NEW YORK, May 29 (Reuters) - The failure of a second high voltage line to New York's Long Island prompted local officials on Wednesday to urge energy savings as they ordered in mobile generators to shore up the island's shaky power supply.

"Conservation will be essential ... especially if Long Island experiences a repeat of last year's record-breaking temperatures and humidity," Long Island Power Authority (LIPA) Chairman Richard Kessel said in a statement.

Last summer, the Island came within a couple hundred megawatts of blacking out during a long heat wave, when air conditioners push power demand to its annual peak.

To ease concerns among Long Island's 1.1 million electric customers, Kessel pointed out that LIPA has "sufficient electricity supplies available to meet normal summer demand" following an aggressive program to upgrade transmission lines and build more power plants.

But much of the hard work has been jeopardized by the loss since mid-May of two of the main undersea cables linking Long Island to the rest of the New York power system.

"The cable problems demonstrate how vulnerable we are to power shortages due to mechanical failures and not necessarily a resource shortfall. Additional on-island generation is critical if we want to keep the lights on," said Kessel.

THE CABLES

Both cables, which supply almost 1,000 megawatts of power to Long Island -- enough to light about a million homes -- run under the Long Island Sound between Westchester County, just north of New York City, and Nassau County on Long Island.

The 400 MW Y-50 cable, owned by LIPA and Consolidated Edison Inc. , New York City's main utility, was shut in mid-May and is likely to be down for several weeks.

"If the Y-50 cable is lost for the summer, the Island's electricity supply could be stressed to its limits, even with the addition of 737 MW of new resources expected to go on line in the weeks ahead," said Kessel.

Those new resources include 10 turbines being developed at six sites around Long Island by energy companies from around the country and a new cable being installed by a Canadian company that will bring in electricity from New England.

The Y-49 cable, shut over the past weekend, brings some 600 MW of power to Long Island. It is owned by the New York Power Authority, the state-owned power generating company.

NYPA has already started repairing line Y-49, already restoring its to about 444 MW, and hopes to have it back at full service within a couple of weeks.

"We're taking every action possible to get Y-50 back as soon as possible, and to repair the Y-49 problem so we can meet the peak summer demand supply requirements," said Kessel.

CONSERVATION PLEA

"Conservation has to be the number one item on every customer's mind," said LIPA spokesman Michael Lowndes, sounding a note heard often last year in California, which also failed to expand its power system fast enough to keep up with demand.

An extreme heat wave could cause peak electricity demand on Long Island to exceed 5,200 MW. LIPA has access to 5,917 MW of energy including its 10 new turbines, the new cross sound cable, and the Y-49 and Y-50 tie lines.

Without line Y-50, LIPA has access to just 5,517 MW.

To make up for the possible long-term loss of Y-50, LIPA said it will add 200 MW of generation, provided by 10 22 MW generators mounted on flatbed truck trailers parked at substations around the Island by July. The mobile generators would be switched on only during an extreme heat wave.

The 10 new turbines are being installed by Florida's FPL Group Inc., New York's KeySpan Corp, California's Calpine Corp. and Pennsylvania's PPL Corp. .

TransEnergie U.S., a unit of Canadian energy giant Hydro-Quebec, is installing the 330 MW cross-sound cable between Connecticut and Long Island. The transmission company expects the line to be in service by the end of June.

"While the new resources will help us meet demand, we urge customers, both residential and commercial, to conserve as much electricity as possible all summer long, especially during extreme heat waves," Kessel said.

--Scott DiSavino, New York Power Desk, +646-223-6072, fax +646-223-6079, e-mail scott.disavino@reuters.com