Cal. Pub. Util. Code § 13172

All sums received as principal and premiums on the sale of any issue of bonds shall be applied to the purposes for which such bonds were issued. The board may provide in the resolution authorizing the issue of any bonds under this chapter that any portion of the proceeds of sale of the bonds may be applied to payment of interest during construction for a period of not to exceed three years from and after the date of such bonds.