The formula is in any good power systems text (eg Stevenson), so it shouldn't be too hard to create your own function. Mathcad's 'gmean' looks like it would work too - each column would be the distance from one wire to every other wire in the conductor - 7 columns of 7 entries for a 7-strand wire. So wire number 0 is in col 0, wire number 1 is in column 1, etc. The distance of a wire to itself is the radius of the wire. (See the manual for a description of gmean, and compare with the general formula for GMR to see what goes where in the array).
But I haven't tried it - I usually find it easier to get GMR from the wire manufacturer's standard tables.
If the central wire is steel, it all falls down because of the magnetic differences between the ferromagnetic core and the diamagnetic aluminium outer conductors.
Bung
Life is non-linear...
