Transients, such as lightning, switching, or faults with a broad frequency spectrum, can lead to overvoltage in electrical equipment and cause insulation failure. To avoid such problems, it is required to design insulation properly that necessitates an exact wide-band frequency model of electrical equipment. Accurate calculation of ac resistance has an essential impact on modeling loss (or damping) in transient simulations. This thesis aims at providing an understanding of resistive loss calculation and simulation of rectangular conductors used in the power system industry. For this purpose, the skin effect of a single conductor is studied. A quasi-analytical approach is introduced for ac resistance calculation of isolated rectangular conductors based on a two-dimensional analytical approach and finite element simulations. When it comes to a system of conductors close to each other, the proximity effect needs to be considered, in addition to the skin effect. The proposed quasi-analytical approach is only valid for single conductors and cannot be used for proximity effect calculations. Due to the skin and proximity effects, very fine mesh size is required at higher frequencies, leading to computational complexities. Therefore, a new computational approach, known as the proper generalized decomposition (PGD), is employed for the skin and proximity effect calculations of rectangular conductors. The PGD decomposes a higher-order system, such as a 2D system, to lower-order ones, such as 1D vectors. Vectors need less computational resources than higher-order matrices. Moreover, an adaptive frequency-dependent meshing technique is added to the PGD approach to solve skin and proximity effect problems. This adaptive meshing technique along with the PGD approach leads to accurate and fast computation. Some applications are electromagnetic transient analysis of electrical equipment such as cables and windings, interpreting frequency response analysis of machines windings, partial discharge localization inside large windings, and high-frequency coil modeling used for wireless charging of electric vehicles.