Northeast power grids survive heat wave (Reuters) - Power companies in the U.S. Northeast and Mid-Atlantic kept electricity flowing to most consumers throughout a three-day heat wave that had air conditioners humming, despite the shutdown of three nuclear plants. New York City electric company Consolidated Edison, which powers the biggest and most congested city in the United States, reduced voltage in a couple neighborhoods in Manhattan on Wednesday, allowing workers to fix some failed equipment. Temperatures in New York City hit 91 degrees Fahrenheit (33 Celsius) on Monday and 96 on Tuesday, and were expected to reach 100 on Wednesday. Thunderstorms were expected to break the heat wave, bringing temperatures closer to normal levels in the 80s by Thursday, according to AccuWeather.com. The nuclear units that shut by early Wednesday included Constellation Nuclear Energy Group's Nine Mile Point 1 in New York and Calvert Cliffs 1 in Maryland, and Exelon Corp's Limerick 1 in Pennsylvania. Calvert Cliffs 1 however was back up and running earlier Wednesday morning. Constellation Nuclear Energy is owned by units of Exelon and French power company Electricite de France SA (EDF). Two of those nuclear units are located in the PJM power grid, which told generators and transmission owners to prepare for the heavy air conditioning demand, but did not take any steps that customers would notice to reduce usage on Wednesday. PJM is the biggest power grid in the United States serving more than 60 million people in 13 Midwest and Mid-Atlantic states and the District of Columbia. On Tuesday, PJM activated its demand response programs in parts of Virginia, West Virginia and Ohio to help maintain the voltage as a lot of power flowed west across those states into the Midwest region. Some of that power was moving into the grid operated by the Midwest Transmission System Operator (MISO), which oversees 11 U.S. Midwest states and Manitoba in Canada. Demand in the MISO on Tuesday was expected to top the region's all-time usage record of 98,526 megawatts (MW) set in July 2011. Officials in the MISO were not immediately available for comment on Tuesday's actual usage. No other power grids forecast peak demand this week would reach all-time levels. In New York, the New York ISO, like PJM activated its demand response programs to reduce relieve some of the stress on the power lines. The biggest power companies in the regions that baked during the latest heat wave include units of Duke Energy, Exelon, FirstEnergy Corp, American Electric Power Co Inc, Xcel Energy Inc, Con Edison, National Grid PLC and Northeast Utilities.