Cal. Code Regs. tit. 22, § 40108
A rural area is: (a) a County Census Division or aggregate of County Census Divisions with a density of less than 250 persons per square mile according to the latest decennial census conducted by the U. S. Bureau of Census, and in which the principal city has a population of no more than 20,000 persons according to said census, or;
1. Amendment of subsection (b) filed 12-9-77; effective thirtieth day thereafter (Register 77, No. 50).