Refunding debentures may be issued in a principal amount sufficient to provide funds for the payment of all debentures or obligations to be refunded thereby and in addition to the payment of all expenses incident to the calling, retiring or paying of outstanding debentures or obligations and the issuance of refunding debentures. Those expenses include any amount necessary to be made available for the payment of interest upon refunding debentures from the date of sale thereof to the date of payment of the debentures or obligations to be refunded, or to the date upon which the debentures or obligations to be refunded will be paid pursuant to the call thereof or agreement with the holders thereof, and the premium, if any, necessary to be paid in order to call or retire the outstanding debentures or obligations and the interest accruing thereon to the date of the call or retirement.