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The BICC Electric Cables Handbook give the formula for inductance as follows:
L=( K+0.2ln 2S d )× 10 −6
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