Thermal radiation is the transfer of heat by electromagnetic radiation. In many cable installations it is not the dominant heat-transfer mechanism, but it can become relevant for bare conductors, busbars, exposed surfaces and enclosures.
This article expands the radiation part of Cable Thermal Analysis. For air-side heat transfer, see the companion article on Convection in Cable Thermal Analysis.
Thermal radiation from surfaces
For a radiating surface, such as a bare conductor or busbar surface in an enclosure, heat transfer can be estimated using:
In this expression, emissivity and area are properties of the radiating surface, and Ta is the enclosure or surrounding temperature. Temperatures used in radiation calculations should be absolute temperatures in Kelvin.
Busbars and enclosures
Radiation heat flow between two large facing surfaces, such as a busbar surface and an enclosure wall, can be calculated from:
Here q is the heat flow between surfaces of area A, ε1 and ε2 are the emissivities of the two surfaces, and T1 and T2 are the surface temperatures in Kelvin.
The radiation effect can be included in a thermal model as a suitable boundary condition. Another practical approach is to convert the radiation heat flow into an equivalent heat removal term for the busbar volume and subtract it from the generated heat.
Symbols
| A | Area, m2 |
| T | Surface temperature, K |
| T1 | Hotter surface temperature, K |
| T2 | Enclosure or second surface temperature, K |
| Ta | Ambient or enclosure temperature, K |
| q | Heat flow, W |
| Heat generated per unit volume, W/m3 | |
| ε | Emissivity of a surface |
| σ | Stefan-Boltzmann constant, W/m2.K4 |
For solar heating of cables installed outdoors, see Solar Radiation Effects on Cable Current Rating.