effective diameter vs overall diameter of multi-strand wire?
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.


  However for much larger numbers of strands elliptical wire shape results in fewer strands for a fixed D.F.

"Effective Diameter" must be the equivalent diameter of a single solid conductor.