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As on Earth, long distance transmission systems often do not employ insulation, instead for hundreds of kilometers they string long bare cables suspended in the air without any insulation, except for porcelain insulators at the pylon attach points. Exactly the same approach could be used on the Moon.
Underground: Lunar soil is somewhat conducting because of the high proportion of native iron, therefore we cannot bury bare wire into the soil. Instead there will have to be insulating conduits. Such conduits could be made from ceramics, or silicon polymers. There is unlimited silicon on the Moon.
For power wires inside habitable areas, it is important to prevent electric shock, or electrical shorts which would drain power or short circuit power supplies, and cause fires. So we still need insulation. One solution would be to import pre-insulated wires from Earth, but that would be expensive primarily due to the high weight of the metal wire conductors. Another approach might be to import empty insulators sleeves, or sleeving material, then via a process such as heat-shrinking or extrusion, attach the Earth-made insulation to lunar-made wires.
It might be possible to develop suitable flexible insulator materials by means of silicon polymer chemistry.