The formulae are accurate providing xp does not exceed 2,8, and therefore applies to the majority of practical cases.

Symbol
$y_{\mathrm{p}}$
Unit
-
Formulae
$0$one single-core cable
$\frac{2.9 d_{\mathrm{c}}^{2} x_{\mathrm{p}}^{4}}{s_{\mathrm{c}}^{2} \left(0.8 x_{\mathrm{p}}^{4} + 192\right)}$two-core cables and two single-core cables
$\frac{2.9 d_{\mathrm{x}}^{2} x_{\mathrm{p}}^{4}}{s_{\mathrm{x}}^{2} \left(0.8 x_{\mathrm{p}}^{4} + 192\right)}$two-core cables with sector-shaped conductors
$\frac{d_{\mathrm{c}}^{2} x_{\mathrm{p}}^{4}}{s_{\mathrm{c}}^{2} \left(0.8 x_{\mathrm{p}}^{4} + 192\right)} \left(\frac{0.312 d_{\mathrm{c}}^{2}}{s_{\mathrm{c}}^{2}} + \frac{1.18}{\frac{x_{\mathrm{p}}^{4}}{0.8 x_{\mathrm{p}}^{4} + 192} + 0.27}\right)$three-core cables and three single-core cables
$\frac{0.666666666666667 d_{\mathrm{x}}^{2} x_{\mathrm{p}}^{4}}{s_{\mathrm{x}}^{2} \left(0.8 x_{\mathrm{p}}^{4} + 192\right)} \left(\frac{0.312 d_{\mathrm{x}}^{2}}{s_{\mathrm{x}}^{2}} + \frac{1.18}{\frac{x_{\mathrm{p}}^{4}}{0.8 x_{\mathrm{p}}^{4} + 192} + 0.27}\right)$three-core cables with sector-shaped conductors
Related
$d_{\mathrm{c}}$