The loadability of short transmission lines (less than $80 \mathrm{~km}$, represented by including only series resistance and reactance) is determined by ; that of medium lines (less than $250 \mathrm{~km}$, represented by nominal $\pi$ circuit) is determined by __ ; and that of long lines (more than $250 \mathrm{~km}$, represented by equivalent $\pi$ circuit) is determined by ________.