The capacity expansion planning (CEP) problem requires an adequate representation of the physical and electrical constraints faced by Independent System Operators (ISO) in practice. Nowadays, researchers employ simplified linear DC-based models to solve the transmission power flow problem. The main challenges for current planning methodologies involve the application of appropriate models to accurately represent the electrical network and incorporate all technical operational variables (voltage and reactive power flow magnitudes). Therefore, this study introduces a detailed framework to model generation and transmission capacity expansion planning considering the alternating current (AC) power flow problem, so that power system engineers can ensure secure and safe operational conditions for long-term scenarios. The study reports the results and effects of nonlinear modeling, contrasting the accomplished model with the traditional DC-based planning approach. Finally, extensive computational experiments are conducted to assess the performance of the capacity planning problem. The solver simulation time and the number of variables and constraints are also included to demonstrate the mathematical complexity.