Transmission loadability includes the thermal and the protective constraints. In case of low voltage and heavy load， the relay loadability may replace the thermal loadability， leading to undesirable relay operation and cascading events. A sensitivity model between the transmission loadability and the field control of generators is newly proposed， thus to quantify the security margins under field control during voltage depression. The numerical results show that the generator overexcitation control depends on its voltage setting; the overload and the underimpedance constraints may underestimate or overestimate the thermal loadability under voltage depression; the relay loadability depends on not only the protection schemes， but also the relay settings.