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In August 2018, EPA proposed to revise the New Source Review (NSR) program in conjunction with the Affordable Clean Energy (ACE) rule to make available to states an NSR applicability test for electric generating unit (EGU) modifications based on hourly emissions. Under the proposed NSR revisions, if a physical or operational change would not increase the hourly emissions of the EGU, it would not trigger major NSR; however, if the EGU's hourly emissions would increase after the change, then the source's projected annual emissions increase would be reviewed using the existing NSR applicability test. These NSR revisions are no longer part of the ACE rule, so this action is intended to finalize the NSR revisions through a separate rulemaking.