Cal. Pub. Util. Code § 99564.3

A petition may be filed with the board, in accordance with its rules and regulations, requesting it to investigate and decide the question of whether the employees wish to decertify an exclusive representative or to reconsider the appropriateness of a unit. The petition may allege that the employees in an appropriate unit no longer desire a particular employee organization as their exclusive representative. The petition shall include proof of a 30-percent showing of interest indicating support for another organization or lack of support for the incumbent exclusive representative.