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Old December 2nd, 2019, 04:51 PM   #12
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danw is offline
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Join Date: Oct 2004
Location: midwest, USA
Posts: 3,095
What you've proposed just won't work.

1. (Echoing others who know what they're doing) Get rid of the panel 2 power supply (for anything to do with the transmitters).

2. Use shielded twisted pair cable.
- Run one conductor for each transmitter from panel 1's 24Vdc (+) power supply to transmitter (+)
- transmitter (-) back to panel 1 AI (+).
- Jumper all the AI (-) together and return them to the 24Vdc power supply 0V (-) return.

3. I'm not sure where 'Ethernet' came from in the discussion, but if you were planning on using 24G CAT 5/6 Ethernet cable with its 24G conductors, its resistance is 26 ohms per 1,000 feet, so its resistance is not a disqualifying factor for 4-20mA at 800 feet round trip.

I've seen a number of places where they've used CAT 5 cable for 4-20mA and it works, but the issue is how non-robust the terminating connections are. 24G wire just doesn't work well under screw terminals. Any vibration and it breaks. There are RJ-45-to-screw terminal adapters that I'd use if I had to.

But you should know that the industry convention for 4-20mA is 18 or 20 gauge shielded twisted pair, not because the loop resistance is less, but because the cable/conductors are more robust.
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