In this paper, a nonlinear theory applicable to the design of nanotube based devices is presented. The role of finite kinematics for a doubly clamped nanotube device is investigated. In particular, we analyze the continuous deformation and instability (pull in) of a clamped-clamped nanotube suspended over an electrode from which a potential differential is imposed. The transformation of an applied voltage into a nanomechanical deformation indeed represents a key step toward the design of innovative nanodevices. Likewise, accurate prediction of pull-in/pull-out voltages is highly needed. We show that an energy-based method can be conveniently used to predict the structural behavior and instability corresponding to the ON/OFF states of the device at the so-called pull-in voltage. The analysis reveals that finite kinematics effects can result in a significant increase of the pull-in voltage. This increase results from a ropelike behavior of the nanotube as a consequence of the stretching imposed by the actuation.