Short answer: A hazardous (classified) location is described by three labels. The Class names what can burn: I for gases and vapors, II for combustible dust, III for ignitible fibers and flyings. The Division says when it is likely there: Division 1 under normal operation, or often during repair, maintenance or leakage; Division 2 mainly when something goes wrong, such as a leak, a failure or lost ventilation, and in areas next to Division 1. The Group narrows the material: A to D for Class I gases and E to G for Class II dusts. Class III does not use lettered groups. Equipment must be identified for all three, and its maximum surface temperature, shown by its temperature code, must not exceed the material’s ignition temperature (NEC 500.5).
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Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Area classification is decided by the people responsible for the facility’s design and safety, not by the installer. Check every rule and table value in your own 2026 NEC, and follow the classification drawings, your supervisor and the authority having jurisdiction.
Read the label in three parts
| Label | What it tells you | Examples |
|---|---|---|
| Class I | Flammable gases, flammable liquid-produced vapors or combustible liquid-produced vapors | Fuel transfer areas, paint spray areas, some process plants |
| Class II | Combustible dust | Grain elevators, flour and sugar handling, some metal powder work |
| Class III | Ignitible fibers or flyings | Textile mills and cotton gins where ignitible fibers or flyings are present |
| Division 1 | The hazard can exist under normal operation, or often during repair, maintenance or leakage | Inside a vapor zone near an open process |
| Division 2 | The hazard is mainly expected under abnormal conditions, such as a leak, a failure or lost ventilation | Areas next to Division 1 or around closed systems |
The classes and divisions are defined in Article 500 (NEC 500.5). Article 501 covers Class I installations, 502 covers Class II and 503 covers Class III.
Groups: which gas or dust
Groups sort materials by how they behave when they ignite (NEC 500.6).
| Group | Type | Typical material |
|---|---|---|
| A | Gas | Acetylene |
| B | Gas | Hydrogen |
| C | Gas | Ethylene |
| D | Gas | Propane, gasoline |
| E | Dust | Combustible metal dusts |
| F | Dust | Carbonaceous dusts such as coal and coke |
| G | Dust | Other combustible dusts, such as grain and flour |
Equipment is identified for specific groups. A fitting identified for Group D is not automatically suitable for Group B.
Zones are a second system
The NEC also allows a Zone system: Zones 0, 1 and 2 for gases and vapors in Article 505, and Zones 20, 21 and 22 for dusts in Article 506 (NEC 505.5). Zone 0 is the most severe gas zone, where the atmosphere is present continuously or for long periods. The Division and Zone systems use different rules and markings, so read which system the drawings use before you choose equipment.
Protection techniques
Equipment can be made safe in several ways (NEC 500.7). The common ones are:
- Explosionproof. The enclosure can contain an internal explosion and cool the escaping gases so they do not ignite the surrounding atmosphere. Used in Class I.
- Dust-ignitionproof. The enclosure keeps dust out and keeps internal heat or arcs from igniting dust on or around it. Used in Class II.
- Purged and pressurized. Clean air or inert gas keeps the hazardous atmosphere out of the enclosure.
- Intrinsically safe. The circuit’s energy is limited so it cannot ignite the atmosphere, even with a fault.
- Nonincendive. Used in Division 2. The equipment or circuit is designed and identified so that in normal operation it cannot ignite the atmosphere. Check the marking and the installation conditions.
Temperature codes: cooler is a higher number
Hot surfaces can ignite a gas without any spark. Equipment is marked with a temperature code that shows its maximum surface temperature, and that temperature must not exceed the autoignition temperature of the gas or vapor present (NEC 500.8). The codes run from T1, the hottest allowed surface, down through subdivided codes such as T2A to T2D and T3A to T3C, to T6, the coolest (NEC Table 500.8(C)(4)). A higher number means a cooler maximum surface.
| Code | Maximum surface temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T2C | 230°C |
| T2D | 215°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
This is a selection of rows, not the full table. Open the full table in your 2026 NEC, which also lists the subdivided codes not shown here.
Wiring methods and seals in Class I, Division 1
- Wiring methods. Class I, Division 1 limits you to specific methods, such as threaded rigid metal conduit, threaded intermediate metal conduit and certain listed cables (NEC 501.10). Check the full list and conditions in 501.10.
- Threaded joints. Threaded conduit and fittings must be made up wrenchtight, and threaded entries into explosionproof equipment generally need at least five full threads engaged (NEC 500.8).
- Seals. Conduit seals limit the passage of gas and vapor and stop flame from traveling through the conduit from one part of the system to another. They are not gas-tight barriers against continuous pressure. A seal is generally required at an enclosure entry that contains arcing or high-temperature devices, placed within the distance the code allows, and where conduit leaves the classified area (NEC 501.15). Seals are poured with a compound identified for the purpose after the conductors are pulled, and the compound needs a minimum thickness. Read 501.15 for the distances, the minimum thickness and the exceptions.
Worked example: picking a temperature code
A Class I, Division 1, Group D area handles a vapor whose autoignition temperature is 220°C. Which temperature codes from the table above can a luminaire have?
- The equipment’s maximum surface temperature must not exceed 220°C.
- T1 (450°C), T2 (300°C) and T2C (230°C) all exceed 220°C. They are not acceptable.
- T2D (215°C) does not exceed 220°C. T3 (200°C), T4 (135°C), T5 (100°C) and T6 (85°C) are cooler still. All of these meet the temperature limit. The luminaire must also be identified for the Class, Division and Group.
- Check: 215 ≤ 220, and 230 > 220. The boundary falls between T2C and T2D.
The luminaire also needs to be identified for Class I, Division 1, Group D. A correct temperature code alone does not make it suitable.
Check yourself
- Grain dust handling area: which class? Class II.
- A gas is in closed piping and escapes only if a joint leaks. Which division does that describe? Division 2. The site’s classification drawings make the final call.
- Which runs cooler, a T3 or a T5 marking? T5. A higher number means a cooler maximum surface.
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Mark Articles 500 and 501 before exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight the class, division and group definitions in 500 and the seal rules in 501, since hazardous location questions usually start with the label. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is the difference between Class I, Class II and Class III?
Class I locations have flammable gases or vapors, Class II locations have combustible dust, and Class III locations have ignitible fibers or flyings. The class tells you what can burn; the division tells you how often it is present.
What is the difference between Division 1 and Division 2?
In Division 1 the hazardous material can be present under normal operating conditions, or often during repair, maintenance or leakage. In Division 2 it is mainly expected only under abnormal conditions, such as a leak, a failure or a ventilation breakdown, and the area next to a Division 1 location can also be Division 2.
What does a T code mean on hazardous location equipment?
The temperature code shows the equipment’s maximum surface temperature. It must not exceed the autoignition temperature of the gas or vapor present. A higher T number means a cooler surface.
Are hazardous locations on the Texas Journeyman exam?
PSI lists special occupancies in the Special Occupancies, Equipment and Conditions subject area, which has 6 scored NEC Knowledge questions. PSI does not say how many cover hazardous locations alone.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 500, 501 and 505
