Is effective diameter the outer overall diameter of a 7-strand earth wire, (about 2.1 mm), or the copper-only diameter based on 7 x 1 mm strands. (2.645751 mm) ? (Refer to answer on multi-strand wire inductance).
Diameter Factor is the ratio of an outer diameter to the diameter of each strand.
It assumes a perfect circle and no insulation. I looked up the value of D.F. and found it was 3.0 for 6 to 7 strands.
"Effective Diameter" must be the equivalent diameter of a single solid conductor.