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Hazardous Location Signage Temperature

T-Codes (Temperature Ratings) for Explosion-Proof Egress Fixtures — Plain-English Guide

T-codes tell you the maximum surface temperature a fixture can reach so it won’t ignite the surrounding atmosphere. In hazardous areas, your emergency lights and EXIT signs must be both hazardous-location listed (ignition safety) and capable of UL 924 egress performance. This guide explains T-codes in plain English—how they work, how ambient temperature (Ta) changes them, differences for gas vs dust, and a simple selection flow you can use with your AHJ.

Last updated: October 2025

Hazardous Emergency Lights Explosion-Proof Combo Buyer’s Guide (Pillar)

Educational Guide UL 844 • UL 924 • NEC 500–516 For Facility Managers & Electrical Contractors

T-Codes (temperature ratings) diagram for explosion-proof egress fixtures

T-Code Basics (What It Is & Why It Matters)

A T-code is a temperature class that caps how hot a device’s surface is allowed to get during normal operation. To be safe, the device’s maximum surface temperature must be below the auto-ignition temperature of the surrounding gas/vapor (or below the dust ignition limits for powders/fibers). In egress applications, you need both:

  • UL 844 hazardous-location listing (device won’t ignite the atmosphere), and
  • UL 924 emergency lighting listing (transfer to battery, ~90-minute runtime, EXIT visibility).

Code context: Hazardous-Location Code Checklist

T-Code Table (°C / °F)

Common North American T-codes for gas/vapor (Class I) areas:

T-Code Max Surface Temp (°C) Max Surface Temp (°F) Plain-English Note
T1 450 842 Hottest allowed
T2 300 572 —
T2A 280 536 —
T2B 260 500 —
T2C 230 446 —
T2D 215 419 —
T3 200 392 Common with some fixtures
T3A 180 356 —
T3B 165 329 —
T3C 160 320 —
T4 135 275 Very common for LED egress
T4A 120 248 Even cooler surface
T5 100 212 —
T6 85 185 Coolest class

Ambient Temperature (Ta) & Nameplate Reading

A T-code only applies at the device’s rated ambient (often shown as Ta on the label). Example: “T4 at Ta 40 °C” means the max surface is ≤135 °C when the surrounding air is ≤40 °C. If your space runs hotter (e.g., 55 °C), you must use a device rated for that ambient (the same model may be marked differently at higher Ta). Always read the full nameplate: Class/Division/Group, T-code, and Ta.

Installation guide: Installing Explosion-Proof Lighting

Gas (Class I) vs Dust/Fibers (Class II/III)

  • Class I (gases/vapors): Select by comparing T-code to the lowest auto-ignition temperature present; ensure the device’s surface temp limit is lower.
  • Class II/III (dusts/fibers): Markings often show a maximum surface temperature in °C rather than a T-code letter. Selection considers both dust cloud and dust layer ignition limits and even layer thickness. Keep surfaces clean—dust layers insulate and raise surface temperature.
  • Groups matter: Ensure the device is listed for the right gas or dust group (e.g., IIA/IIB/IIC or E/F/G) in addition to temperature limits.

Primer: Class, Division & Group — Plain-English Primer

Simple Selection Flow

  1. List hazards & AITs: Identify the lowest auto-ignition temperature (or dust ignition limits) in each zone.
  2. Pick temperature class: Choose a T-code (or °C marking for dust) with a comfortable margin below that value.
  3. Confirm ambient (Ta): Ensure the device’s nameplate T-code applies at your hottest expected ambient.
  4. Verify Class/Div/Group: The hazardous-location listing must match your area classification.
  5. Egress performance: For lights/signs, verify UL 924 (transfer + ~90-minute runtime) and plan aiming to meet AHJ foot-candle targets.

Inspection prep: What Inspectors Check

Installation & Maintenance Effects on Surface Temp

  • Dust layers & grime: Act like a blanket—increase surface temperature. Clean lenses and housings on a schedule.
  • Seals & flame paths: Damaged gaskets or contaminated flame-path surfaces can change thermal behavior—keep them pristine and properly torqued.
  • Ambient drift: Seasonal heat or enclosed mezzanines may exceed Ta—verify conditions and adjust spec if needed.
  • Battery heat: Charging can add heat; follow OEM guidance and allow recharge before long tests.

Upkeep tips: Maintenance Tips for Explosion-Proof Fixtures

Quick FAQ

Is T4 always “better” than T3?

“Cooler” classes (e.g., T4/T4A) allow lower surface temperatures, which is safer for many gases. But you must still match Class/Division/Group and ambient.

Do dust areas use T-codes?

Often they use a maximum surface temperature in °C instead. Use the lower of the dust cloud or dust layer limits (accounting for layer thickness) and keep surfaces clean.

Why does my device show different T-codes in different catalogs?

The same family may carry different T-codes at different ambients (Ta) or configurations. Always go by the installed unit’s nameplate.

Printable T-Code Cheat Sheet

  • Identify lowest AIT (gas) or dust ignition limits per zone
  • Select T-code / max surface °C with margin below those limits
  • Confirm Ta (ambient) on the nameplate matches site conditions
  • Verify Class/Div/Group and UL 844; for egress, verify UL 924
  • Keep surfaces clean; log monthly 30-sec & annual 90-min tests

This guide is informational and not a substitute for professional engineering judgment or the authority of your AHJ.