The thesis presents theoretical background for calculation of the electromagnetic field, losses and heat transfer in 3-phase metal-enclosed busbars at rated current. On the example of a segregated generator phase bus, the distribution of electromagnetic field and losses was computed on a 2D model using the finite element method. The losses in conductors and enclosure calculated in this way were the basis for analytical and numerical calculations of temperature rise. Thermal network method was used for analytical calculation of steady-state temperature rises in conductors, the enclosure and the air in the enclosure. Temperature distribution on a 2D model was computed using the finite element method, taking into account all the three modes of heat transfer: heat conduction, convective and radiative heat transfer. In these computations it was assumed that the losses had uniform distribution along the cross sections of conductors and the enclosure. To take into account the convective heat transfer in air in the enclosure, the heat equation and the fluid dynamics equations were coupled and solved simultaneously. Convective heat transfer from outer surface of the enclosure was described using empirical relations. The results of analytical and numerical calculations are compared with measurement results obtained on a test model. Due to large number of simplifications, the results of analytical calculation are in poor agreement with measurement results. On the other hand, the results of numerical calculation are in very good agreement with measurement results. It is concluded that the presented numerical procedure can be applied to the design of 3-phase metal enclosed busbars.