by more than 15 percent of its demonstrated capacity in megawatts for serving the utility's Georgia retail customers without first obtaining a certificate or an amendment to a certificate, as appropriate, that public convenience or necessity requires or will require such increase or decrease; provided, however, no certificate shall be required if the increase or decrease is caused by a rule, regulation, or law mandated by any duly constituted local, state, or federal governmental body or agency or is caused by power pooling, forced or maintenance outages, or short-term sales for a period of less than one year.
OCGA § 46-3A-3