The explosion rattled windows, blew out power and for a time, shut down traffic at 4th and Market Street.
The first explosion was at 10:00 a.m. on Monday morning and another one happened 20 minutes later as fire fighters attempted to smother the electrical transformer fire with carbon dioxide.
Witnesses say the initial blast was much bigger.
"It sounded like a bomb. It really did it shook the building," said Joe Speel from Kone Elevators.
Speel's crew was unloading a truck right next to the manhole when the underground transformer blew.
"Like a 5.0 on the Richter scale shaking that building it really did," said Speel.
Thirty minutes after firefighters arrived, smoke stopped pouring out of the manhole. It was a much quicker resolution than one month ago when on June 5th a vault fire at Polk and O'Farrell Streets burned for nearly two hours.
It drew an angry response from San Francisco's State Senator Mark Leno, who called on the state's public utility commission to investigate Pacific Gas and Electric Company.
According to leno's statement, the average PG&E customer is without power seven hours a year, which is more than twice the national average.
"Something needs to be done. So I am hoping that PUC is going to pay some attention to this and again answer some questions," aid Sen. Leno.
The San Francisco Fire Department admits transformer failures like this are pretty common.
"I don't have a count but it's not infrequent, but it's not every day," said Mindy Talmadge from the SFFD.
A spokesman for PG&E couldn't tell ABC7 how many transformer blowups San Francisco has seen this year. But he says it's not a maintenance issue.
"Well first of all we take the safety of our system very seriously, and we have a very regular and rigorous safety inspection program we're constantly inspecting though out our vaults in San Francisco and our electrical," said Joe Molica from PG&E.
"Do you think you need to be doing more or are you doing enough?" asked ABC7's Mark Matthews.
"We need to be doing more, we'll certainly look into that," said Molica.
San Francisco Supervisor David Chu says he'd like to take PG&E at its word, but he won't.
"We have seen a pattern not just in recent years, but now in recent weeks of outages and problems that seem to stem from ageing infrastructure and I have very deep concerns," said Chu.
Power is still out in the Ross Dress for Less building, and when we spoke to Leno earlier, ABC7 asked him whether he had heard from the PUC and he said 'no.' The California Public Utilities Commission also did not answer ABC7's calls.