Radiation is the transfer of heat by electromagnetic radiation. 

Luckily in cable installations, we rarely encounter the need to consider radiation.  One exception is that of bare conductors in an enclosure, where the effect of radiation can be taken into account by:

$q=\epsilon \sigma A\left(T^4-{T_a}^4\right)$

with ε and A properties of the busbar, and T_a the temperature of the enclosure

Busbar & Enclosures

Radiation heat flow between two plates (busbar surface to enclosure for example), can be calculated from:

$q=\frac{\delta A\left({T_1}^4-{T_2}^2\right)}{{\displaystyle \frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_2}-1}}$

q is the heat flow in W between plates of area A
ε₁, ε₂ are the emissivities of the two surfaces
T₁, T₂ are in K (not °C)

The effect of radiation is included in any solution as a suitable boundary condition.  An alternative approach would be to adjust the heat flow for the volume of the busbar and subtract this directly from the heat generated within the busbar.

Symbols

A         - area, m²
T₁       -  temperature of surface (hottest), K
T₂       -  temperature of enclosure

q         - heat flow, W (or Joules/second)
$\dot{q}$        - heat generated per unit volume, W/m³

ε         - emissivity of a surface