Arizona Public Service Company is the largest electric utility in Arizona, United States and the principal subsidiary of publicly traded S&P 500 member Pinnacle West Capital Corporation (NYSE: PNW), which in turn had been formerly named AZP Group, when Arizona Public Service reorganized as that holding company in 1985. The utility company also operates three nuclear reactors. Its Palo Verde Nuclear Generating Station in Arizona, the largest nuclear plant in the U.S., came under scrutiny by the Nuclear Regulatory Commission in 2005 when operational problems began to cause prolonged outages.