| Publisher | NFPA |
|---|---|
| Type | Code (installation code, part of NFPA 70) |
| Scope | Requirements for electrical and electronic equipment and wiring in locations where fire or explosion hazards may exist due to flammable gases, vapors, liquids, combustible dusts or ignitible fibers. |
| Who uses it | Electrical engineers, electricians, inspectors and owners of refineries, chemical plants, grain facilities, fuel handling sites and other hazardous locations in the United States. |
| Edition note | Published within NFPA 70, which is revised on a three-year cycle; the edition enforced depends on local adoption. Refer to NFPA and the authority having jurisdiction. |
What It Is
Articles 500 through 506 of the National Electrical Code form the core of the US rules for hazardous (classified) locations. Article 500 establishes the traditional Class and Division system: Class I for flammable gases and vapors, Class II for combustible dusts and Class III for ignitible fibers and flyings, each divided into Division 1 and Division 2 according to the likelihood of the hazard. Articles 501, 502 and 503 then give wiring and equipment requirements for Class I, II and III locations respectively.
Article 504 covers intrinsically safe systems, including the control drawings and separation of intrinsically safe circuits. Article 505 provides an alternative Zone 0, 1 and 2 system for Class I locations, and Article 506 provides a Zone 20, 21 and 22 system for combustible dusts and fibers. Both zone articles align with the international IEC approach, allowing US facilities to use equipment and methods familiar on global projects. Later articles in Chapter 5 apply these principles to specific occupancies such as commercial garages, motor fuel dispensing facilities and spray application areas.
Why It Matters
Correct classification and equipment selection are what stand between a routine electrical fault and an explosion. The NEC has legal force wherever a state or local jurisdiction adopts it, and OSHA's general industry rule for hazardous locations follows the same framework. Insurers and owners also hold facilities to these requirements.
Practical mistakes are common: equipment rated for the wrong group or temperature, missing seals, mixing Division and Zone methods without following the rules for doing so, and intrinsically safe circuits installed without regard to the control drawing. Engineers and technicians who understand the logic of these articles avoid such failures.
Key Concepts
- Class and Division — a two-level classification combining the type of material (Class I, II or III) with the likelihood of its presence (Division 1 or 2).
- Material groups — Class I gases and vapors are assigned to Groups A, B, C and D, and Class II dusts to Groups E, F and G, so equipment can be matched to the specific hazard.
- Zone system — Article 505 (gases and vapors) and Article 506 (dusts and fibers) offer zone-based classification and protection techniques aligned with IEC practice.
- Temperature code — equipment is marked with a temperature code that must not exceed the autoignition temperature of the material present.
- Explosionproof and dust-ignitionproof equipment — enclosure-based protection techniques that contain or exclude ignition, used widely in Division 1 locations.
- Sealing — conduit and cable seals limit the passage of gases, vapors and flames through wiring systems between locations and enclosures.
- Intrinsic safety — Article 504 rules for circuits that are inherently incapable of releasing enough energy to cause ignition.
Who Should Know It
- Electrical engineers designing installations in process and fuel-handling facilities
- Electricians installing and maintaining equipment in classified locations
- Electrical inspectors and authorities having jurisdiction
- Process safety engineers performing area classification
- Instrument technicians working with intrinsically safe loops
Related Standards
- NFPA 70 (NEC) — the parent code containing these articles.
- IEC 60079 Series — the international standards aligned with the zone approach of Articles 505 and 506.
- OSHA 29 CFR 1910 Subpart S — includes the federal hazardous location requirement in 1910.307.
- NFPA 497 and NFPA 499 — NFPA recommended practices used to classify locations for flammable materials and combustible dusts.
- API RP 500 and API RP 505 — API recommended practices for classifying petroleum facility locations under the Division and Zone systems.
How ASEEE Uses It
ASEEE hazardous area programs map learning outcomes and assessments to the concepts in NEC Articles 500-506, so that candidates can work confidently with both the US Division system and the international Zone system. See related ASEEE programs.
Educational summary only. ASEEE is not the publisher of this document and this page does not reproduce or replace it. Always consult the current official edition from NFPA for requirements.