Industry officials have long warned that the North American power transmission system, which saw its greatest expansion in the years following World War II, is groaning under the weight of the heavy loads it carries today.

"We're a superpower with a third-world grid. We need a new grid," New Mexico Gov. and former Energy Secretary Bill Richardson told the CNN television network. "The problem is that nobody is building enough transmission capacity."

According to the Electric Power Research Institute in Palo Alto, California, US power demand has surged 30 percent in the last decade, while transmission capacity grew a mere 15 per cent.

And during muggy weather like the kind blanketing the Northeast on Thursday, air conditioning accounts for a hefty 30 per cent of all power flowing over the lines, severely taxing an already overworked transmission network.

It was the biggest blackout in North American history, according to US power grid operators. It eclipsed the 1965 blackout in the United States and Canada that affected about 30 million people.

It spread in a matter of seconds, tripping circuit breakers from the Great Lakes to New England to protect costly electrical equipment from a sudden voltage jolt.

Power was returning to some of the affected areas. By early (on) Friday, power had been restored to New York's Bronx borough and suburban Westchester County, TV networks reported.

The blackout was reminiscent of the infamous 1965 outage, triggered by a lightning strike on a high-voltage line running from Canada to New York, stranding about 30 million people without electricity along the populous New York-New England corridor.

The 1965 outage shocked the nation and gave birth to the North American Electric Reliability Council, a New Jersey-based industry group that works to ensure reliable service on the 500,000-mile network of high-voltage lines that serve 270 million US and 31 million Canadian customers.