If you are networking two or more PLC's together using Ethernet (i.e TCP/IP, UDP) use a good quality Plenum rated CAT-5E, UTP, 24 guage. BERK-TEK makes a good cable, but any manufacturers cable will suffice (as long as it is CAT-5E UTP). Cabling specifications state that the cable is fully rated to 100Mbit out to 100M (330 ft).
If you require longer distances, go with multi-mode fiber. MM fiber is good up to 2Km, depending on the ehrtnrt to fiber interface you choose.