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One-alarm, two-alarm, three-alarm fires, etc., are categories of fires indicating the level of response by local authorities. The term multiple-alarm is a quick way of indicating that a fire is severe and is difficult to contain. This system of classification is common in the United States and in Canada among both fire departments and news agencies.
A common misconception is that a "3-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.
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 because the incident is persisting long enough that firefighters on scene need to be relieved.
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 depend 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 intensity by number. For example, a code commonly used throughout the US was four rings, a pause, and another four rings to indicate a particularly intense fire, giving rise to the phrase "four alarm fire."
Typical alarm levels
Below is a list of the alarm levels used in the response policy of the New York City Fire Department (FDNY). This is a basic example of how alarm levels are categorized in a fire department, how many fire apparatus or fire units respond to each alarm level, etc. In New York, however, additional special alarm levels are utilized, aside from the conventional 1st alarm fire, 2nd alarm fire, 3rd alarm fire, 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 alarm, 2nd alarm, 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. Engine companies, Ladder Companies, Rescue Companies, etc. respond to these disasters. Some units can act as Firefighter assist and search team (FAST) units
Below is how alarm levels are categorized in order per protocol. Each apparatus count is in an addition per alarm (a five alarm fire assignment has 21 engine companies total). Each total is the total number of units on scene. 
|Alarm levels and units assigned||1st Alarm fire / Box Alarm||1st Alarm fire/ "All Hands" Box Alarm||2nd Alarm fire||3rd Alarm fire||4th Alarm fire||5th Alarm fire|
|Engine Companies||3 Engine companies||4 engine companies||8 Engine companies||12 Engine companies||16 Engine companies||21 Engine companies|
|Ladder Companies||2 ladder companies||3 Ladder Companies
(operating as Firefighter assist and search team (FAST))units
|5 ladder companies||7 Ladder companies||9 Ladder companies||11 ladder companies|
|Battalion Chief||1 Battalion Chief||2 Battalion Chiefs
(one as a Firefighter Assist and Search Team (FAST) unit)
|5 Battalion Chiefs||6 Battalion Chiefs||6 Battalion Chiefs||6 Battalion Chiefs|
|Squad Companies||1 squad Company||1 Squad Company||1 Squad Company||1 Squad Company||1 Squad Company|
|Rescue Companies||1 Rescue Company||1 Rescue Company||1 Rescue Company||1 Rescue Company||1 Rescue Company|
|Division Chief||1 division Chief|
|Deputy Chief||1 Deputy Chief||1 Deputy Chief||1 Deputy Chief||1 Deputy Chief|
|RAC Unit||1 RAC Unit||1 RAC Unit||1 RAC Unit||1 RAC Unit|
|Satellite||1 Satellite||1 Satellite||1 Satellite||1 Satellite|
|Safety Battalion||1 Safety Battalion||1 Safety Battalion||1 Safety Battalion||1 Safety Battalion|
|SOC Battalion||1 SOC Battalion||1 SOC Battalion||1 SOC Battalion||1 SOC Battalion|
|Tactical Support Unit||1 Tactical Support Unit||1 Tactical Support Unit||1 Tactical Support Unit||1 Tactical Support Unit|
|Field Communications Unit||1 Field Communications Unit||1 Field Communications Unit||1 Field Communications Unit||1 Field Communications Unit|
|Field Communications Battalion||1 Field Communications Battalion||1 Field Communications Battalion||1 Field Communications Battalion||1 Field Communications Battalion|
|Communications Unit||1 Communications Unit||1 Communications Unit||1 Communications Unit||1 Communications Unit|
|Mask Service Unit||1 Mask Service Unit|
|Air Recon Chief
(on Brooklyn Box)
|1 Air Recon Chief||1 Air Recon Chief||1 Air Recon Chief|
|Mobile Command Unit||1 Mobile Command Unit||1 Mobile Command Unit|
|Planning Section Chief||1 Planning Section Chief||1 Planning Section Chief|
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 four engine companies, three ladder companies, one squad company, one rescue company, two battalion chiefs, and one division chief operating at the scene. If the fire is not large enough to require a 2nd alarm, but there is a need for more equipment and manpower, 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, one squad company, one rescue company, six battalion chiefs, one division chief, one deputy chief, one 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 firehouse. 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.
- MacIntyre-Yee, Tina (January 27, 2015). "Demolition of building near MAG to continue Wednesday". Democrat & Chronicle. Retrieved January 27, 2015.
- Engber, Daniel (4 May 2006). "How Big Is a '10-Alarm Fire'?". Slate. Retrieved 15 November 2016.
- "Paying For Donated Blood, Ethanol In Lawn Mowers, Numbered Alarm Fires". NBC4.com. 12 May 2006. Archived from the original on 23 November 2007. Retrieved 15 November 2016.
- "FDNY Dispatch Policy". FDNewYork.com. Retrieved 15 November 2016.