Public Utilities Code § 13152

Each issue of bonds issued pursuant to this chapter shall conform to the following requirements:

(1) Such bonds shall be serial bonds or sinking fund bonds, or a combination of serial and sinking fund bonds.

(2) Provision shall be made for the retirement of such bonds through annual payments on principal, and such payments shall begin not more than 10 years, and end not more than 20 years, after the date of such bonds.