Hazardous-area markings exist in two parallel worlds. North America uses the NEC Class/Division system (NEC Article 500) and, in parallel, the Zone system (Articles 505/506). The rest of the world uses the IEC/IECEx Zone system, and the European Union adds the ATEX marking on top. The systems describe the same physics with different letters and numbers, and the translation between them is not a clean one-to-one map.
This guide explains what the Hazardous Area Code Translator decodes: how to read a NEC marking and an Ex string, why the gas-group lettering runs backwards between the two systems, why a Division is not a single Zone, and what the temperature class and the equipment protection level actually mean. It is marking interpretation only, not area classification, equipment approval, IEC 60079 installation review, ATEX conformity assessment, or AHJ/notified-body acceptance.
1. Two Systems for the Same Physics
Both systems classify three things: what the hazard is, how often it is present, and how severe it is.
NEC Class/Division: the Class is the hazard (Class I gas, Class II dust, Class III fibres). The Division is how often the hazard is present (Division 1 in normal operation, Division 2 only under fault). The Group is the severity (gas Groups A through D; dust Groups E, F, G). A marking reads "Class I Division 1 Group D T3".
IEC/IECEx Zone: the equipment Group is the hazard category (Group II surface gas, Group III dust, Group I mining). The Zone is how often the hazard is present (Zone 0 continuous, Zone 1 occasional, Zone 2 fault-only for gas; Zone 20/21/22 for dust). The gas subdivision is the severity (IIA, IIB, IIC). A marking reads "Ex db IIB T4 Gb".
ATEX: the EU marking adds an equipment block before the Ex string: "II 2 G" means equipment group II (surface), category 2, gas. The category maps to the zone with an off-by-one (category 1 for Zone 0, 2 for Zone 1, 3 for Zone 2).
2. The Gas Groups Run Backwards
This is the single most error-prone part of any hazloc cross-reference. The gas-group lettering runs in opposite order between the two systems.
| NEC group | Example gas | IEC group | Direction note |
|---|---|---|---|
| A | acetylene | IIC | A and B both map to IIC |
| B | hydrogen | IIC | IIC back to NEC is ambiguous (A or B) |
| C | ethylene | IIB | unambiguous both ways |
| D | propane, methane | IIA | unambiguous both ways |
In the NEC, Group A (acetylene) is the most easily ignited and the severity drops to Group D (propane). In the IEC, the order is reversed: IIA is the least severe and IIC is the most severe. So NEC Group D maps to IEC IIA, and NEC Group A maps to IEC IIC.
It gets worse going one way. NEC Group A (acetylene) and Group B (hydrogen) both collapse into the single IEC group IIC. So a US nameplate marked "Groups A, B, C, D" becomes "IIC" in IEC terms and loses the A-versus-B detail. Coming back, an IEC IIC device is ambiguous in NEC terms: it could be Group A or Group B, and you need to know whether the application is acetylene or hydrogen to pick. The translator flags this loss and ambiguity on every hop.
3. A Division Is Not a Single Zone
The NEC Division system has two area grades; the IEC Zone system has three. They do not line up one-to-one.
- Class I Division 1 spans both Zone 0 (hazard present continuously or for long periods) and Zone 1 (hazard present occasionally in normal operation).
- Class I Division 2 corresponds to Zone 2 (hazard present only under fault conditions).
So a Division-to-Zone translation is approximate. Division 1 alone cannot tell you whether the Zone-side requirement is Zone 0/EPL Ga or Zone 1/EPL Gb. For dust, Division 1 spans Zone 20 and Zone 21, and Division 2 corresponds to Zone 22. The translator flags the Division-to-Zone hop as lossy and leaves suitability to the certificate, governing code basis, and qualified review.
One more terminology trap: NEC "Class II" means combustible dust, while IEC "Group II" means surface gas. Same Roman numeral, opposite meaning. Read the whole marking, not the token.
4. Reading an Ex String Position by Position
An IEC Ex string is read left to right. Take "Ex db IIB T4 Gb":
- Ex is the prefix (IECEx). AEx is the US Article 505 zone marking; EEx is the legacy CENELEC marking.
- db is the type of protection: "d" is flameproof (it contains an internal explosion and cools the escaping gases below ignition), and the "b" sets the equipment protection level Gb. Other common types are e (increased safety), i / ia / ib / ic (intrinsic safety), p (pressurization), m (encapsulation), o (oil immersion), q (powder filling), n (type n for Zone 2), and t (dust protection).
- IIB is the gas group (ethylene).
- T4 is the temperature class (maximum surface temperature 135 degrees C).
- Gb is the equipment protection level: gas, high protection, normally associated with Zone 1 and ATEX category 2G in the marking scheme.
Translated to the NEC system, "Ex db IIB T4 Gb" is "Class I Division 1 Group C T4", unambiguous because IIB maps cleanly to Group C and Gb (Zone 1) corresponds to Division 1. Change the group to IIC and the NEC side becomes "Group A or B", ambiguous.
Hazardous Area Code Translator
Decode a NEC Class/Division marking, an IEC/IECEx Ex string, or an ATEX marking, and translate between the systems. Surfaces the reversed gas-group lettering (NEC Group A is IEC IIC) and the lossy or ambiguous hops most cross-references hide.
5. Temperature Classes Are Identical
The temperature class is the one part that is the same in both systems. It is the maximum surface temperature the equipment is allowed to reach, and it must stay below the autoignition temperature of the gas or dust present.
| T-class | Max surface temp |
|---|---|
| T1 | 450 degrees C |
| T2 | 300 degrees C (NEC subdivisions T2A 280, T2B 260, T2C 230, T2D 215) |
| T3 | 200 degrees C (NEC subdivisions T3A 180, T3B 165, T3C 160) |
| T4 | 135 degrees C |
| T5 | 100 degrees C |
| T6 | 85 degrees C |
The lower the number, the hotter the surface allowed. The NEC adds the finer subdivisions (T2A through T3C) that the IEC does not use. On a dust marking the surface temperature is often written directly (T80C means 80 degrees C) instead of a T-class, and it must stay below the dust layer and cloud ignition temperatures.
6. Using the Translator With This Guide
Open the Hazardous Area Code Translator and type a marking. NEC: "Class I Div 1 Group D T3". IEC: "Ex db IIB T4 Gb". ATEX: "II 2 G Ex d IIC T4 Gb". Dust: "Ex tb IIIC T80C Db". It decodes each position, names the system and the hazard, and translates to the other system with the lossy and ambiguous hops flagged and explained.
Use the translation to understand what an imported marking appears to say. Do not use it to classify a hazardous area, select or approve equipment, verify listings, approve installation, energize equipment, authorize hot work, or replace governing-code review. Those decisions require the current code basis, product certificate, manufacturer data, and qualified AHJ/notified-body review. PDF and CSV export carry the full decode, source pointers, and source-boundary warnings for records.