Process Signal Wire Size

[GaDawg]

Can anyone suggest a recognized standard (ISA, IEEE,...) that provides information on sizing 4-20mA process control wire? I am specifically looking for a recommended maximum voltage drop for the wire using a 24VDC, two conductor circuit.

Regards,
GaDawg

 

[dpc]

Not aware of any specific guidance on this. I have run 4-20 mA signals some surprisingly long distances (miles), but I did run #10 AWG. You just need to make sure there is adequate voltage at the transmitter.

 

[JLSeagull]

The NEC includes resistance tables. If you calculate voltage drop you will find that the typical conductor sizes drop very little. Intrinsic safety barriers drop lots.

You may find short distances within a DCS cabinet between 18-22 AWG. Home run cables from the marshaling cabinet to junctin boxes may be 16 AWG. Individual instrument runs to the JB may also be around 16 AWG.

Several ISA standards mention conductor gauges for various applications. Examples include 12.06.01 for IS wiring, 50.02 part 2 for Foundation Fieldbus, etc.

 

[xnuke]

ANSI/ISA-50.00.01-1975 (R2002) - Compatibility of Analog Signals for Electronic Industrial Process Instruments has what you're looking for in it.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GaDawg]

Thank you all very much...this has been helpful!
GaDawg

 

[JimCasey]

Also, consult the catalog info for the device you are using. Usually they need 12 volts to work. If you power the loop with 24V, that leaves you with 12 volts to lose through resistance in the wire and the device that reads the signal.
Through the courtesy of Ohm's law, R=v/i=12/0.020=600 total ohms. The receiver device typically has about 125 ohms impedance. That leaves 475 ohms. So you can select whatever wire you want that will give you less than 475 ohms in the distance you need to run it. It takes a long run of wire to make that much resistance. Most instrument loops use vast overkill in selecting wire that will carry 20 amps instead of 20 milliamps.

 

[dpc]

Wire size in instrumentation is based on more than current-carrying capacity. Very small gauge wire is more susceptible to damage during installation via excess pulling tension and other risks.

For single twisted pair cable, we generally specify a minimum size of #16 AWG. For multi-pair cable, we specify #18 AWG.

 

[JLSeagull]

The 24 Vdc source is normally current protected at a level substantially higher than 20 mA. Each DCS and SIS is different. The Fisher ProVox bulk supplies were connected to the distribution bus by a 45A fuse. The optional power distribution panel may have also provided current protection, perhaps 15 Amps. The DCS I/O modules would limit the signals to something above 20 mA on the return side but the 24 Vdc conductors should be rated for short circuit protection that could occur between the source and ground or within the I/O. This would not be an issue for intrinsically safe signals.