Element C is only relevant for cables in air. The factor $T_{4iii}$ is the external thermal resistance due to air in the steady-state, calculated according to IEC 60287, expressed as a quantity per conductor or equivalent conductor.

$T_{\mathrm{C}}$

K.m/W

$\frac{T_{\mathrm{4iii}} q_{\mathrm{4}}}{n_{\mathrm{c}}}$

$q_{\mathrm{4}}$

$T_{\mathrm{4iii}}$