The Northeast faced a fourth day Wednesday of a record-breaking heat wave, after triple-digit temperatures tested power supplies throughout the region and tried the patience and resilience of anyone who dared to venture outside.

While temperatures were expected to moderate some from Tuesday’s peak — which saw a record set at 103 degrees for the day in New York City — utilities warned that the length and intensity of this heat wave was testing the limits of the power grid.

Con Edison said it was working to restore power to about 6,300 customers in New York City on Wednesday; in Washington nearly 2,000 customers were without power, while New Jersey’s Public Service Electric and Gas Company reported about 6,500 customers without electricity.

Transportation officials cut the speed of commuter trains on Tuesday in New York and suburban Washington when the tracks got too hot, and rail riders in New Jersey were advised to expect delays again Wednesday.

The heat broke several records in the Northeast, as Boston, Providence and Philadelphia all saw triple-digit temperatures Tuesday that eclipsed previous highs. In Philadelphia, a 92-year-old woman was found dead in her home on the second floor, where all but one window was shut. The medical examiner ruled that extreme heat was a factor in her death.

Forecasters predicted that Wednesday would offer only limited relief, with temperatures in the mid-90s in New York City. The National Weather Service blamed a high-pressure system along the East Coast that drew hot, humid air from the south and will probably stay put until late this week.

Con Edison officials used automated calls Tuesday night to appeal to customers in New York City to turn off “non-essential” appliances as power consumption reached record levels.

The heat’s effects on the five boroughs were unsparing: Some city pools were filled to capacity within an hour or so of opening, sending seekers of respite to libraries, cooling centers and other public havens from the heat. Hospitals set out jars of ice water as their waiting rooms filled with wheezing elderly patients and exhausted firefighters.

The city’s police commissioner, Raymond W. Kelly, said that the Police Department was mindful that more crime tended to occur in the extreme heat of night, and was also prepared to send extra officers to places that had lost power.

But even as Con Edison officials were optimistic that the city would survive the day without widespread power failures, they acknowledged that the intensity and duration of the heat wave could have a cumulative effect on the cables and transformers. In short, they said, the worst may be yet to come.

“It’s Round 1 in a prizefight,” said John Miksad, Con Edison’s senior vice president of electric operations. “Round 1 looks O.K., but the bell hasn’t rung yet.”

In Washington, commutes were longer after trains were ordered to operate at least 20 miles per hour under maximum speeds because of the heat. In Baltimore, officials planned to distribute bottled water to the homeless over the coming days.

In Rhode Island, some residents could not even savor a cool drink from their faucets. The Department of Public Health urged residents in parts of Narragansett and South Kingstown to boil their water before drinking it as a precaution after water pressure dropped below acceptable limits.

“I just turned on my outdoor shower and there’s no water,” said David Cunningham, 46, of South Kingstown. “It’s a trickle.”

But New York seemed most concerned about the prospect of brownouts or blackouts after having gone through three major blackouts over the last half century.

And it has been four years since the utility’s equipment in Long Island City failed in a cascade of blown feeder lines and left tens of thousands of Queens residents without power for more than a week. The power system’s ability to withstand a sustained surge in demand has not really been tested since that summer.

“We haven’t had a real New York heat wave in a while,” said Mr. Miksad, who admitted to having fretted through a “nervous weekend” knowing what was coming. Con Edison dispatched extra crews to Staten Island where a main feeder cable failed early in the day, and by late afternoon more than 4,000 customers were without power in the Fox Hills neighborhood.

By late Tuesday evening, Con Edison had reduced voltage in neighborhoods throughout Brooklyn and Queens because of problems with electrical cables. Mr. Miksad said that more people might have lost power if not for a set of demand-reduction programs that were used on Tuesday, including voluntary cutbacks by big corporate customers and the utility’s ability to control the thermostats in some residents’ homes. All told, those programs saved as much as 400 megawatts and kept total demand from surpassing the all-time high, he said.

Some office buildings, including Con Edison’s headquarters near Union Square, shut down banks of elevators, lowered the lights and turned up thermostats. Con Edison requested that all of its customers conserve electricity by turning off equipment not being used, keeping air-conditioners at 78 degrees and running washers, dryers and dishwashers late at night.

Robert Madden, 25, a waiter who lives in Astoria, said that his electronic equipment shut down in a specific order.

“First the PlayStation turns off, then the refrigerator, then the computer, the lights, then the pilot lights on the stove, then the fan,” he said.

He added that he had advised his wife, who is both pregnant and a student, to stay at school. “There is air-conditioning there,” he said. “You can’t have a pregnant wife at home baking.”

By late afternoon on Tuesday, New York-Presbyterian Hospital/ Weill-Cornell Medical Center had seen only scattered cases of heat-related illnesses, but doctors there said that the numbers would most likely increase.

Plasma Cutters, Adult Halloween Costumes, Fantasy Football