California Code, Public Utilities Code - PUC § 16643

The board shall annually levy and collect a tax sufficient (a) to pay the annual interest on bonds, (b) to pay such part of the principal as will fall due within the succeeding year, and (c) to provide for the sinking fund payments of the next succeeding fiscal year.

When the interest and sinking fund payments for any fiscal year on bonds issued for a public utility or public utility works can be met out of the surplus earnings of such public utility, or out of moneys in the general fund of the district theretofore appropriated and transferred to the sinking fund of the public utility, no tax shall be levied for such purpose.