Hazardous Location Classifications
The NEC Article 500 system in three look-ups: what each Class covers and what separates Division 1 from Division 2, which gases and dusts sit in material groups A through G, and the T-code surface-temperature limits of Table 500.8(C). A complete area marking combines all three — Class, Division, Group, T code — and equipment must match every element.
Classes & Divisions
| Class | Material | Division 1 — normal conditions | Division 2 — abnormal only | Typical facilities |
|---|---|---|---|---|
| I | Flammable gases & vapors | Ignitible concentrations exist under normal operation, frequently during repair or leakage, or breakdown could release them while making equipment an ignition source | Liquids/gases confined in closed systems, kept clear by mechanical ventilation, or adjacent to a Division 1 area — hazardous only on accident or failure | Fuel transfer, spray booths, open solvent vats, drying rooms |
| II | Combustible dusts | Dust in suspension under normal operation in explosive quantities, or mechanical failure could create a cloud plus an ignition source, or Group E metal dust is present in hazardous quantities | Dust airborne only from abnormal operation, or accumulations sufficient to block heat dissipation or be ignited by equipment failure | Grain handling, coal prep, metal powder, flour & feed mills |
| III | Ignitible fibers & flyings | Easily ignitible fibers or materials producing combustible flyings are handled, manufactured, or used | Easily ignitible fibers are stored or handled, other than in the process of manufacture | Textile mills, cotton gins, woodworking, flax & rayon plants |
Material groups A–G
| Group | Class | Defining atmosphere | Representative materials |
|---|---|---|---|
| A | I | Acetylene | Acetylene (the only member) |
| B | I | Hydrogen-class · MESG ≤ 0.45 mm or MIC ratio ≤ 0.40 | Hydrogen & gases over 30% H₂, butadiene, ethylene oxide, propylene oxide, acroleinNEC exceptions allow Group C/D equipment for the four named vapors with explosionproof seals within 18 in. |
| C | I | Ethylene-class · MESG 0.45–0.75 mm or MIC 0.40–0.80 | Ethylene, ethyl ether, carbon monoxide, hydrogen sulfide, cyclopropane |
| D | I | Propane-class · MESG > 0.75 mm or MIC ratio > 0.80 | Propane, gasoline, natural gas / methane, ammonia, benzene, butane, ethanol, acetone, hexane |
| E | II | Combustible metal dusts | Aluminum, magnesium, and their commercial alloysDivision 1 only — the NEC lists Group E under 500.5(C)(1) with no Division 2 counterpart. |
| F | II | Carbonaceous dusts, > 8% entrapped volatiles | Coal, coke, charcoal, carbon black |
| G | II | Combustible dusts not in E or F | Flour, grain, wood, plastic, chemicals |
Temperature codes (T codes)
| T code | Max surface °C | Max surface °F |
|---|---|---|
| T1 | 450 | 842 |
| T2 | 300 | 572 |
| T2A | 280 | 536 |
| T2B | 260 | 500 |
| T2C | 230 | 446 |
| T2D | 215 | 419 |
| T3 | 200 | 392 |
| T3A | 180 | 356 |
| T3B | 165 | 329 |
| T3C | 160 | 320 |
| T4 | 135 | 275 |
| T4A | 120 | 248 |
| T5 | 100 | 212 |
| T6 | 85 | 185 |
How the four-part marking composes
Every classified area resolves to a four-part statement — Class (what form the material takes), Division (how often it's there), Group (which specific atmosphere), and the T code the equipment may not exceed. Class I, Division 1, Group B, T4 is a hydrogen process area where ignitible gas exists in normal operation and no surface may pass 135°C. Equipment carries the same four marks, and suitability is element-by-element: equipment suitable for Division 1 may serve Division 2 of the same class and group (equipment marked "Division 2" works in Division 2 only), the marked group must cover the gas actually present, and a cooler T code always satisfies a hotter requirement. What the tables above can't do is classify the area itself — drawing the Division 1/Division 2 boundaries around a tank, vent, or pit is engineering work under NFPA 497/499 and the facility standards (NFPA 30, 58, 820, API RP 500).
Why the groups are ordered by severity
The gas groups are a flame-propagation ranking, measured two ways: the maximum experimental safe gap (how tight an enclosure joint must be to quench an internal explosion before it lights the outside atmosphere) and the minimum igniting current ratio (how little spark energy the gas needs, relative to methane). Acetylene and hydrogen propagate flame through gaps that would safely quench a propane burn — which is why a Group D explosionproof box is not a Group B box, and why Group B gear is heavier, tighter, and costlier. The dust groups rank differently: Group E metal dusts are conductive (an electrical hazard on top of the ignition hazard, and the reason Group E exists only as Division 1), Group F carbonaceous dusts smolder, and Group G covers the organics and plastics that make up most dust-handling occupancies.
Common questions
What is the difference between Division 1 and Division 2?
When the hazard is present. Division 1 means ignitible concentrations exist under normal operating conditions — during ordinary production, transfer, or maintenance. Division 2 means the material is there but controlled: confined in closed systems, kept dilute by ventilation, or adjacent to a Division 1 area — it becomes ignitible only through accident, rupture, or equipment failure. The same plant usually contains both: Division 1 at the vent or open vat, Division 2 in the surrounding envelope.
How do I read a marking like "Class I, Division 2, Group D, T3"?
Four independent facts: the material form (Class I = gas/vapor), how often it is present (Division 2 = abnormal conditions only), how severe the specific atmosphere is (Group D = propane-class gases like gasoline and natural gas), and the hottest the equipment surface can run (T3 = 200°C max). Equipment is suitable only if every element matches the area classification — and the T code must not exceed the autoignition temperature of the actual gas present.
What do the T codes actually limit?
Maximum surface temperature of the equipment, in a 40°C ambient — because a hot surface can ignite a gas or dust with no arc at all. T1 allows up to 450°C, down to T6 at 85°C. Per NEC 500.8(D), the marked T code must not exceed the autoignition temperature of the specific gas, and for organic dusts that can dehydrate or carbonize the marking cannot exceed 165°C (T3B or lower). A lower number is not better equipment — it is a hotter allowance.
Where do Zones fit in?
Zones are the IEC-style alternative in NEC Article 505/506. Roughly: Zone 0 + Zone 1 together cover what Division 1 covers (Zone 0 being the continuously-hazardous core), and Zone 2 corresponds to Division 2. Gas groups map too: Groups A and B fall under IIC, Group C under IIB, Group D under IIA. A facility picks one system per area — the two schemes classify the same physics with different granularity.
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