Fire Capt. Jeffrey Thompson says firefighters responding to the PECO building on Wednesday found that a transformer had malfunctioned.
The call came in around 2:14 p.m. for a high-rise building fire, but when fire crews arrived they found no flames or smoke.
Upon further investigation they determined the power outage. The incident was placed under control at 2:31 p.m.
PECO serves 1.6 million electric and 486,000 natural gas customers in southeastern Pennsylvania. It is a subsidiary of Exelon Corp.
The Associated Press contributed to this report.