Busbar Design Considerations
TABLE 3. EFFECT OF EMISSIVITY* AND NUMBER OF BUSSES ON AMPACITY
|Number of 1/4 x 4" Busses*||Ampacity, Amp|
|30 °C Rise||50 °C Rise||65 °C Rise|
*Emissivity is the ratio of radiation emitted by a blackbody or a surface and the theoretical radiation predicted by Planck’s law. Blackbody emissivity is frequently referred to as a single number. To be more scientifically correct one needs to be more explicit.
A material's surface emissivity is a measure of the energy emitted when a surface is directly viewed. Surface emissivity is generally measured indirectly by assuming that e = 1 - reflectivity. A single energy bounce is measured and the reflected energy measured.
A mirrored surface may reflect 98% of the energy, while absorbing 2% of the energy. A good blackbody surface will reverse the ratio, absorbing 98% of the energy and reflecting only 2%.
Effective emissivity is the ratio of the total amount of energy exiting a blackbody to that which is predicted by Planck’s law. This is the value most frequently referred to as "emissivity". Effective emissivity of a cavity type blackbody will normally be much higher than the surface emissivity due to the multiple energy bounces inside the body cavity.
Additional refinements to the term "emissivity" may be made by defining it in terms of the wavelength of interest, changes due to temperature affects, etc. The simple concept of emissivity can very quickly become a very complex topic!