NFPA 72 Table 17.6.2.1 defines seven fixed-temperature heat detector classifications with paired rating ranges and color marks: Low (100 to 134 degrees Fahrenheit, 39 to 57 degrees Celsius), Ordinary (135 to 174 degrees Fahrenheit, 58 to 79 degrees Celsius), Intermediate (175 to 249 degrees Fahrenheit, 80 to 121 degrees Celsius), High (250 to 324 degrees Fahrenheit, 122 to 162 degrees Celsius), Extra High (325 to 399 degrees Fahrenheit, 163 to 204 degrees Celsius), Very Extra High (400 to 499 degrees Fahrenheit, 205 to 259 degrees Celsius), and Ultra High (500 to 575 degrees Fahrenheit, 260 to 302 degrees Celsius) [S3][S5].
Color marks apply only from Intermediate up: white, blue, red, green, and orange, respectively. Low and Ordinary detectors carry no color mark, a fact that routinely trips first-time inspectors who assume every class has a hue [S2][S5].
Rating range versus maximum ceiling temperature
Selection is governed by the maximum-ceiling column, not the rating range: Low allows a 80 degrees Fahrenheit ceiling, Ordinary 100 degrees Fahrenheit, Intermediate 150 degrees Fahrenheit, High 225 degrees Fahrenheit, Extra High 300 degrees Fahrenheit, Very Extra High 375 degrees Fahrenheit, and Ultra High 475 degrees Fahrenheit [S5]. The detector's actual alarm setpoint sits well above the ceiling limit, so normal ambient swings never nuisance-trip the device. NFPA 72 further requires the rating to clear the highest expected ambient by at least 20 degrees Fahrenheit, a margin that drives the step from Ordinary to Intermediate in boiler rooms, kitchens, and unconditioned attics [S2][S5].
This dual-column design is the reason a boiler room running at 130 degrees Fahrenheit ambient must specify an Intermediate (white) detector rather than an Ordinary unit, even though the fire signature a designer wants to catch is identical in both cases.
Where each classification fits a real space
Ordinary (uncolored, 135 to 174 degrees Fahrenheit rating, 100 degrees Fahrenheit ceiling cap) is the default in offices, corridors, and most occupied areas. Intermediate (white, 175 to 249 degrees Fahrenheit rating, 150 degrees Fahrenheit ceiling) is specified in boiler rooms, commercial kitchens, laundry areas, and attics where summer peaks push ambient above 100 degrees Fahrenheit. High (blue, 250 to 324 degrees Fahrenheit rating, 225 degrees Fahrenheit ceiling) is used in foundries, industrial drying ovens, and similar processes where 150 degrees Fahrenheit ambient is routine. Extra High (red, 325 to 399 degrees Fahrenheit, 300 degrees Fahrenheit ceiling) and above are reserved for foundries, kiln rooms, and steel-mill ladle areas where the rating step is the only way to keep the detector in a non-alarm state during normal operation [S2][S5].
The link to heat detector selection is direct: the color code is not a fire-severity dial, it is a thermal-budget indicator tied to the highest normal ambient the enclosure can reach.
Fixed, rate-of-rise, and rate-compensation detectors under the same code
Color codes apply to fixed-temperature elements regardless of the detection family. Rate-of-rise detectors sense a temperature climb above a preset slope, typically 15 degrees Fahrenheit per minute, and pair the rate element with a fixed-temperature backup. Rate-compensation detectors differ by reading the surrounding air temperature rather than an internal element, reducing thermal lag in outdoor or unconditioned enclosures where ambient swings widely [S2]. All three families, when shipped as fixed-temperature units, must carry the Table 17.6.2.1 color mark of the class they belong to.
Engineers weighing spot-type versus line-type coverage can compare field layout and spacing rules in the line-type vs spot-type selection guide, which addresses the same classification table but from a coverage-geometry angle.
Common field errors and inspection traps
The most frequent misread is treating an uncolored detector as a missing label rather than a Low or Ordinary unit. The second is reading the rating range as the ceiling cap, which leads to specifying an Ordinary detector in a 110 degrees Fahrenheit attic and watching it fault on the first hot afternoon. The third is conflating Extra High (red) with Very Extra High (green): at a glance, both read as red under shop lighting, but the 75 degrees Fahrenheit gap in rating range matters in any process space where ambient routinely crosses 300 degrees Fahrenheit [S2][S5].
UL Solutions' testing program for in-service heat detectors references NFPA 72 Chapter 14 inspection, testing, and maintenance requirements, which rely on the color code as the primary visual cue for the replacement class when a device is being swapped without paperwork [S1].
Standards and verification chain
NFPA 72 Table 17.6.2.1, 2022 edition, is the published reference for the seven classifications, the rating ranges in Fahrenheit and Celsius, the maximum-ceiling values, and the color codes [S3][S5]. UL Solutions publishes the testing service that laboratories use to verify detector performance against that table, including response within the rated temperature window [S1]. Where the application is also a safety instrumented function on a fired process, the same selection flows into the heat treatment furnace protection layer, but the color-code table itself is unchanged.
Both run on the standard's normal three-year revision cadence rather than a fixed calendar date.
Component reference pages worth checking: coding machine.