So severe is the problem, city bosses say, that they are keeping street lights burning round the clock to deter the thieves, many of them drug addicts.
An official said it was unlikely that the thieves would risk their lives by tackling live wires.
Clive Justus said it was cheaper to keep the lights burning than to replace stolen cable and vandalised equipment.
Mr Justus, a mayoral committee member in Cape Town, said: "Unfortunately, the city is suffering from an unprecedented onslaught from cable thieves, most of whom are ‘tik’ (methamphetamine) addicts desperate for drug money.
"An effective deterrent is to keep the street lights burning, as thieves rarely risk their lives by hacking into live wires," he was quoted as saying by AFP news agency.
National blackouts
The city will keep lights burning in districts which have been hardest-hit by the thieves.
Mr Justus said that under normal circumstances there was no excuse for wasting power.
But he said the cost of burning energy-efficient bulbs, which are being introduced in the city’s 320,000 street lights, was much cheaper than replacing stolen cable and vandalised equipment.
South Africa, which hosts the World Cup next month, suffered national blackouts in January 2008 that paralysed much of the country.
The 10 World Cup stadiums each have generators designed to maintain power supplies during matches, even during a blackout.
Source: BBC