Additional external thermal resistance caused by heating from other cables in a group.

Where $T_4$ is the external thermal resistance of the hottest cable of the group if isolated, $T_4 + ΔT_4$ is the external thermal resistance of the hottest cable of the group taking into account the losses, assumed to be equal, in all cables of the group, and the conductor losses $W_c$ and cable joule losses $W_I$ per unit length of cable for the hottest cable of the group correspond to all cables working at a sustained 100% current.

Note: The thermal resistance to ambient $T_{4iii}$ already contains this additional external thermal resistance: $T_{4iii} = T_4 + ΔT_4$.

$\Delta T_{\mathrm{4}}$

K./W

$\frac{\rho_{\mathrm{4}}}{2 \pi} \ln{\left (F_{\mathrm{eq}} \right )}$

$\pi$

$\rho_{\mathrm{4}}$

Thermal resistivity of soil [K.m/W]