UPDATE 2-Power blackouts hit swaths of Peru's capital

(Adds power seen restored, paragraphs 5-6)

LIMA, Aug 7 (Reuters) - Blackouts hit Peru's capital Lima on Thursday after the power grid agency Coes told distributors to reduce electricity deliveries, the strongest sign yet that the Andean country's supply cushion has worn thin.

It was the second time since Saturday that mechanical trouble at generators prompted Coes to ask distributors or big consumers, especially mines, to help trim overall consumption. But it was the first time that residential neighborhoods were affected.

"It's a blackout because of a generation failure," said Hans Berger, of Luz del Sur, one of the two main power distributors in Lima.

He said at least 120,000 homes were without power on the south side of the city, and a similar number was likely in the dark on the north side.

The blackouts were caused when a valve failed on a gas line to the Etevensa power plant, shutting down a generator that can produce up to 10 percent of the country's power.

An Etevensa official said the valve was later fixed and that power would be fully restored until 2200 local time (0300 GMT).

Still, the grid operator, Coes, said it has little cushion as power demand has surged during seven years of economic growth. A dry rainy season has also suddenly reduced output by hydroelectric plants.

"Reserves have fallen and are very small. They aren't sufficient to guarantee that there won't be electricity interruptions when there are problems with generators," Cesar Butron, the head of Coes, told Reuters.

The economy is expected to grow 8 percent this year and even though Peru has large natural gas deposits, it has few pipelines to feed thermoelectric plants.

A wider pipeline into Lima, which would help boost power reserves, is not expected to be ready for another year.

On Saturday, Coes asked several large mining companies -- the backbone of Peru's economy -- to reduce consumption for several hours.

The companies included, among others, Southern Copper (SPC.LM: Quote, Profile, Research, Stock Buzz) (SPC.N: Quote, Profile, Research, Stock Buzz), the Antamina mine of BHP Billiton Ltd (BHP.AX: Quote, Profile, Research, Stock Buzz)(BLT.L: Quote, Profile, Research, Stock Buzz) and Xstrata (XTA.L: Quote, Profile, Research, Stock Buzz), and Newmont's (NEM.N: Quote, Profile, Research, Stock Buzz) Yanacocha gold mine.

(Reporting by Marco Aquino, Jean Luis Arce and Terry Wade; editing by Richard Chang)