Those affected include several industrial and commercial firms along the lower parts of the busy Enterprise and Likoni Roads, parts of Lunga Lunga Road and Diamond Estate among others.
Kenya Power says the fire took about 45 minutes before it was brought under control with the help of the Nairobi County Fire Brigade.
“Kenya Power electrical engineers are on site to restore power to customers. However, some customers will spend the night without power as sections of the substation will require extensive and time consuming repair,” Kenya power said in a statement.
In the meantime investigations have commenced to determine the cause of the fire, suspected to be a fault that occurred from one of the lines emanating from the sub-station.
“Kenya Power’s management has apologised to affected customers for the inconvenience caused by the unfortunate incident, and appealed for patience as the matter is addressed,” the company said.