The splice, a piece of equipment that connects two overhead wires, came apart and fell to the ground shortly before 5:20 p.m. Monday, minutes before the game between the 49ers and Pittsburgh Steelers was set to start, PG&E spokeswoman Katie Romans said.
A secondary source of power for the stadium switched on immediately, but the park's metal halide lights "took several minutes to cycle back on," leaving fans and players in the dark for roughly 20 minutes, Romans said.
PG&E officials initially on Tuesday blamed a city-owned automatic transfer switch for malfunctioning and failing to prevent the outage, but backed away from that claim later in the day.
After the game got under way, the lights went out again briefly at about 6:45 p.m. during the second quarter. The cause of that second outage remains under investigation by PG&E and city officials.
Mayor Ed Lee, who was at the game, called the outages "a national embarrassment" the following day and pledged a full investigation into what caused them.
Romans said PG&E does not yet know what caused the splice to fail, but said since Monday, the utility has repaired the downed line on Egbert Avenue, located a couple of blocks away from the stadium, as well as six other splices in the neighborhood.
The equipment was replaced as part of "an active investigation into all of our surrounding infrastructure," she said.
The 49ers won Monday's game 20-3 over the Steelers, improving their record to 11-3 on the season.
The team, which clinched a playoff spot earlier this month, plays on the road for the final two weeks of the regular season, but is assured of at least one home playoff game in January.