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Cross Section Area of 4-20 mA Transmitting Cable

06/25/2018 11:48 PM

Hi everyone,

I work in installation of water treatment plant and have to select a cable for transmitting 4 - 20 mA analog signal from pressure sensor to control panel over 30 m. In the given specification of the client, it has been mentioned to use 2 wire system.

In a supplier catalog of analog signal transmitting cable, there are several cross section areas that can be selected.

How do I select correct cross section area?

What are the parameters I should consider selecting in the cross section area (for example, for power supply cable, we have to consider load current grouping factor, location, temperature etc. and have to use BS Code to select proper cross section area)?

I would be grateful if someone could assist me with this.

Thanks & regards,

Kaushika

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#1

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 12:19 AM
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#2

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 2:39 AM

Selection of cable is done on the basis of its mechanical properties rather than its electrical ones, as the proverbial "two bits of wet string" will transmit <...4-20mA...over 30m...>.

  • There must be, somewhere, a client-based cable standard that will give details of the cable to be used for such a task. In the absence of a published standard, copy the specification of the cable from a previously-installed one that also carries <...4-20mA...>, for the new cable will then be acceptable on the basis of precedent.
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#3

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 5:07 AM

You need to know the voltage driving the current loop and the resistance at the receiver to ensure that the cable resistance isn't too high to be able to attain 20mA at full scale. Then allow a safety margin.

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#4

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 6:52 AM

A commonly used shielded twisted pair (STP) cable is Belden 8760. Spec sheet here:
https://catalog.belden.com/techdata/EN/8760_techdata.pdf

for other gauge cable with other features, use a cable finder guide
https://cdn2.hubspot.net/hubfs/282305/Old/assets/migrated-products/pdfs/02-2_29.pdf

On page 8, there are lots of choices for 18g, shielded twisted pair instrument cable with other features.

Other vendors have similar offerings.

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#6
In reply to #4

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 12:06 PM

18 AWG for a 20 mA current source? That's a 25 mV for 1000 feet of cabling when the voltage drop does not matter.

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#5

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 7:41 AM

Check out the BS EN 50288-7:2005BS for instrumentation cable.

https://www.egpet.net/vb/attachment.php?attachmentid=10621&d=1529446792

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#7

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 12:14 PM

A shielded twisted pair is probably what you need. At only 100 ft (30 m) cable lengths you probably can use 24 AWG cable. Connect the shield drain at only one end.

If you have a connector that will only clamp onto a certain range of cable diameters then go by that.

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#17
In reply to #7

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/27/2018 11:33 AM

I will second connecting the shield drain to just one end, otherwise you can get a ground loop and your equipment will go kooky-kitty on you.

Connect the shield to the end that has the power source ground, otherwise you can pick up noise from the far end and transmit it back to the main equipment, or worse, a static charge.

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#8

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 1:02 PM

Go big, or go home!

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#9

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 1:59 PM

The whole point of current loop is that you don't need to be concerned about IR drop. All the current that goes into one end of the wire comes out the other. Look up ohms per 1000 m and multiply by your round trip distance/1000 to get the resistance. Multiply the resistance by .02 to get the maximum voltage drop.

For example, 60m round trip x 84.2/1000 = 5.052 ohms for 24 ga. 5 ohms x .02 amps = 0.1volts.

"Common Wire Gauges

The common U.S. wire gauges (called AWG gauges) refer to sizes of copper wire. The resistivity of copper at 20 C is about

This table uses this value of resistivity, but it is known to vary by a few percent based on purity and process of manufacture."

AWG wire
size (solid)
Diameter
(inches)
Resistance per
1000 ft (ohms)
Resistance per
1000 m (ohms)
240.020125.6784.2
220.025416.1452.7
200.032010.1533.2
180.04036.38520.9
160.05084.01613.2
140.06402.5258.28
120.08081.5885.21
100.10190.9993.28

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html

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#10

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 3:17 PM

It's not about voltage drop. The use of 18 or 20 gauge is common because

- it works for long runs as well as short runs

- it's in stock (both stores and vendor) because it's so common

- because of its mechanical strength and robustness with screw terminal connections.

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#12
In reply to #10

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 8:01 PM

Exactly, the voltage drop is insignificant. Any twisted shielded pair that is available would work just fine.

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#11

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 5:54 PM

If there were only one correct size, why would catalogs sell other sizes? There is no such thing as just one correct size.

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#13

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/26/2018 11:33 PM

As your location is in a water treatment plant, the more important aspect of cable design is to be waterproof and suitable for long term immersion as the air is likely to be high humidity constantly.

As others have said, the largest diameter conductor which will fit in the connectors will give the longest service.

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#14

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/27/2018 12:25 AM

Dear All,

Thank you every one for helping me and I have gone through all of your suggestions and selected cable as follows.

