The D.C. Fire Department since January has been dispatching city ambulances according to a new ranking system designed to better ensure that life-support equipment is available when needed, but response to less serious calls may be delayed by up to 20 minutes.

Fire officials did not announce the new system when it was put into effect but discussed it last week during hearings before the City Council on the department's fiscal 1987 budget request.

The new Priority Dispatch Program has five rankings for calls ranging from life-threatening cases -- "priority M" -- to those involving minor injuries -- "priority four." The new system replaces a system that had only two categories, one for life-threatening situations and one for all other calls.

"The problem with the old system is that in many cases we were dispatching advance life-support systems for calls that only needed basic life-support systems," said Dr. Victoria Countee, medical director for the Fire Department.

"We never turn down a person who calls for an ambulance, that is not the objective of this program," said Countee. " . . . The majority of our calls are the lowest nonemergency , and under this new system those people may have to wait 20 minutes for an ambulance."

To determine the seriousness of a call, ambulance dispatchers are asking callers more questions. Countee said the questioning could take up to four minutes, but if initial questions indicate a life-threatening situation, an ambulance is immediately dispatched.

The new program was prompted by increases in the number of calls for ambulance service, Countee said. In 1984 a committee was formed by Mayor Marion Barry to study how the city could better rank calls for dispatching ambulances. Committee members recommended the new program in a report issued in August 1984.

The city has 19 ambulances on call 24 hours a day. Of those units, 14 provide basic life support, and five provide advanced life support. There are also 32 fire engine companies, which send out trained medics for high priority calls and when no ambulances are available.

Basic life-support units have emergency medical technicians trained to control bleeding, bandage, splint and perform cardiopulmonary resuscitation (CPR). Advanced life-support units cost about $80,000 each and have highly skilled paramedics that can start an intravenous line to administer medicine, mechanically monitor a heart, or insert a breathing tube.

"The City Council has complained in the past that some people were using ambulances as a taxi service," said Sharon Ambrose, executive assistant to City Council member Betty Ann Kane (D-At Large).

The department also hopes to improve its response time to serious cases now that it has computerized its ambulance dispatch services, Countee said. With the computer, an ambulance can be dispatched while a caller is still being questioned, she said.

In 1975 the city logged 75,000 ambulance calls. Five years later the number had grown to 85,000 calls and by 1984 it was up to 100,000. Last year it had mushroomed to 112,842.

Under the new program dispatchers are trained to ask for an address and call back phone number immediately, because upset callers would often hang up before giving the address, Countee said.

Callers are also asked for such basic information as whether the victim is breathing and conscious, or bleeding heavily. "The questions are ordered so that you exclude the most serious conditions first," Countee said.

Advanced life support units are located in Wards 1, 2, 4, 6 and 8. "They are situated to cross over to other wards," Countee said. There are 280 emergency medical technicians, including paramedics, who operate the city's 19 ambulances.

The fire department dispatches ambulances for 300 to 350 calls a day. Last year city ambulances took 64,332 patients to hospitals. The average response time was nine and a half minutes, according to city records. The response time has not changed significantly under the new program, Countee said.

"About 15 to 20 percent of the calls actually have advanced life support performed, but we dispatch" the sophisticated ambulances more often, Countee said.

Asked about the new system, Stanley Allen, chairman of the Mount Pleasant Advisory Neighborhood Commission, said "I just hope that if it is a true emergency that there won't be a delay if call takers ask more questions."

Medical technicians and people who answer the 911 emergency number began a 40- to 56-hour training session for the new program last summer.