The analysis and calculation of insertion loss in multistage next generation photonic switches is presented in this paper for the first time. The theory is based on using the so-called connection diameter of the photonic switch for evaluation of loss. Furthermore, the expression for calculating the diameter of Close scheme has been obtain for the first time. The comparison of insertion losses in multistage photonic switches such as Close, Banyan, dual schemes and crossbar switch is presented. The evaluation of crosstalk in the photonic switches on example of dual switches based on physical principles of their functioning is considered.