UPDATE 2-TransCanada restricts Keystone volumes for August

* 20 pct restriction compared with 19.28 pct for July

* Work expected to be done this summer (Adds expected volumes, impact on Canadian crude markets)

CALGARY, Alberta, July 15 (Reuters) - TransCanada Corp said on Friday it will cut nominated crude volumes on its Keystone pipeline to the United States from Canada by 20 percent next month as it works on the line following two spills in May.

The restriction compares with a 19.28 percent cut for July on the 591,000 barrel a day capacity pipeline.

"It's a continuation of the reliability enhancement maintenance program we have been conducting," TransCanada spokesman Terry Cunha said in an email. "It should be concluded this summer."

Cunha said the cuts will result in volumes of 400,000 to 450,000 barrels a day for the month on the system, which extends to Wood River and Patoka, Illinois, and the Cushing, Oklahoma, storage hub from Hardisty, Alberta.

Trade sources said word of the restriction had no impact on Canadian heavy crude discounts, which remain narrower than was the case over the past two months. Keystone has not run at capacity rates since starting up last year.

Western Canada Select heavy blend for August was quoted at $17.75 a barrel under benchmark West Texas Intermediate light oil, about 25 cents tighter than spreads seen earlier this week.

The maintenance work comes after spills at pumping stations in North Dakota and Kansas that shut down the pipeline, bolstering oil prices and providing fodder to opponents of the company's $7 billion plan to expand the system as far as the U.S. Gulf Coast. (Reporting by Jeffrey Jones; editing by Peter Galloway)