The factor for two-core cables with round conductors is equal to 2.
The factor for three-core cables with round conductors is equal to $2.1547=1+2/\sqrt 3=1/(2 \sqrt 3 -3)$.
$1$ | Single-core cables, round conductors & Multi-core cables, sector-shaped conductors |
$1+\frac{1}{sin\left(\frac{\pi}{2}\right)}$ | Two-core cables, round conductors |
$1+\frac{1}{sin\left(\frac{\pi}{3}\right)}$ | Three-core cables, round conductors |
$1+\frac{1}{sin\left(\frac{\pi}{4}\right)}$ | Four-core cables, round conductors |
$1+\frac{1}{sin\left(\frac{\pi}{5}\right)}$ | Five-core cables, round conductors |