The National Union of Metalworkers of SA (Numsa), the country’s largest union with more than 400,000 members, has joined other workers, including public servants, in demanding above-inflation wage increases, which employers say are unsustainable.

Numsa said it would embark on a wage strike at ArcelorMittal SA — Africa’s largest steel producer — in support of a 10% wage demand from Wednesday after serving the company with a 48-hour strike notice...