Power utility Eskom says it will move from stage 2 to stage 4 load shedding at 15:00 on Wednesday due to "exceptionally high demand".
Stage 4 power cuts will continue until 22:00.
Eskom said cold weather had placed additional demands on the country's grid which was already severely constrained because of breakdowns.
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"Load shedding will persist throughout the weekend as Eskom teams work around the clock to return as many of these generation units to service as possible," the utility said in a power update on Wednesday afternoon.
Eskom said unplanned breakdowns amounted to 11 300MW of capacity, roughly a quarter of the group's total nameplate capacity. Another 5 040MW is currently out on planned maintenance.
"We urge the people of South Africa to help reduce electricity usage in order to assist Eskom to reduce the instance of load shedding."