Class/Division/Group (plus T-codes) is the language AHJs use to decide what can be installed in hazardous areas. If you’re choosing emergency lights, EXIT signs, or combos for a classified space, this plain-English primer explains how the system works, what to read on the nameplate, and a simple selection flow that helps facility managers and electrical contractors avoid red tags.
Last updated: October 2025
At-a-Glance: How the System Works
Think of it like an address:
- Class = the type of hazard (gas/vapor, dust, fibers).
- Division = the likelihood the hazard is present (normal vs abnormal).
- Group = the specific substance family (e.g., IIA/IIB/IIC gases; E/F/G dusts).
- T-code = the maximum surface temperature the device can reach without igniting the atmosphere.
Classes (I/II/III)
- Class I: Flammable gases/vapors (refineries, paint mix rooms, pump stations).
- Class II: Combustible dusts (grain mills, flour processing, coal handling).
- Class III: Ignitable fibers/flyings (textiles, woodworking, some packaging lines).
Divisions (1 vs 2)
- Division 1: Hazard likely/continuous during normal operations (e.g., spray booth interior, digester headspace).
- Division 2: Hazard present only abnormally/occasionally (e.g., adjacent storage or transfer areas).
Groups (IIA/IIB/IIC, E/F/G)
- Gases/Vapors (Class I): IIA (propane), IIB (ethylene), IIC (hydrogen/acetylene — most stringent).
- Dusts (Class II): E (metals), F (carbon/coal), G (grain/wood/flour).
Match your fixture’s hazardous-location listing to the exact Group in your area classification—more than one Group may apply across a plant.
Temperature ratings: T-Codes for Explosion-Proof Fixtures
Reading the Nameplate (UL 844 • UL 924 • T-code • Ta)
- UL 844 (hazardous-location): Class/Division/Group marking must match your area.
- UL 924 (egress): Required for emergency lights/EXIT signs—transfer to battery and ~90-minute runtime.
- T-code: Must be below the auto-ignition temperature (or dust ignition limit) in your atmosphere.
- Ambient (Ta): T-codes apply at a rated ambient (e.g., “T4 at Ta 40 °C”). Hotter spaces may require different models.
Compliance steps: Hazardous-Location Code Checklist
Selection Flow for Emergency Lights & Exit Signs
- Identify hazards per area (Class, likely Division, and Group).
- Pick T-code (or dust °C limit) with margin below ignition thresholds.
- Confirm Ta (ambient) on the nameplate fits your site’s hottest conditions.
- Verify listings (UL 844 + UL 924 for egress) on the exact model/option set.
- Plan photometrics: combos at doors; add hazardous-location lights for long runs/high bays; verify 90-minute performance.
- Document: cut sheets, nameplate photos, and test forms for the AHJ.
Installation: Installing Explosion-Proof Lighting
Myths vs Facts
- Myth: “NEMA 4X/IP66 means explosion-proof.” Fact: Weather ratings ≠ hazardous-location listing.
- Myth: “Any Class I device works anywhere with gas.” Fact: Group and T-code still must match the specific gas and ambient.
- Myth: “UL 924 alone is enough.” Fact: You need UL 844 and UL 924 for egress fixtures in classified areas.
Compare terms: Explosion-Proof vs Wet-Location vs Outdoor
Quick Reference Table
| Element | Defines | Options | Common Examples |
|---|---|---|---|
| Class | Hazard type | I (gas/vapor) • II (dust) • III (fibers) | Refinery (I), grain mill (II), textile (III) |
| Division | Likelihood | Div 1 (normal) • Div 2 (abnormal) | Spray booth (1), adjacent storage (2) |
| Group | Substance family | IIA/IIB/IIC (gases) • E/F/G (dusts) | Propane (IIA), ethylene (IIB), H₂ (IIC); grain (G) |
| T-code | Max surface temp | T1 (450 °C) → T6 (85 °C) | T4/T4A common on LED egress fixtures |
Quick FAQ
Do I always need UL 924 and UL 844?
For egress lights and EXIT signs in classified areas, yes—UL 844 for ignition safety and UL 924 for emergency performance.
Can a Div 2 device be used in Div 1?
No. Div 1 requires equipment listed for Div 1 (or the equivalent Zone rating). Always match the nameplate to the area classification.
Where do Zones fit?
Zones are another classification system (Articles 505/506). Many AHJs accept either, but stay consistent within a project and match listings.
Printable Cheat Sheet
- Identify Class, Division, and Group per area
- Select T-code / °C limit with margin; confirm Ta (ambient)
- Verify UL 844 (haz-loc) and UL 924 (egress) on the exact model
- Plan photometrics and 90-minute testing
- Save cut sheets, nameplate photos, and test logs for the AHJ
This guide is informational and not a substitute for professional engineering judgment or the authority of your AHJ.