The ratio of thermal resistance to ambient for group of cables in air is quickly converging equation is solved by iteration, starting with a value of $h_{lg}$.

Symbol
$h_{\mathrm{T4}}$
Unit
-
Formulae
$h_{\mathrm{lg}} \left(k_{\mathrm{l}} + \frac{1}{h_{\mathrm{T4}}} \left(- k_{\mathrm{l}} + 1\right)\right)^{0.25}$
Related
$h_{\mathrm{lg}}$
$k_{\mathrm{l}}$
Used in
$T_{\mathrm{4iii}}$