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.

Symbol
$T_{\mathrm{C}}$
Unit
K.m/W
Formulae
$\frac{T_{\mathrm{4iii}} q_{\mathrm{4}}}{n_{\mathrm{c}}}$
Related
$n_{\mathrm{c}}$
Used in
$T_{\mathrm{B}}$