Can I Run 14 AWG Control Wire 400 Feet From a Pump to a PLC?
Yes, with the usual caveat that voltage drop, not the Code, is what decides the wire size. A 400 foot run of 14 AWG copper carrying a half amp 24 volt DC signal drops about 1.3 volts, which is roughly 5 percent and is workable for most PLC inputs. Push the current higher, or use a device with a narrow supply window, and 12 AWG becomes the safer choice.
Running the voltage drop
NEC 2023 Chapter 9, Table 8 gives DC resistance for uncoated copper at 75 degrees C. Stranded 14 AWG is 3.14 ohms per 1000 feet. Stranded 12 AWG is 1.98. A 400 foot run is 800 feet of conductor round trip, so 14 AWG gives 0.800 times 3.14, or 2.51 ohms, and 12 AWG gives 0.800 times 1.98, or 1.58 ohms.
At 0.5 amperes the 14 AWG drop is 1.26 volts, or 5.2 percent of 24 volts. The 12 AWG drop is 0.79 volts, or 3.3 percent. At 1 ampere, which is realistic for a relay coil plus a pilot light, 14 AWG drops 2.51 volts and you are at 10.5 percent, which is where relays start chattering and where PLC input thresholds get marginal. Section 210.19 and 215.2 carry informational notes recommending 3 percent on a branch circuit and 5 percent total, and while informational notes are not enforceable, they are the right target for control wiring.
If the feedback signals are 4 to 20 milliamp analog loops, voltage drop stops being the problem. A 20 milliamp loop through 2.51 ohms drops 0.05 volts. What matters there is loop compliance voltage: the transmitter supply must cover the sensor, the wire resistance, and the receiving resistor. Any 24 volt supply handles 800 feet of 14 AWG without noticing.
The Code side of a Class 1 control run
In the 2023 NEC, Class 1 circuits moved to Article 724. Section 724.43 sets overcurrent protection for Class 1 control conductors at not more than 7 amperes for 18 AWG and 10 amperes for 16 AWG, with larger sizes following 240.4. Section 724.49 permits 18 AWG and 16 AWG conductors for Class 1 circuits where they supply loads that do not exceed the ampacities in 724.49(A), and conductors 14 AWG and larger follow 310.14. Section 724.48 permits Class 1 circuits from different sources in the same raceway, cable, or enclosure where all conductors are insulated for the maximum voltage of any conductor present.
That last point is the one that matters for a pump panel: the 24 volt DC feedback, the 120 volt AC control, and the alarm wiring may share a raceway as long as everything is rated for the highest voltage present, which in practice means 600 volt rated conductors throughout. Section 300.3(C)(1) says the same thing for power and control conductors in general.
Two practical additions for a 400 foot run. Use shielded twisted pair for analog signals and ground the shield at one end only, at the PLC panel, to avoid a ground loop. And keep the control conductors out of the same raceway as the pump motor leads where a variable frequency drive is involved, since drive output carries fast switching transients that couple into adjacent conductors regardless of what the Code permits.