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0-20mA and 4-20mA signals vs wire length

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  • 0-20mA and 4-20mA signals vs wire length

    Does anyone know maximum distances you can run these type of signal wires? I'm using an analog card for a DL06 and the signal is about 100 feet away. Thanks.

  • #2
    The thing you need to look at is total resistance. The 4-20 transmitter will be able to drive up to X ohms. Each reciever will have Y ohms of resistance. The wire will have Z ohms / foot.

    For example...

    The max resistance (if memory serves) the 4-20 output cards in the 205 series can drive is 900 ohm.

    You'll need to add up the resistance of the devices you're feeding the 4-20 signal into (normally 250 ohm) and then add in the resistance of the wire you're useing for the connections (usually stated in the wire specs as resistance per 1000 foot).

    [This message has been edited by marksji (edited 11-12-2003).]


    • #3
      So in other words, if one were to use 18 AWG wire (approx 7 ohms per 1000'), he could most likely run a very very long distance of wire. Thank you for your help.


      • #4
        A similar question came up on this forum:

        "A 4-20 mA signal will read correctly up to the point you exceed the total load limit of the transmitter. This is usually 500 to 1,000 Ohms. Since a typical PLC input or indicator resistance is 250 Ohms, you can run a lot of wire. Back to basics - Ohms law says V = I x R. The resistance, in Ohms per thousand feet at 149 degrees F, of various sizes of wire is:

        AWG = Ohms / 1,000 feet
        22 = 19.0
        20 = 11.9
        18 = 7.51
        16 = 4.73
        14 = 2.97
        12 = 1.87

        If you have 1,200 feet of 18 AWG wire for example, your extra load is 9 Ohms, and the additional voltage drop with is 0.18 Volts. I routinely run 4-20 mA signals for this kind of distance without problem."

        [This message has been edited by Tom Jenkins (edited 11-13-2003).]


        • #5
          Now that's a pretty useful chart, thanks!