BICC Electrical Cables Handbook

The BICC Electric Cables Handbook give the formula for inductance as follows:

$L=\left(K+0.2\ln \frac{2S}{d}\right)\times {10}^{-6}$

Where:
L - cable inductance, H.m^-1
K - conductor formation constant
S - axial spacing between conductors within a cable, mm
- axial spacing between conductors in trefoil, mm
- 1.26 x phase spacing of flat formation conductors, mm
d - conductor diameters, mm

For 2, 3 or 4-core circular or sector-shaped cables, multiply the above by 1.02 to obtain the inductance.  Multiply by 0.97 for 3-core oval conductors.

Typical values of K