In the one of given link it has been mentioned that even though same current flow throughout the cable, between transmitter and received, the voltage drop should be less than 10%. Receiver provide 24Vdc and resistance of receiver is 100 ohm. Considering worst case I have calculated the wire resistance for 30m (loop is 60m) under 24V and 20 mA with 10% voltage drop. (Receiver is VFD and has in built 24Vdc supply. In here the wire part that has been used to loop GND terminal DC supply to GND terminal of signal receiving end was not considered because it is too small. This is 2 wire sensor model). Calculated resistance is 10 ohm for 30m and resistance of selected cable for 30m should be less than 10 ohm

In the cable supplier catalog, there are details about max. dc resistance (ohms per km) at 200C. I have selected 2C x 1mm2 cable (Type: Cu/PVC/SCR/PVC, Standard: BS 6500) so that resistance at 200C for 30m is 0.585 ohm for 30m. (0.5 mm2 also can be used but considering mechanical strength, I selected 1 mm2)

(In the given specification of the client for electrical wiring system, it has been mentioned that when wires are selected ambient temperature should be considered as 450C. So ground temperature is likely to become 350C (This cable will be laid in ground). Variation of resistance due to this temperature is very small and can be avoided).

This is Class 4 cable and it will be laid in the ground within rigid PVC duct. With this cable route, class 1 and class 2 cables will be routed. I need separate this cable from other disturbing cable class 1 and 2. I used following link to get idea about this and I am struggling to select correct spacing between 30 cm (according to paragraph) and 1m (according given image) and would grateful if someone provide a guidance to select proper value.

http://www.electrical-installation.org/enwiki/Wiring_recommendations

Thanks & regards,

Kaushika

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#15
In reply to #14

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/27/2018 9:21 AM

<...This cable will be laid in ground...> would dictate a steel-wire-armoured cable.

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#16
In reply to #15

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/27/2018 9:55 AM

The OP specified it will be in PVC conduit. It therefore is already durably protected so no need of armor. Also no hazardous voltages and currents will be applied to the cores.

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#19
In reply to #14

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/30/2018 3:21 PM

Electrostatic shielding is effective, especially if it is solid, like a steel pipe or conduit.

Magnetic shielding is expensive & ineffective & only justified on sensitive instruments.

There is no substitute for distance to avoid magnetic coupling, it is the amps/frequency in the "culprit" cable which matter.

The magnetic flux density at x metres from an isolated straight wire in air is, per amp carried...

μ0/2πx where π = 3.142 :μ0 = 4πx 10-7 N.B. doubling distance will halve flux.

1m spacing instead of 1 cm cuts trouble 100 times

Inserting values , that is 4 x 10-7 webers/square metre/amp for x = 1 metre.

Consider 1000m run of instrument twin with 2mm separation between cores, that is a coupling loop of 2 square metres. So for 1 amp you have a flux of....

8 x 10-7 webers

Of course, AC is much more trouble, take a sine wave current of 1 amp peak at frequency f...

i = sin(2πft), then rate of change di/dt = 2πfcos(2πft); peak value 2πf which is 314.2 amps/second at 50 Hz.

Combining the two figures you have a peak induced emf of....

8 x 10-7 webers x 314.2 amps/second at 50 Hz = 2.5 x 10-4 volts peak per amp peak.

So for 1000 amps peak at 50 Hz you will get 0.25 volts peak induced in your 1000m cable run with 1m separation - which might be 5 volts peak for a 20 kA peak short circuit.

If you have a twisted pair instrument cable, most of that will cancel out, say a 1 metre run for 360 degrees twist might leave you with 5 mV peak (if power cable was not laid with a twist, practically it will have some twist & snaking).

You may have noticed that an isolated conductor is theoretical - usually a power cable has a return conductor [equal & opposite current] only cm from the go conductor which at 1 metre seperation will cancel a lot of the field (100cm:1cm reduction for 1 cm between power conductors in cable) - however, if an overhead line shorts to ground it may be far from the return path in earth. Many industrial installations have metal skids and earth bonding of lower resistance than earth core in power cable, so most of earth fault current may not return in power cable [think ship].

In practice, cables to e.g. variable frequency drives might carry 10 kHz and switching loads can make high frequency, but short duration, bursts.

So difference between 300mm & 1m is about 10:1 in pickup (1/x2, inverse square law). I believe 1m is adequate even with 1000 amp power cables, which are about biggest likely.

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#18

Re: Cross Section Area of 4-20 mA Transmitting Cable

06/28/2018 8:11 AM

I see a lot of good answers about gauge, and they are correct - voltage drop is insignificant, as I've seen a 4-20ma device work at 18 volts when my 4 to 20ma loop tester batteries are getting very old.

What is important here is shielding with 30 meters of run through a plant environment. Be sure you get a shielded cable and connect the shielding at one end or the other to ground. There is a big disagreement as to grounding at the instrument or at the receiving end (usually a PLC I/O card) and I've seen both work fine in 25 years of designing/building process system skid control circuits, so I'll not try to stir that up. Just ground it at either end.

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