NEC 500 ↔ Zone Classification Converter
Translate hazardous location classifications between the US system and the international Zone system. Built for engineers, procurement teams, and anyone trying to understand a marking they've never seen before.
What is this for?
The United States uses one system to label hazardous areas (NEC Article 500 — the "Class, Division, Group" system). Most of the rest of the world — and NEC Article 505, the alternative US system — uses the Zone system (also called IEC, ATEX, or IECEx, depending on the certifying body).
Both systems describe the same kinds of hazards — flammable gases, combustible dusts, and ignitable fibers — but they use different terminology and slightly different category boundaries. If you're buying European equipment for a US facility, reading an ATEX datasheet, or specifying for an international project, you need to translate between them.
Pick the system you have a classification for, enter what you know, and this tool will translate it to the equivalent in the other system — with explanations of what each marking actually means, the materials and facility types it applies to, and important caveats about why "equivalent" doesn't always mean "interchangeable."
⚠ Important: "Equivalent" is not "Interchangeable"
This tool translates between two classification systems that describe hazards in different ways. It does not certify that equipment rated under one system is acceptable for installation under the other. Equipment certified for ATEX or IECEx is not automatically approved by a US Authority Having Jurisdiction (AHJ), and vice versa. Always confirm equipment listings, marking requirements, and acceptance with a qualified electrical engineer and your local AHJ before installation.
Complete Conversion Reference
The full mapping between the US (NEC 500) and international (NEC 505 / IEC / ATEX) systems:
Class I — Flammable Gases and Vapors
| US (NEC 500) | International (NEC 505 / IEC) | Representative Materials |
|---|---|---|
| Class I, Div 1, Group A | Zone 0 or 1, Group IIC | Acetylene |
| Class I, Div 1, Group B | Zone 0 or 1, Group IIC | Hydrogen, butadiene, ethylene oxide |
| Class I, Div 1, Group C | Zone 0 or 1, Group IIB | Ethylene, ether, cyclopropane |
| Class I, Div 1, Group D | Zone 0 or 1, Group IIA | Propane, methane, gasoline, natural gas |
| Class I, Div 2, Group A | Zone 2, Group IIC | Acetylene |
| Class I, Div 2, Group B | Zone 2, Group IIC | Hydrogen, butadiene, ethylene oxide |
| Class I, Div 2, Group C | Zone 2, Group IIB | Ethylene, ether, cyclopropane |
| Class I, Div 2, Group D | Zone 2, Group IIA | Propane, methane, gasoline, natural gas |
Class II — Combustible Dusts
| US (NEC 500) | International (NEC 506 / IEC) | Representative Materials |
|---|---|---|
| Class II, Div 1, Group E | Zone 20 or 21, Group IIIC | Combustible metal dusts (aluminum, magnesium) |
| Class II, Div 1, Group F | Zone 20 or 21, Group IIIB | Carbonaceous dusts (coal, coke, carbon black) |
| Class II, Div 1, Group G | Zone 20 or 21, Group IIIB | Grain, flour, sugar, plastic, wood, chemical dusts |
| Class II, Div 2, Group E | Zone 22, Group IIIC | Same as Div 1, but only present abnormally |
| Class II, Div 2, Group F | Zone 22, Group IIIB | Same as Div 1, but only present abnormally |
| Class II, Div 2, Group G | Zone 22, Group IIIB | Same as Div 1, but only present abnormally |
Class III — Ignitable Fibers and Flyings
| US (NEC 500) | International (IEC) | Representative Materials |
|---|---|---|
| Class III, Div 1 | Zone 22, Group IIIA | Cotton, rayon, jute, sawdust, textile fibers |
| Class III, Div 2 | Zone 22, Group IIIA | Storage and handling of fiber/flying materials |
Background: How the Two Systems Work
NEC Article 500 — The US System
NEC Article 500 has been the US standard for classifying hazardous locations since the 1930s. It uses a three-part label: Class (what hazard type), Division (how often it's present), and Group (which specific materials).
- Class I = flammable gases, vapors, or mists are present
- Class II = combustible dust is present
- Class III = ignitable fibers or flyings are present
- Division 1 = hazard is present continuously, intermittently, or periodically under normal operating conditions
- Division 2 = hazard is present only under abnormal conditions (equipment failure, accidental release)
Groups divide further by the actual material — see the reference tables above.
NEC Article 505 / IEC / ATEX — The Zone System
The Zone system was developed by the International Electrotechnical Commission (IEC) and adopted by the European Union as ATEX, by most other countries as IECEx, and added to the US National Electrical Code in 1996 as NEC Article 505.
Instead of "Class + Division," the Zone system uses a single number (0, 1, or 2 for gases; 20, 21, or 22 for dust) that combines the hazard type and likelihood:
- Zone 0 / 20 = hazard present continuously or for long periods
- Zone 1 / 21 = hazard likely present in normal operation
- Zone 2 / 22 = hazard not likely under normal operation, only abnormally
Groups in the Zone system are IIA, IIB, IIC for gases (with IIC the most demanding, covering hydrogen and acetylene), and IIIA, IIIB, IIIC for dusts (with IIIC for conductive metal dusts).
Why don't the systems map cleanly?
The two systems were developed independently and chose different boundaries for their categories. Two specific places they don't line up cleanly:
- US Division 1 = IEC Zone 0 OR Zone 1. The US lumps "continuously present" and "present in normal operation" into one category. The IEC splits them apart. So going from Division 1 to the Zone system, you need to know which zone applies based on whether the hazard is truly continuous.
- US Groups A and B both = IEC Group IIC. The IEC put hydrogen and acetylene into the same group (IIC) because they have similar minimum ignition energies. The US separates them into Group A (acetylene) and Group B (hydrogen). Going from IIC back to the US system, you need to know whether the actual material is acetylene or hydrogen.
This tool surfaces these ambiguities in the result so you don't get caught by them.
For a deeper explanation of the practical differences between these systems, see our NEC 500 vs. NEC 505 comparison guide.
Need help specifying explosion-proof HVAC across systems?
We design and modify explosion-proof air conditioning for facilities in the US and internationally, including dual-system documentation when you need both NEC 500 and IEC/ATEX markings for cross-border projects.