Energy Insurance Mutual said it will drop its reserve premium requirements for gas and electric utilities wanting to join the mutual insurer.

The Bridgetown, Barbados-based company, which offers excess liability coverage, had required a one-time payment equal to the first year's premium for utilities wanting to join the mutual.Now new members will not have to pay the reserve premium requirement. This effectively cuts the first-year costs in half, a company spokesman said.

The offshore insurer provides general liability and directors and officers liability coverage for losses in excess of $25 million for gas and electric utilities in the United States.

The company was formed in July 1986 in response to a sharp decline in the availability of liability insurance and a steep rise in the cost of coverage experienced by the energy industry.

This move was an attempt by the members to open up the company to more utilities and make the coverage more available, the company spokesman said.

Outgoing Chairman Irene M. Moszer said at the company's annual meeting March 9 that eliminating the reserve premium requirement may encourage more utilities to obtain excess liability coverage.

Now EIM has reached a level of strength and maturity which no longer necessitates as great a financial commitment by the companies it insures, said Ms. Moszer, who also is vice president, administrative services at Virginia Power.

The company also said it will open a Tampa, Fla. branch office as soon as its Florida license application is approved by the state insurance department. The approval is expected around the end of the month, the spokesman said.

In July 1987, Florida joined other states in passing a new captive insurance law giving incentives to encourage captives to operate in the state.

Like Vermont's captive law, the Florida law does not require a captive insurer to be incorporated in the state.

The federal Tax Reform Act of 1986 eliminated many of the tax incentives for operating a captive insurer outside U.S. borders.

The spokesman said that operating in Florida will free the company up from many restrictions - having to deal with offshore brokers, no direct sales and other technical requirements.

Ms. Moszer said the elimination of the reserve premium requirement is a profound act by EIM's members who want to put excess liability coverage within the reach of as many utilities as possible.

The insurer also reduced from three years to two years the commitment a new utility must make to the company for coverage.

In February, the mutual obtained all of the reinsurance package it sought, the spokesman said.