California Code, Government Code - GOV § 20815

(a) Notwithstanding any other provision of this part, including, but not limited to, Sections 20225 and 20790, the board shall not combine the assets and liabilities of public agency employers into a single account for the purpose of setting a uniform rate of employer contributions for all public agency employers. The rate at which a public employer's contribution to this system shall be fixed shall be based upon its own experience. Provisions of law that provide authority for this system to combine the assets and liabilities of public employers into a single account for purposes of establishing a uniform rate are superseded to the extent that they provide that authority. For purposes of this section only, references to public employers shall not be construed to include school employers.

(b) Notwithstanding subdivision (a), the assets and liabilities of a county and a trial court jointly contracting with the board under Section 20460.1 shall be combined for purposes of setting the employer contribution rate for both the county and the trial court.