NEW YORK, June 25 (Reuters) - The Electric Reliability Council of Texas (ERCOT), grid operator for most of the state, asked consumers to reduce power use Thursday afternoon as air conditioning demand during a brutal heatwave pushed demand to near record levels, testing the reliability of the system.

"We are expecting high electricity demand today due to the heat advisory in effect for much of the state," Vice President of System Planning and Operations Kent Saathoff said in a release.

The heatwave has targeted Texas and the lower Mississippi Valley with high humidity and temperatures reaching over 100 degrees Fahrenheit (38 degrees Celsius) all week.

"We don't anticipate any electricity emergencies but due to the effect of some unplanned generation outages, we expect to have minimal reserves available," Saathoff said.

Demand on ERCOT broke the June record for a fourth day in a row but did not break the all-time record on Thursday. Demand on Thursday reached 62,298 megawatts. The all-time record still stands at 62,339 MW set in August 2006.

The grid operator was not expecting record breaking demand on Friday.

One megawatt typically powers about 200 average homes during hot summer weather. ERCOT said.

ERCOT manages the flow of power to about 20 million people in Texas and administers the wholesale electric market with a generating capacity of about 83,000 MW. (Reporting by Scott DiSavino; Editing by Marguerita Choy)