UPDATE 1-Calif power grid curbs maintenance during heatwave

(Recasts to show no major heat-related outages, changes dateline, previous NEW YORK)

LOS ANGELES, June 20 (Reuters) - California ISO restricted maintenance operations at power plants and transmission lines on Friday throughout California as hot weather boosted air conditioning demand, the state's power grid operator said.

That is one of the first steps the grid operator takes during a heatwave and does not indicate problems on the system.

Gregg Fishman of Cal ISO and the state's big utilities say while demand will be much higher than usual on Friday, the power system is expected to hold without significant outages, as long as no major plant or transmission outages occur.

The heat will stretch most of the 770-mile (1240-kilometer) length of California. San Francisco will have a high of 91 degrees Fahrenheit (33 Celsius), which is 21 F above the normal high, Los Angeles will reach 93 F, 13 degrees above normal, and in Sacramento, the high will be a near-record 104 F, where the normal high is 88 F, according to AccuWeather.com.

Cal ISO forecast electricity demand would reach 46,657 megawatts Friday afternoon, but there is enough of the key reserve margin. There was an estimated 49,000 MW of generation expected to be available at the Friday peak, Cal ISO said on a year-round average.

One megawatt powers 650 to 700 homes in California, on a year-round average. Less homes per megawatt are served during heat waves.

While none of the state's big three investor-owned utilities noted heat-related outages on Friday morning, they were asking customers to mind their use of electricity.

The meteorologists forecast temperatures would moderate over the weekend.

Southern California Edison, a unit of Edison International (EIX.N) said that demand on Thursday of 21,203 MW was the highest so far this year.

SCE's demand was forecast to near 23,000 MW on Friday, and the peak load for Pacific Gas & Electric Co. was forecast to near 20,000 MW. PG&E is a unit of PG&E Corp (PCG.N).

The Los Angeles Department of Water and Power said it also was not experiencing problems on Friday morning. The LADWP is the biggest municipal utility in the United States.

The California ISO operates the grid on which flows about 80 percent of the state's power and serves about 30 million people in California. It administers the wholesale electricity market with an internal generating capacity of more than 47,000 megawatts. The grid also has access to thousands of megawatts of imports from outside California.

The big power transmission lines that the Cal ISO administers are owned by the state's investor-owned utilities, SCE, PG&E and San Diego Gas & Electric Co, a unit of Sempra Energy (SRE.N). (Reporting by Bernard Woodall and Scott DiSavino; Editing by Marguerita Choy)