Cal. Pub. Util. Code § 17006

The proceeds from the sale of bonds shall be placed in the treasury to the credit of the proper fund, and shall be applied exclusively to the purposes and objects mentioned in the ordinance authorizing their issue until such objects are fully accomplished, after which, if any surplus remains, the surplus may be transferred to the general fund, except that a surplus exceeding five thousand dollars ($5,000) shall be wholly transferred to the appropriate fund to pay interest and maintain the sinking fund, or provide for the retirement of the bonded indebtedness in connection with which the surplus remains.