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One-alarm, two-alarm, three-alarm fires, or higher, are categories of fires indicating the level of response by local authorities, with an elevated number of alarms indicating increased commitment of resources. The term multiple-alarm is a quick way of indicating that a fire was severe and difficult to contain. This system of classification is common in the USA among both fire departments and news agencies.
A common misconception is that a "three-alarm fire," for example, means that three firehouses responded to the fire. This is not the rule behind the naming convention, although some cities may use the number of firehouses responding for multi-alarm designations because that is the simplest way to determine an alarm number. A three alarm fire means that there was a serious accident. For example, someone could have died or been injured.
The most widely used formula for multi-alarm designation is based on the number of units (firetrucks for example) and firefighters responding to a fire; the more vehicles and firefighters responding, the higher the alarm designation. (Note: In most cities, a "unit" can be anything from a tanker or ladder truck to rescue vehicles to even cars driven by the chief and deputies.)
With this unit/firefighter alarm designation, the initial dispatch is referred to as a "first alarm" and is typically the largest. Subsequent alarms are calls for additional units, usually because the fire has grown and additional resources are needed to combat it, or that the incident is persisting long enough that firefighters on scene need to be replaced due to exhaustion.
Requests for units and firefighters from outside jurisdictions do not normally occur in multi-firehouse urban areas until elevated alarms are reached (alarm three and above), but will depending on the location of the incident and the condition of the authority having jurisdiction at the time of the incident.
The system of classification comes from the old tradition of using pull stations to alert the local departments to a fire in their area. The "box" would send a message to all local stations by telegraph that there was a fire, indicating the location as a number: (station area)-(box number), e.g., 11-2. Fires are still dispatched as "box alarms," following this tradition, with maps broken up into a grid of "box areas."
Below is a list of the alarm levels used in the response policy of the New York City Fire Department. This is a basic example of how alarm levels are categorized in a fire department, how many fire apparatuses respond to each alarm level, etc. In New York, however, additional special alarm levels are utilized, aside from the conventional 1st Alarm, 2nd Alarm, 3rd Alarm, etc. Examples of such alarm levels are the Signal 10-75 Assignment, the Signals 10-76 and 10-77 Assignments, and the Signal 10-60 Assignment. A 10-75 is a Working Fire (i.e., there is fire visible from a building), the 10-76/10-77 Assignments are the alarm levels separate from the 1st, 2nd, 3rd Alarms, etc. that are the standard fire department responses to fires in high-rise buildings. The Signal 10-60 is a separate response to major disasters. Below is how the alarm levels are categorized in order per protocol and each apparatus count in an addition per alarm.
If the incident commander decides that the incident does not require a higher alarm level to be requested, they can specially request an additional unit to the scene without requesting a full alarm level assignment. For example, at a Working Fire, there are 4 Engine Companies, 3 Ladder Companies, 1 Squad Company, 1 Rescue Company, 2 Battalion Chiefs, and 1 Division Chief operating at the scene. If the fire is not large enough to require a 2nd Alarm, but a need for more equipment and manpower is needed, the commanding Chief can request additional units to respond "Specially Called" to the scene.
Thus, at the scene of a 5th Alarm Fire in New York, there are a total of 21 Engine Companies, 11 Ladder Companies, 1 Squad Company, 1 Rescue Company, 9 Battalion Chiefs, 1 Division Chief, 1 Deputy Chief, 1 Assistant Chief, and the Chief of Operations, as well as multiple specialized units and or specially called units operating on the scene.
All of these companies come from many firehouses to the scene. Some companies, however, are quartered together at the same firehouses. So, it is not a matter of how many firehouses respond to a fire, as popularly believed, but rather, how many companies/units and how many firefighters are operating on scene.