HOUSTON (Reuters) - Hourly workers at independent western U.S. refiner Tesoro Corp's largest refinery are scheduled to vote on Wednesday on a contract proposal reached after three days of talks between the company, the United Steelworkers union (USW) and a federal mediator aimed at averting a strike, a union spokeswoman said.

Members of USW local 5 have already authorized a strike by workers at Tesoro's 166,000 barrel per day (bpd) San Francisco Bay area refinery in Martinez, California, if a new contract is not ratified by its 170 members.

USW spokeswoman Lynne Hancock on Sunday declined to disclose the terms of the proposal.

A Tesoro spokeswoman said the company expects ratification.

"We have reached a tentative agreement which is subject to membership ratification," said Tesoro spokeswoman Jill Saletta. "We look forward to achieving ratification at Martinez, just as we have at all other locations."

Workers at six of Tesoro's seven refineries are represented by the USW. Five of those refineries have approved contract proposals, some after strike authorization votes.

The union has been battling Tesoro over worker safety issues and proposed changes in benefits.

Tesoro has said in the event of a strike at the Martinez refinery, it would halt production temporarily to prepare temporary replacement workers who would then resume production.

(Reporting by Erwin Seba; Editing by Maureen Bavdek and Diane Craft)