Abstract In practical projects，while an asynchronous motor starting，the drop of voltage on the bus bar must be within the lim its which is stipulated by the national standard． By establishing a equivalent circuit and mathem atics deduction，a form ula，by which we can calculate the largest rated starting capacity that permitted，was obtained．A nd on the basis of the result，the largest rated power of a motor that perm itted on a variety of conditions can be determined．M eanwhile，several factors that change the final result were discussed，as well as the error due to the approximation．
