I am working on a double leaf bascule bridge and installing a PLC I/O rack on the farside. The power transmission between the two piers (nearside and farside) is via a single submarine cable (installed 1987). Unfortunately, the submarine cable is made up of #10 AWG xhhw-2 control wires and no ethernet cables. Would it be possible to use 8 individual #10 AWG wires to make up the "4-pair" cable required for PLC communication? I would like to avoid wireless communication, and modifying existing/installing a new submarine cable is not an option.
This almost certainly won't work.
What makes twisted pair work for high speed data transmission is not just the diameter of the conductor, but also that the two wires in each pair are kept a constant distance apart along the length of the cable.
Wires in Cat-5 pairs are also twisted together, but this is done more to reduce cross-talk and interference (because the interference in one part of the twist will cancel the interference in another part of the twist) than to enable high-speed transmission in the first place.
So if your wires are not kept at a constant, controlled separation center-to-center, they won't provide a good path for high-frequency signalling. And if they aren't twisted together in pairs, they will be subject to cross-talk and interference.
As far as I can see, the "XHHW-2" spec is about the chemical and water-resistance of the insulation, and will have very little to do with how well these wires can carry high-frequency signals. The dielectric constant of the insulation will affect the characteristic impedance of the wire pair, but this effect could be compensated for with a matching circuit at each end of the cable if necessary.