I have done it the easy way -through computer modelling. The model used is valid, external to the conductors, for multiple conductors above a ground plane. The assumption in the program is that the charge of a conductor is at its center, which is excellent for the typical distances involved -the distance between the line charge location within the conductor to make it an equipotential surface, and the true center of the conductor is negligable. Corrections can be made but except for special cases such as cables where distances are short, there is no point in doing so. Input information is dimensional data (radii, height above ground plane, spacing, etc) and voltage (instantaneous or rms) of each conductor with respect to ground. There will be a reduction in surface E field and in a short distance from the conductor bundle,-say 2 or 3 bundle radii- the field will be near that of a single conductor of much large radius . Here is a simple case. Single conductor radius 2 cm at height 10m and voltage of 100kv surface field under conductor is 724.+ kV/m and at side it is 723+ kV Two conductors, same size and voltage, spaced 30cm apart field below =453kV/m, inside =418 kV/m and outside 478 kV/m I also see that the charge on each conductor is appreciably lower than that on the single conductor at the same voltage although the total charge is greater. The effective radius of the bundle is about 7.75cm in this case. The ground level field is increased slightly in the bundled case as the field is a bit more uniform. The line capacitance is increased by bundling and the inductance is decreased. The decrease in line inductance is of more importance, in most cases, than the increase in C. Note that in general capacitance is based on the conductor radius while, for inductance, the concept of GMR is considered and this takes into account internal flux linkages. For ACSR, the GMR is measured but it can be calculated different geometries in the absence of magnetic materials.