Any county in the state of Alabama may from time to time authorize, sell and issue refunding warrants for the purpose of refunding any outstanding warrants theretofore issued under the provisions of this chapter (whether or not the principal of the outstanding warrants to be refunded shall have matured at the time of the issuance of the refunding warrants). Refunding warrants may not, however, be issued in a principal amount that will result in the principal proceeds to be derived by the issuing county therefrom exceeding the sum of (a) the outstanding principal of the warrants being refunded, (b) the interest thereon accrued to the date of issuance of such refunding warrants, (c) the interest to accrue on such outstanding warrants to their respective maturities or to the earliest date on which they may, under their terms, be redeemed (whichever is earlier), (d) any premium necessary to redeem or retire the outstanding warrants being refunded, and (e) the expenses estimated to be incurred by the issuing county in connection with the sale and issuance of such refunding warrants. Warrants may also be issued under the provisions of this chapter for the combined purpose of so refunding any such outstanding warrants and financing the costs of erecting necessary public buildings, bridges and roads, in which event those provisions of this section relating to refunding warrants shall apply to that portion of such combined issue being issued for refunding purposes. All the provisions of this chapter relating to warrants issued to finance the costs of erecting necessary public buildings, bridges or roads shall be applicable to warrants issued solely or partly for refunding purposes.