, over here in the UK we have RS components and they stock Belden cable in various forms. I've used this stuff before now, for signal wiring sensors to PLC's & VFD's / VSD's, with no problems.


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## oliquir (Jan 13, 2011)

Any basic shielded cable works for 4-20ma signals (don't even need shield most of the time), shield are more important for very low voltage signals or communications.


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## micromind (Aug 11, 2007)

oliquir said:


> Any basic shielded cable works for 4-20ma signals (don't even need shield most of the time), shield are more important for very low voltage signals or communications.


Agree. 

I've used just about every type of cable there is for 4 - 20. It's a rugged signal that'll tolerate a lot of different cables. 

I've run 24 AWG over 200' without issue. 

If I spec it, it'll usually be any type of 20AWG shielded twisted 1 pair but I have run 2 signals in 2 pair as well.


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## JRaef (Mar 23, 2009)

If you are required to use PLTC, there really is no acceptable functional equivalent (per the NEC). The differences are mostly about fire ratings.


You might want to keep this chart handy:
http://www.cabletool.net/Carol/225-226.PDF


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## splatz (May 23, 2015)

I would normally use 18 gauge unshielded for 4-20ma unless there's a reason to use something else. 



Unshielded because why bother, doesn't matter at all with 4-20ma, and 18awg because it's cheap, readily available in a variety of configurations, and easy to terminate and splice - works with anything from dolphin type crimps to stakons to scotchloks to little blue wire nuts.


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## glen1971 (Oct 10, 2012)

splatz said:


> I would normally use 18 gauge unshielded for 4-20ma unless there's a reason to use something else.
> 
> 
> 
> *Unshielded because why bother, doesn't matter at all with 4-20ma*, and 18awg because it's cheap, readily available in a variety of configurations, and easy to terminate and splice - works with anything from dolphin type crimps to stakons to scotchloks to little blue wire nuts.


Beg to differ with ya on that one.. I've had to go back and fix countless shields that were improperly terminated that it caused noise on the 4-20 mA signals and had levels out to lunch, pressures off, etc which messed up other control loops.. Shielded cable isn't that tough to use and it's a good piece of mind, if it's done right..


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## telsa (May 22, 2015)

I have to second Valdes.

ALWAYS run to the equipment techs.

Get THEM to opine.

They've seen a LOT more of their installations than you ever will.

BTW, saving your boss some $$$$ is a fruitless task.

He's the only one allowed to save himself money. 

He -- subconsciously -- would object if you did. 

Yes, I've been there. Your genius is NOT wanted.

This has to do with emotions and self-respect -- and your role in the organization.


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## splatz (May 23, 2015)

glen1971 said:


> Beg to differ with ya on that one.. I've had to go back and fix countless shields that were improperly terminated that it caused noise on the 4-20 mA signals and had levels out to lunch, pressures off, etc which messed up other control loops.. Shielded cable isn't that tough to use and it's a good piece of mind, if it's done right..


I will have to reconsider ... maybe I've just been lucky. 

Do you think it's possible the improperly terminated shield cause trouble because it became a parallel conductor in the 4-20ma loop? That would of course be a mess. Rather than a failure to shield interference. The sensor is passing the right current, it's just being divided between the shield and the loop conductor. 

That's been my experience with shielded in general - one thing wrong and it causes trouble in places you'd have had no trouble with unshielded. 

Have you ever seen a loop malfunction with unshielded cable then be fixed when replaced with shielded?


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## glen1971 (Oct 10, 2012)

splatz said:


> I will have to reconsider ... maybe I've just been lucky.
> 
> Do you think it's possible the improperly terminated shield cause trouble because it became a parallel conductor in the 4-20ma loop? That would of course be a mess. Rather than a failure to shield interference. The sensor is passing the right current, it's just being divided between the shield and the loop conductor.
> 
> ...


The biggest issues I've seen was when shields were grounded at both ends. Depending on what else is onsite will usually influence the amount of problems. Where there are a lot of VFD's or large motors, it is a bigger deal. 

There are tons of transmitters in the area I'm in now that are ran with underground telephone cables and there's no issues. But on those sites there is a 1/3 hp motor and at best a 10 hp motor off a phase converter. There aren't any noticable issues on those ones, but I typically only carry shielded wiring and replace as I go.

I think it also depends on what kind of technology you are working with where your 4-20 signal is received. Some of the older controllers are probably not as sensitive as some of the newer PLC's and DCS systems.


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## CoolWill (Jan 5, 2019)

You'd have to have an exceptionally noisy environment to affect a 4-20 signal since it is current based. A current would have to be induced in the loop in order to change the signal.


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## splatz (May 23, 2015)

CoolWill said:


> You'd have to have an exceptionally noisy environment to affect a 4-20 signal since it is current based. A current would have to be induced in the loop in order to change the signal.


It's my understanding, and maybe I am off on theory, but really currents are not induced, voltages are; but the sensor adapts to the voltage present and regulates the current, so even in the presence of induced voltage, the current should be correct because the sensor corrects for it.


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## MechanicalDVR (Dec 29, 2007)

Can't recall not using shielded pair, mostly by manufacturer specs.


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## CoolWill (Jan 5, 2019)

splatz said:


> It's my understanding, and maybe I am off on theory, but really currents are not induced, voltages are; but the sensor adapts to the voltage present and regulates the current, so even in the presence of induced voltage, the current should be correct because the sensor corrects for it.


Yes, I agree about voltages, but it's really a matter of semantics. Voltages are induced and cause currents to flow in closed circuits.

I don't know, but I would think that controllers have some inherent delay, so fast transient voltages could induce currents that couldn't be compensated for. Or if the induced voltage was additive to the control loop voltage, would that not cause a momentary change in the current?


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## gpop (May 14, 2018)

CoolWill said:


> You'd have to have an exceptionally noisy environment to affect a 4-20 signal since it is current based. A current would have to be induced in the loop in order to change the signal.


In 20 years i have only seen one major problem with a 4-20 signal and to be honest it was probably more to do with induced voltage affecting the rtd hockey puck (loop powered converter). 

We spent days trying to fix it as the readings changed when ever contractors opened or closed. we finally added 250ms of dampening to the converter as a band add and the problem went away. To this day i would love to have had a scope to see what was really going on. 

As you have stated the loop may except a lot of noise but there is a possibility that a loop powered device may be more sensitivity to it.


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## micromind (Aug 11, 2007)

splatz said:


> It's my understanding, and maybe I am off on theory, but really currents are not induced, voltages are; but the sensor adapts to the voltage present and regulates the current, so even in the presence of induced voltage, the current should be correct because the sensor corrects for it.


Think of the PLC as a power supply and the instrument as a variable resistor. As the resistance changes, the current will change proportionally.


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