Understanding voltage drop in control cables
If all it will ever carry is a control signal, then consider the resistivity of the wire. Also find the manual for the PLC and determine it's 120v input impedance; it should be in the specifications. Plug these into the following.

Wire resistivity:

\$\frac{4.016Ω}{1000ft} \cdot 300ft = 1.2Ω\$. 

If the PLC's 120v input presents a load of 10kΩ, then use the voltage divider rule: 

\$120v\cdot \frac{10k\Omega}{1.2\Omega + 10k\Omega} = 119.98v\$ 

That said, I'd shy away from using very small wire for long control signals, even though it is much less expensive. Typically, traditional hook-up wire is used (like 18 or 16 gauge), as to be robust and tough in service. Thinner wire usually has thinner insulation, and thus a lower voltage rating. If anywhere near 480v, then use 600v-rated wire. It has more (and better) insulation.