Num spokesman Lesiba Seshoka says that if the strike continues, it may result in load shedding.
"It could have very serious negative effects on the entire economy. The bulk of the activities depend on electricity, although Eskom says it has six weeks of stockpiles. The people on the ground are telling us they have only three weeks' stockpile.
"It could result in load shedding," he said.
On Monday Eskom said it had 38-day coal stockpiles at power stations around the country.
The Federation of Unions of SA (Fedusa) on Tuesday condemned employers, claiming they had failed to negotiate in good faith with its affiliated trade union Uasa.
"It is clear to us that it is the intention of the employer to provoke workers into strike action by deliberately coming to the negotiating table with minuscule and unacceptable wage offers to compensate workers," Fedusa chief negotiator Franz Stehring said in a statement.
He was concerned employers would employ the "age-old tactic of retrenchment" during talks, to threaten the job security of workers.
Chamber of Mines negotiator Frans Barker said over 50 percent of workers joined the strike, which began on Monday.
"I think production is quite severely impacted."
The chamber was in talks with the National Union of Mineworkers (Num), Solidarity and the United Association of SA (Uasa).
The chamber is negotiating on behalf of companies including Anglo Thermal Coal SA, Delmas Coal, Exxaro, Kangra Coal, Optimum Coal and Xstrata Coal.
Barker declined to comment on the offer to members, saying negotiations were at a "sensitive stage".
The chamber's last offer was 8.5 percent for the lowest paid worker and seven percent for other employees.
The Num and Uasa wanted 14 percent, and Solidarity 12.
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