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.

$d_{\mathrm{ct}}$

mm

$d_{\mathrm{c}}$ | 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 |

$D_{\mathrm{i}}$

$n_{\mathrm{c}}$

$\pi$

$\rho_{\mathrm{i}}$

$T_{\mathrm{1}}$

$t_{\mathrm{1t}}$

$T_{\mathrm{1t}}$