METALS-Copper at 3-wk high on buying, lower inventories

* Chinese arbitrage buying boosts prices

* Corporate and macroeconomic data cap gains

* Coming Up: Fed chairman Ben Bernanke 1800 GMT

(Updates official prices)

By Michael Taylor

LONDON, July 21 (Reuters) - Copper rose to a three-week high on Wednesday due to strong physical and Chinese buying and falling inventories, but gains were capped by uncertainty over economic growth.

Benchmark copper for three-month delivery CMCU3 on the London Metal Exchange traded at $6,790 a tonne from $6,637 at the close on Tuesday and compared with a session high at $6,830.

Shanghai's benchmark third-month copper SCFc3 gained 0.6 percent to close at 53,350 yuan a tonne. The contract peaked at 53,600 yuan, also its highest since July 15.

Copper prices rose 140 percent last year but are down about 8 percent in 2010.

"We're seeing more activity from commercials -- fabricators, manufacturers, consumers," Brebner said. "The physical market is taking advantage and buying, and physical markets remain tight."

Zinc CMZN3 traded at near two week highs at $1,895 a tonne in exchange rings from $1,875 and tin CMSN3 was untraded but last bid at $18,315 from $18,240.

Steel-making ingredient nickel CMNI3 traded at three month highs at $19,320 in LME rings from $19,125, while battery material lead CMPB3 was untraded but last bid near two month highs at $1,865 from $1,837. More...