For calculation of the cyclic and emergency current rating of cables acc. IEC 60853, the multi-core cable is replaced by an equivalent single-core construction dissipating the same total conductor losses. The equivalent single-core conductor has a diameter as calculated by the formula below where $D_i$ is the value of diameter over insulation and $T_1$ the thermal resistance for the multi-core cable.

The actual conductors are considered to be completely inside the diameter of the equivalent single-core conductor, the remainder of the equivalent conductor being occupied by insulation.

Symbol
$d_{\mathrm{ct}}$
Unit
mm
Formulae
 $d_{\mathrm{c}}$ single-core cables and multi-core cables with sector-shaped conductors $D_{\mathrm{i}} e^{- \frac{2 T_{\mathrm{1}} \pi}{n_{\mathrm{c}} \rho_{\mathrm{i}}}}$ three-core cables with round conductors $\sqrt{2} d_{\mathrm{c}}$ two-core cables with round conductors
Related
$d_{\mathrm{c}}$
$D_{\mathrm{i}}$
$n_{\mathrm{c}}$
$\pi$
$\rho_{\mathrm{i}}$
$T_{\mathrm{1}}$
Used in
$p_{\mathrm{i}}$
$t_{\mathrm{1t}}$
$T_{\mathrm{1t}}$