Eskom will implement Stage 3 load shedding on Wednesday from 8am to 11pm as a result of a shortage of capacity, the power utility said.

The shortage was a result of a number of generating units that are out of service due to breakdowns, it said.

"Our maintenance teams continue to work hard and through the night to bring units back online. Four units totalling 1 675MW were returned [on Tuesday] and we expect an additional 600MW to return to service [on Wednesday]," Eskom said in a statement on Tuesday night.

"Despite the return of these generating units as planned and some replenishment of diesel supplies, the water and diesel emergency reserves are still very low and we remain concerned about the risk to our operations as a result of Wednesday’s planned national strike action by [Congress of South African Trade Unions] Cosatu."

On Tuesday, the labour federation called for a national shutdown in eight provinces. It called on workers to stay at home on Wednesday and not go to work while others would picket countrywide.

Eskom said it activate contingency plans for Wednesday.

"We have activated contingency measures in the event that employees participate in the strike action and should there be further impact on our operations. However it is important to note that in terms of Section 77(1) of the Labour Relations Act, only employees who are not engaged in an essential service or a maintenance service have the right to participate in the strike action."

Stage 3 calls for 3000MW to be rotationally load shed nationally at a given period. 

"Load shedding is conducted rotationally as a measure of last resort to protect the power system from a total collapse or blackout. We continue to appeal to residents and businesses to use electricity sparingly during this period. Please switch off geysers as well as all non-essential lighting and electricity appliances to assist in reducing demand," the power utility said.