Multiple-alarm fire

One-alarm fires, two-alarm fires, three-alarm fires, etc., are categories classifying the seriousness of fires, commonly used in the United States and in Canada, particularly 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 used by both fire departments and news agencies.

The most widely used formula for multi-alarm designation is based on the number of units, (for example firetrucks, tankers, rescue vehicles and command vehicles) and firefighters responding to a fire; the more vehicles and firefighters responding, the higher the alarm designation.

In terms of understanding the relative severity of an incident, the government of Rochester, New Hampshire has reported in a statement that a "typical fire in a small structure will usually require only one or two alarms". In contrast, however, a "large-scale fire in a commercial building would require three alarms or more."

Definition

When identifying the 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.

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.