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4-20ma Instrument Cable Thickness Calculation

10/18/2014 10:14 PM

Experts,

We are implementing some transmitters in water treatment plant, the distance between the transmitter and the receiver is approx 120 meters. Can you please suggest the calculation of cable size(thickness).

Please also suggest which type of cable will be more suitable: Shielded/screened. Twisted pair or untwisted etc.

Thanks,

Manish

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#1

Re: 4-20ma Instrument cable thickness calculation

10/18/2014 10:24 PM

No!

Your instrumentation tech can, if you have one.

If you don't, ask the supplier of the undefined instruments to help you.

Or, go wireless.

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#2

Re: 4-20ma Instrument cable thickness calculation

10/18/2014 10:48 PM

4/0 will surely be big enough.

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#3

Re: 4-20ma Instrument cable thickness calculation

10/18/2014 11:08 PM

What calculation?

I'll give you a very big hint, you have given us insufficient information to help you. The first thing needed is where these cables will be placed. This will then identify the wiring codes for your region. Then within these codes the environment the cables will be subjected to will narrow the choices even more. Then comes the parameters of signal bandwidth along with precision, resolution and noise isolation. Along the way somebody should consider fire prevention. Then there is the possible exotic concerns of lightning, EMP, RFI, electrocution, etc.

You could try contacting the transmitter and receiver manufacturers for some guidance.

Here's another hint, they might even have a web site with installation information.

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#4

Re: 4-20ma Instrument cable thickness calculation

10/19/2014 5:30 AM

The original idea behind 4-20ma scheme was to be totally insensitive to parameters like cable size. And being very interference tolerant shielding is not required, but since distance is long, forming cables as twisted pairs won't hurt. Just comply with local codes and you're OK. S.M.

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#5

Re: 4-20ma Instrument cable thickness calculation

10/19/2014 6:08 PM

The calculation involves how much resistance the loop's power supply can drive 22mA through ( 21+ mA is a common fault condition indicator that should be accounted for).

The total loop resistance has to be less than the maximum that the loop's power supply can drive.The loop resistance is the sum of the analog input resistance and the wire resistance.

Analog input resistance is many times 250 ohms but I've seen about a dozen different values from 1 ohm to 500 ohms. No guarantees, just a matter for research.

The voltage in a 2 wire loop powered scheme is typically a 24Vdc power supply. No guarantees, but it's typical.

The voltage for a 3 or 4 wire device is sometimes known, other times there's a spec for the total (maximum) loop resistance that the analog signal can tolerate, which is the value that is needed anyhow.

The 2 wire loop powered devices either have a formula or a chart known as a 'load line' that shows how much resistance a given power supply voltage will drive.

Most 2 wire transmitters will drive 500 ohms with a 24Vdc power supply.

Most 4 wire AC powered magmeter active analog outputs will drive at least 750-800 ohms.

All that said, for a typical 250 ohm load (analog input resistance), 240 meters (~800 feet) (double the distance for one wire out and one wire back) of CAT 5 ethernet cable which is 24g (AWG) has 0.188 ohms/meter resistance for a total resistance of 46 ohms, giving a total loop resistance of 286 ohms (250 + 46), well below the typical maximum 500 ohms that most 2 wire loops can work with and definitely below the 750 ohms that an active output from a magmeter can drive.

CAT 5 cable for Ethernet has a 100m limitation, but that limitation does NOT apply to 4-20mA which operates at a near DC frequency, not Ethernet 100MHz frequency.

If you don't use shielded, twisted pair (STP), you will regret it due to noise problems.

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#6
In reply to #5

Re: 4-20ma Instrument cable thickness calculation

10/19/2014 7:53 PM

Right on the money again Iris, GA.

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#7
In reply to #5

Re: 4-20ma Instrument cable thickness calculation

10/19/2014 8:56 PM

I agree that you are spot on the money here. However, this rudimentary calculus should be performed by somebody locally that can service the installation.

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#8

Re: 4-20ma Instrument Cable Thickness Calculation

10/19/2014 9:04 PM

The basic rule I learned for 4 - 20 ma signal transfer was that anything good grade of solid copper wire 22 ga or larger will work for a round trip distance of over 2 miles (~ 3 Km).

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#9

Re: 4-20ma Instrument Cable Thickness Calculation

10/21/2014 3:13 PM

Friend,

The following book has excelent information and example with calculus regarding your issue

Programmable Controllers

Thomas A. Hughes
regards
JP

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#10

Re: 4-20ma Instrument Cable Thickness Calculation

10/27/2014 2:05 AM

If it's smaller than 0.75mm2 then it's too small for an industrial installation on the grounds of mechanical resilience.

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#11
In reply to #10

Re: 4-20ma Instrument Cable Thickness Calculation

11/01/2014 9:30 PM

Dear All,

In our industry for instrumentation signal cable we use 1.5mm2 regardless of length.

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#12
In reply to #11

Re: 4-20ma Instrument Cable Thickness Calculation

11/01/2014 9:58 PM

How many miles wide is Beverly Hills?

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#13
In reply to #11

Re: 4-20ma Instrument Cable Thickness Calculation

11/04/2014 7:47 AM

Don't know your industry but usually 1.5 mm2 for 4-20 mA doesn't mack much sense, it's a waste of money and space and also in many cases you won't be able to connect correctly because cable glands will be too small and instruments terminals don't always accept 1.5 mm2. And how common are shielded n x 2x 1.5 mm2 twister pair cables anyway? I suppose you aso use 1.5 mm2 for PLC signals. Ever tried to connect 33 wires of 1,5 mm2 ea. in a S7-400 digital input connector?

0.75 mm2 is a good compromise for analog signals (general purpose uses).

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#14

Re: 4-20ma Instrument Cable Thickness Calculation

08/23/2024 10:12 AM

Calculation?

Mechanical strength is the usual selection criterion.

Let the end Client's standards prevail.

"Two bits of wet string" will carry 4-20mA!

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#15
In reply to #14

Re: 4-20ma Instrument Cable Thickness Calculation

08/23/2024 10:28 AM

Wet strings only work with impure strings. A little salt can help.

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#16
In reply to #15

Re: 4-20ma Instrument Cable Thickness Calculation

08/29/2024 4:24 AM

Quite.

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