Class I locations*

Flammable gases, vapors or liquids

Locations in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. (“Gases” refers to materials that are in a gaseous state under normal atmospheric conditions. Examples of this would be hydrogen or methane. “Vapors” refers to the gases over a material that is a liquid under normal atmospheric conditions but emits gases within the flammable range under these atmospheric conditions.)

Division 1

Locations are those where the volatile flammable gases or vapors exists under normal conditions, or where volatile flammable gases or vapors may exist frequently because of repair or maintenance operations or leakage, or where breakdown or faulty operation of electrical equipment or processes might release ignitable concentrations of flammable gases or vapors, and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become the source of ignition. An example of this might be an area where a flammable liquid is stored under cryogenic conditions, and a leak directly into the electrical equipment could cause a failure of the electrical equipment at the same time the vapors of the evaporating liquid are within the flammable range.)

Division 2

Locations are those locations where ignitable concentrations of flammable gases or vapors are not likely to exist under normal conditions, or in locations that are adjacent to Class I, Division 1 locations where there are no barriers or partition to separate the Division 1 space from a non-hazardous location or where ventilation failure extend the area where flammables exist under normal conditions.


The gases and vapors of Class I locations are broken into four groups (NEC 500-503), by the codes A, B, C & D. These divisions are because different materials have different explosion and ignition characteristics. The grouping permits equipment to be tested based on the type of flammable material in which it is intended to be used. It also permits area classification to be based on the type of material anticipated in that location. The grouping is based on two major factors: the explosion pressure generated during an explosion; and the maximum gap between ground flat mating metal surfaces that will prevent propagation of an explosion through the gap to a flammable atmosphere of the same flammable material and concentration. This chart shows common gases in Class I and how they differ by ignition temperature and explosive pressure:

  NFPAC IgnitionF IgnitionHydrostaticZone
Class I Group Temperature Temperature Pressure * Gas Groups
Acetylene A 305 581 6,000 PSI IIC
Hydrogen B 520 968 6,000 PSI IIC
Ethylene C 450 842 1,200 PSI IIB
Gasoline D 280 536 600 PSI IIA

* Typical tested hydrostatic pressure required for electrical fittings such as unions, elbows, seals, etc.

Ambient temperature is the surrounding temperature of the environment in which a piece of equipment is installed, whether it is indoors or outdoors. A heat producing product is considered acceptable for the location, provided the minimum ignition temperature of the hazardous material present and the ambient temperature of the location do not exceed the limits set by the manufacturer. If the ambient temperature is higher than the maximum stated on the nameplate, it might still be acceptable to use the product under certain conditions, providing the minimum ignition temperature of the hazardous material has not been exceeded. In all cases, consult the factory for assistance.

Operating temperature
The rated operating temperature for hazardous (classified) products is determined by conducting laboratory tests in an ambient temperature of 40 ° C. Products certified by the various agencies consider products certified to their standards to be suitable for different temperature ranges. The range for CSA ( Canada) is -50 ° C to +40 C to +40 ° C; the range for UL ( United States) is -25 ° C to +40 ° C; the range for IEC and CENELEC (International) is -20 ° C to +40 ° C.

Temperature Codes (T-Codes)
The ignition temperature or auto-ignition temperature (AIT) is the minimum temperature required to initiate or cause self-sustained combustion in a substance without any apparent source of ignition. The lowest published ignition temperature should be the one used to determine the acceptability of equipment. This is of particular concern when selecting heat producing equipment such as lighting fixtures or motors which could generate sufficient heat to ignite the surrounding atmosphere.

Class I and Class II areas use T-Codes or are subject to maximum temperature limitations as shown in the following chart. North America and the IEC are consistent in their temperature or T-Codes. However, unlike the IEC, North America includes incremental values as shown. Equipment tested must have nameplates and marked showing class, group and operating temperature based on operation in a 40 ° C ambient (see NEC 500-3 for exceptions). Non-heat producing equipment does not have this requirement.

North America Temp. Codes
(NEC-500) & CSA
Maximum Temperature
° C° F
T1 T1 450 842
T2 T2 300 572
T2A   280 536
T2B   260 500
T2C   230 446
T2D   215 419
T3 T3 200 392
T3A   180 356
T3B   165 329
T3C   160 320
T4 T4 135 275
T4A   120 248
T5 T5 100 212
T6 T6 85 185


The rewritten Article 505 of the 1999 National Electrical Code (NEC) provides for the use of the Zone Classification System from the Division System. Prior to insertion of the Zone Classification, the IEC/CENELEC (for the European Union) utilized this classification system whereas the Division System was utilized only by the NEC (for the United States). Article 505 was rewritten because the Division Classification and the Zone Classification were difficult to merge.

