Gauge Size 1

[PPC800]

I would like to use a 28 awg shielded cable over a distance of 50 meters. Will it work? Or is it better to use 22 or 24 awg.

This is for trasmission of digital and 4-20mA analog signals.

 

[sreid]

28 gage has a resistance of about 64 milliohm per ft so a 350 ft round trip woud be about 22 Ohms and drop about 1/2 volt at 20 ma. That should be OK.

 

[PPC800]

I did my own calc and for 50meters, I got a 0.21 V drop.

I am not quiet sure what a 0.21 volt drop means in this context?

You said in your example that 1/2 volt drop would be OK for your example, what threshold are considering when stating that?

Thanks,

 

[IRstuff]

Don't you know what the compliance voltage on your system is?

TTFN

FAQ731-376

 

[PPC800]

compliance voltage... do you even know what you are talking about?

 

[itsmoked]

Yes I can assure you he does know.

Save the attitude ask questions instead.

Doing your resistance calc did you include both directions?

Compliance is the standard term for voltage over the minimum needed in a 4-20mA signal chain.

Since it is a CURRENT loop voltage just tags along and makes everything work - its value doesn't matter precisely.

You have a power supply that puts out some voltage to run your loop. Say 24VDC

Then you have a sensor that needs to see the 4-20mA signal. It will consume/demand some of the voltage lets say 10V now you have 14V left or available for other voltage drops in the loop. Like your wire size voltage drop. If that wire were much longer and added up to 14V you would be out of compliance voltage and the circuit would not work.

IRstuff is just surmising that 1/2V is not likely to knock most systems out of compliance.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

We need a smiley eating popcorn... these types of threads always seem to have an amusing element to them... ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[PPC800]

lol

 

[PPC800]

Compliance voltage=Vmax from constant-current power supply

 

[itsmoked]

That's incorrect.

not constant current

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Most of the components of a 4-20 ma system have a voltage drop rating at 20 ma. The sum of the device voltages must be less than the power supply voltage.
Anecdote. We had a loop with a theoretical device load of about 22 volts at 20 ma. The input resistor on a replacement alarm relay was off spec. As a result, the 24V power supply could not develop 20 ma. across the loop. We became aware of the problem when the shutdown alarm failed to function in an over temperature event. The stainless steel thermo well in a 100' stack was melted off. Fortunately the refractory material held up. One crack in the refractory and we may have lost the stack.
Yes, voltage ratings of the devices in a loop are important, as is a field check of device calibration and function, rather than a quick and easy shop calibration.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

The manufacturers of 4-20 mA devices normally give you a formula for calculating the maximum loop resistance based on the voltage across the transmitter at 20 mA
e.g. Rosemount 3051 Pressure Transmitter
Max Loop Res. = 43.5 (P.S voltage - 10.5)
So at 24 VDC you are allowed 587 Ohms
Assuming your reciever is 250 Ohms then your cable can be 337.
So 28 AWG would be good for a very long way.
You can also boost the PS voltage up to allow even greater distances.
Hope this Helps
Old Instrument Guy