Modification or modified municipal waste combustion unit means a municipal waste combustion unit you have changed after June 6, 2001 and that meets one of two criteria:
(1) The cumulative cost of the changes over the life of the unit exceeds 50 percent of the original cost of building and installing the unit (not including the cost of land) updated to current costs.
(2) Any physical change in the municipal waste combustion unit or change in the method of operating it that increases the emission level of any air pollutant for which new source performance standards have been established under section 129 or section 111 of the CAA. Increases in the emission level of any air pollutant are determined when the municipal waste combustion unit operates at 100 percent of its physical load capability and are measured downstream of all air pollution control devices. Load restrictions based on permits or other nonphysical operational restrictions cannot be considered in the determination.
40 CFR § 60.1940
Terms used but not defined in this section are defined in the CAA and in subparts A and B of this part.