The process leading to the quench of superconductivity in a nonideal type II superconducting wire carrying an audio-frequency alternating transport current in a normally applied static magnetic field was studied. The quench of superconductivity was found to initiate from particular points of 10-mil diameter Nb25 percent Zr wire in repeated tests. In the course of the experimentation a nucleation site was closely bracketed by potential probes and the voltage response of this segment at currents less than the quench current was studied. Utilizing a limited current density model to describe the ac conduction process, the previous voltage response measurements were used to calculate values for the model parameter Jc. The model was extended to include heating effects and a regenerative process that leads to a quench of superconductivity. Quench current predictions based on voltage measurements prior to the quench at a single frequency agree favorably with measured quench currents over a relatively wide frequency range.