Both the Zone System and the Division group classify the different gases in ascending order of “more easily ignitable”; Group “A” in the NEC (Division System) is the most easily ignitable – Acetylene, whereas Group “A” in the IEC (Zone System) is the least easily ignitable. Further, the IEC uses three groups for the representative gases and the NEC uses four. AD to this confusion, the IEC and NEC do not use the same terminology and neither have the same number of categories to identify the likelihood of the hazard being present.

Both Division and Zone Classification Systems start with a definition if what the hazard is and the probability the hazard will be present. The NEC Division Classification Systems uses Classes and Groups to identify hazards:

  • Class I represents gases & vapors
  • Class II represents explosive dusts
  • Class III represents hazardous (flammable) fibers

Groups further define the hazard in Class I and Class II areas. In Class I, there are Group A (Acetylene and similar gases), Group B (Hydrogen and similar gases), Group C (Ethylene and similar gases) and Group D (Propane and similar gases).
Hazardous dusts and fibers (Class II & III respectively) are not treated in Article 505.

The IEC Zone Classification System identifies the hazard by 2 Main Groups

  • Group I for mining (underground locations and Group II for surface (not underground) locations. [Further Section 90-2(b)(2) states the NEC does not cover underground mining installations.)
  • Group II is divided into 3 sub-groups in order of hazard to threat of ignition – Group A – the least likely to ignite (such as Propane); Group B (gases such as Ethylene) and Group C – the easiest to ignite (such as Acetylene and Hydrogen).


A Class I, Division 1 location is one where an explosive atmosphere is presumed to be present in normal operation either all or part of the time.

A Class I, Division 2 location is one where volatile flammable liquids or gases are handled, processed or used, but which are normally enclosed in containers from which they can only escape in the case of accidental rupturing or abnormal operation of equipment.


Class I locations can further be divided into Zones based upon the frequency of occurrence and duration of and explosive gas or vapor atmosphere as follows:

Zone 0 areas are locations in which explosive gas or vapor atmospheres are present continuously or for long periods of time. Zone 0 areas are those where there is a flammable mixture typically more than 1,000 hours per year.

Zone 1 areas are those in which (a) explosive gas atmospheres are likely to occur in normal operation, or (b) explosive gas atmospheres may exist frequently because of repair or maintenance operations or because of leakage, or (c) the location is adjacent to a Class I, Zone 0 location from which explosive gas atmospheres could be transmitted. Zone 1 areas are those where there is a flammable mixture more than 10 hours per year and less than 1,000 hours per year.

Zone 2 areas are those in which (a) explosive gas or vapor atmospheres are not likely to occur and if they do occur, will exist for a short period of time, or (b) flammable liquids, gases or vapors are handled, processed or used, but in which liquids, vapors or gases are normally confined within closed containers or closed systems from which they can escape only as a result of accidental rupture or breakdown of the containers or systems or the abnormal operation of the equipment by which the liquids, gases or vapors are handled, processed or used, or (c) explosive gas atmospheres are normally prevented by adequate ventilation but which may occur as a result of failure of the ventilation system, or (d) the location is adjacent to a Class I, Zone 1 location from which explosive gas atmospheres could be transmitted unless this is prevented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against failure of the ventilation system. Zone 2 areas are those where explosive gas atmospheres will exist for less than 10 hours per year.


ZONE 0 (Continuous Grade – 1,000 hours per year)

Areas within process equipment developing flammable gas or vapors
Areas within enclosed pressure vessels or storage areas
Areas around vent pipes which discharge continually or for long periods
Areas over or near the surface of flammable materials

ZONE 1 (Primary Grade – 100 hours per year)

Areas above roofs outside storage tanks
Areas above floating storage tanks
Areas within specified radii around the outlet pipes and safety valves
Rooms without ventilation openings from a Zone 1 area
Areas around flexible pipelines and hoses
Areas around sample taking points
Areas around seals of pumps, compressors and similar primary sources

ZONE 2 (Secondary Grade – 10 hours per year)

Areas around flanges and connecting valves
Areas outside Zone 1 around outlet pipes and safety valves
Areas around vent openings from Zone 2.

*CEC/NEC (North American) Area Classifications