The main objective of this paper is to propose an algorithm for the determination of the allowable ampacities of single rectangular-section bus bars without the occurrence of correction factors. Without correction factors, the ampacity computation of the copper and aluminium bus bars is fully automatized. The analytical algorithm has been implemented in a computer program code that along with the allowable ampacity can compute the bus bar temperature and the individual heat transfer coefficient for each side of the bus bar, as well as their corresponding power losses. Natural and forced convection correlations for rectangular bus bars are applied. Effects of the solar radiation and radiation heat losses from the bus bar surface are taken into consideration as well. The finite element method (FEM) has been used for the linear/non-linear steady-state thermal analysis, i.e. for validation of the analytical algorithm. All FEM-based numerical computations were carried out using the COMSOL Heat Transfer Module. [Projekat Ministarstva nauke Republike Srbije, br. TR33046]