* Temperatures to reach 100 degrees in Chicago * Demand high but not expected to reach record levels * Con Edison reduces voltage in parts of Manhattan By Scott DiSavino July 16 (Reuters) - The power grids in the U.S. Midwest and Northeast said they have more than enough resources to keep the lights on this week as customers crank up their air conditioners to escape another brutal heat wave. The grid operators said power usage would be high but was not expected to break any demand records. "Demand will be high but will likely not even reach the level we forecast for this summer. We have adequate resources and are not expecting any issues," Paula DuPont-Kidd, a spokeswoman at PJM, the biggest power grid in the United States, told Reuters. PJM serves more than 60 million customers in 13 U.S. Mid-Atlantic and Midwest states and the District of Columbia. High temperatures in Chicago were expected to reach 96 degrees Fahrenheit (36 Celsius) on Monday and 100 on Tuesday before dropping to near normal levels in the mid 80s on Wednesday, according to AccuWeather.com. In New York, the mercury was expected to hit 92 degrees on Monday, 95 on Tuesday and 94 on Wednesday before falling to near normal levels in the 80s on Thursday, AccuWeather.com said. New York power company Consolidated Edison Inc, which locked out its 8,000 member union workforce on July 1 due to a contract dispute, reduced the voltage, also known as a brown out, in parts of Manhattan to take the load off the system and allow workers to fix heat stressed equipment. The company said it was alright for the homes and businesses in the affected areas to continue running their air conditioners, but asked all of its 3.2 million customers in New York City and Westchester County to use power wisely. Con Edison also reduced the voltage in several other neighborhoods during the last two heat waves to bake the Big Apple so far this summer. The company said voltage reductions had nothing to do with the lockout, noting the utility reduced the voltage before the lockout and after. The company and union were expected to meet again later Monday. DEMAND HIGH BUT NO RECORDS Three regions expect power usage to approach record breaking demand levels this week, New York, New England and the Midwest. The Midwest Independent Transmission System Operator (MISO), which operates the power grid in all or parts of 11 U.S. Midwest states and the province of Manitoba in Canada, forecast demand would approach 96,600 megawatts (MW) on Tuesday, which is close to the system's all-time peak of 98,526 MW in July 2011. The New York ISO (NYISO), which operates the grid in New York State, forecast demand Tuesday would approach 31,900 MW, which is still below the state's all-time peak of 33,939 MW in August 2006 before the recession of the late 2000s cut mostly industrial and commercial demand for power. ISO New England, which operates the grid in the six New England states, forecast demand would approach 26,800 MW on Tuesday, still shy of its 28,130 MW record set in August 2006. For a factbox on record peaks, see One megawatt can power about 1,000 homes. SOLAR STORMS, GAS RESTRICTIONS In addition to the heat, the U.S. Northeast is also dealing with solar magnetic storms and natural gas restrictions. NextEra Energy Inc reduced the output of its 1,247-MW Seabrook nuclear power plant in New Hampshire to 68 percent by early Monday from 85 percent early Friday because solar magnetic activity was affecting its transformer. In addition, Spectra Energy said the 1,000-mile Algonquin Gas Transmission natural gas pipeline restricted service for some interruptible customers in New England. Interruptible customers typically pay less for gas with the understanding that service may be cut during times of peak demand. The Algonquin system connects with five major interstate pipelines, delivering 1.5 billion cubic feet per day of gas from supply areas in the Gulf Coast to New England. Despite the slight power reduction at the Seabrook reactor and the natural gas restrictions, ISO New England said power system conditions were normal Monday afternoon. CONSERVATION URGED To take some of the stress off the system, some grid operators have already asked generators and transmission owners to put off unnecessary maintenance and some local utilities, like Con Edison, have called on consumers to conserve power during the heat wave. Later in the day as temperatures rise, some utilities may also call on customers who signed up for demand response programs to reduce power usage. Demand response programs pay consumers to cut back on electric use during peak times or when power prices are high by shutting off unnecessary lights, elevators and other equipment, reducing air conditioning and even turning on backup generators to reduce the amount of power they take from the grid. The biggest power companies in the regions baking in the latest heat wave include units of Duke Energy, Exelon Corp, FirstEnergy Corp, American Electric Power Co Inc, Xcel Energy Inc, Con Edison, National Grid PLC and Northeast Utilities.