Cal. Pub. Util. Code § 11851

In any district in which the number of directors is required to be increased from five to seven as provided in Section 11850, the board of directors in office on January 1, 1974, shall, by resolution or ordinance adopted within 90 days after such date, divide the territory of the district into seven wards, and fix the boundaries thereof. The boundaries shall be fixed such that the wards are as equal in number of registered voters as may be.