Power utility Eskom on Wednesday called on consumers to switch off geysers, pool pumps and all non-e.
The union is among the organisations that have been loud in waging war against corruption.
Num general secretary Frans Baleni told a media briefing in Johannesburg yesterday that while it endorsed efforts by the state security agencies to root out corruption, they had detected "tenderprenuers" within the union.
"Corruption is not limited only to government.
"We have had instances where companies contracted to us, especially catering companies cooking at the hostels, telling us that they had been paying bribes to get contracts.
"They said they wanted to come clean. Shop stewards involved in corruption mobilise against these individuals so that they no longer get tenders from us," Baleni said.
He could not say how much corruption within its ranks had cost the union.
In a briefing that followed the union's central committee meeting last week, Baleni also came out in support of the ANC's decision to research on the issue of nationalising the mines.
He said Num's position was that no decision about the mines must be taken without the involvement of mineworkers.
He said while Num supported nationalisation as expressed in the Freedom Charter, the matter should be decided based on scientific research and not emotions.