Answer / carl_ellis

It depends on how much voltage the field transmitter needs to operate and the total loop resistance of the loop circuit .

Every 2 wire loop powered transmitter has a specification for the voltage required to drive a given loop resistance. Sometimes that is a formula, sometimes a load line graph, sometimes a statement like "minimum liftoff voltage is 17.7V", or rarely, a vague statement of maximum loop resistance (common with active analog outputs, rare with loop powered transmitters)

Excessive wire resistance in a 2 wire loop powered transmitter circuit can affect the operation by
- preventing the transmitter from turning on and operating
- allowing partial operation at lower current outputs, but failing to operate at higher current outputs. That failure can be either an output that just never exceeds a certain value or a transmitter that cycles on-off-on-off an never initializes.

If there is sufficient voltage to run drive the total loop resistance, then the transmitter could function normally.

You need to calculate the wire resistance over its total wire run length (out and back) and know what the receiver's analog input resistance is. Then look up the specification for how much resistance the transmitter's can drive and see whether your loop resistance is lower than the maximum.

Carl Ellis
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