Cal. Pub. Util. Code § 30902

If any proposition is defeated by the electors, the board shall not call another election on a substantially similar proposition to be held within six months after the prior election. If a petition requesting submission of such a proposition, signed by 15 percent of the district electors, as shown by the votes cast for all candidates for governor at the last gubernatorial election, is filed with the board, it may call an election before the expiration of six months.