With more than 263,000 members, NUMSA is demanding wage increases of 13 percent from the metal and engineering industries, down from an initial demand of 20 percent.
The mass action is scheduled to start on Monday after wage negotiations broke down and when an attempt by the Plastics Converters Association to have a labour court interdict the strike failed.
“The effect of the Labour Court ruling is to the effect that all workers in the engineering and metal workers, whether union members or not, can partake in strike action,” NUMSA said in a statement.
“This shall have the effect that 320,000 can take part in strike action.”
Unions and employers in Africa’s biggest economy are in their mid-year bargaining session known locally as “strike season”, with many labour groups seeking a wage increase more than double the country’s 4.6 percent inflation rate.