Ind. Code § 8-1-2-70
In its order upon any investigation made under the provisions of this chapter, IC 8-1.5-3, or IC 8-1.5-6, either upon complaint against any municipal utility, upon the petition of any such municipal utility, or upon the initiation of the commission, the commission shall ascertain and declare the expenses incurred by it upon such investigation, and the municipal utility affected thereby shall pay into the commission public utility fund account described in IC 8-1-6-2 the amount of the expenses, so ascertained and declared, within a time to be fixed in the order, not exceeding twenty (20) days from the date thereof. The commission shall cause a certified copy of all such orders to be delivered to an officer or agent of the municipal utility affected thereby, and all such orders shall, of their own force, take effect and become operative twenty (20) days after service thereof unless a different time be provided in said order. Any order of the commission as may increase any rate of such municipal utility shall not take effect until such expenses are paid into the commission public utility fund account described in IC 8-1-6-2.
IC 8-1-2-70