Officials are calling it a two-alarm fire. Paramedics say three patients were taken to a local hospital with minor smoke inhalation.
Toronto firefighters and hydro workers are on scene. The cause of the transformer fire is not immediately known.
Firefighters say the hydro vault has been "de-energized."
Coun. Kristyn Wong-Tam tweeted hearing "a thunderous boom" before the building was evacuated.
More to come