Emergency services have extinguished the fire at Robertsham substation in southern Johannesburg, which started on Monday evening.
The cause is unknown at this stage.
Several areas in the south are in darkness.
Affected areas include: Robertsham, Booysens, Crown Gardens, Ridgeway, part of Mondeor, Gillview, Glenada, Ophirton, Evans Park, Ormonde, Winchester Hills, Southdale, Aeroton and Nasrec.
City Power said two main transformers were gutted.
“The substation needs to be dry-ice cleaned.
“After the cleaning, we’ll assess the ability to restore supply. No ETR [estimated time of repair] given at this stage.”
No-one was injured.