8 mm (in copper, which is rarely used for powerlines, if at all, it is super expensive and heavy) is huge for a single conductor, and your typical overhead powerline is concentric shells of tens of conductors. Negligible: has no practical effect on the construction. It's in the 4th significant decimal or so for a typical powerline segment, dwarfed by plain resistive losses.

You want those multiple conductor arrangements anyway to reduce the corona discharge.

If you go up to multiple KHz then it will become a problem.