Zenith National Insurance Corp. said Tuesday that its workers?compensation insurance subsidiary has filed for a 4 percent base rate increase.

The Zenith Insurance Company rate increase would be effective July 1. The State Compensation Insurance Fund, a not-for-profit that has a 23 percent share of state? workers?compensation market, raised its rates average 8.9 percent in January.

Zenith, based in Woodland Hills, did not give a reason for the increase, but carriers doing business in the state have increasingly complained about rising medical treatment costs and recent unfavorable decisions by a state board that hears appeals from workers disputing their permanent disability awards.

The Workers?Compensation Insurance Rating Bureau in March recommended a 24 percent increase in employer premiums. State Insurance Commissioner Steve Poizner plans to hold a hearing next month on the recommendation and rising worker? comp medical treatment costs.

A base rate only establishes a floor for premium rates charged businesses. Other factors that go into the premium are the industry and a company? safety record.