The National Electric Code (NEC/ NFPA 70) is a model code produced by the National Fire Protection Association (NFPA). It is adopted by various governing bodies to regulate electrical equipment and wiring. As such, it details requirements for electrical installations in hazardous locations. The NEC classifies areas as Class I, II, or III locations, based on the presence of fire or explosion hazards. Areas are further separated into Divisions and Groups, based on the likelihood of exposure from specific materials. Areas lacking hazards are unclassified. Each classification has corresponding requirements to safeguard electrical equipment and wiring. Thus, to prevent a fire or explosion, a specifier should electrically classify hazardous portions of a building to determine appropriate safeguards for electrical installations.
Per NFPA 70, Class I locations are hazardous because of the presence of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors in quantities sufficient to produce explosive or ignitable fuel-air mixtures. Class I locations are further separated into Divisions 1 or 2. Generally, Class I, Division 1 locations can produce ignitable fuel-air mixtures under normal operating conditions. Conversely, Class I, Division 2 locations produce ignitable fuel-air mixtures during accidents or unusual operating conditions. Class I locations include the following material groups1:
1. Group A: Acetylene
2. Group B: Flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors that can burn or explode when mixed in air (w/ MESG† ≤ .45 mm or MIC ratio‡ ≤ .4); e.g., hydrogen.
3. Group C: Flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors that can burn or explode when mixed in air (w/ .45 mm < MESG ≤ .75 mm or .4 < MIC ratio ≤ .8); e.g., ethylene.
4. Group D: Flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors that can burn or explode when mixed in air (w/ .75 mm < MESG or .8 < MIC ratio); e.g., acetone, butane, and propane.
Per NFPA 70, Class II locations are hazardous because of the presence of dust in quantities sufficient to produce explosive or ignitable dust-air mixtures. Class II locations are further separated into Divisions 1 or 2. Generally, Class II, Division 1 locations can produce ignitable dust-air mixtures under normal operating conditions. Conversely, Class I, Division 2 locations produce ignitable dust-air mixtures during unusual operating conditions. Including, but not limited to, poor housekeeping. Class II locations include the following material groups1:
1. Group E: Combustible metal dusts; e.g., aluminum and magnesium dusts. Per NEC commentary, Group E dusts are particularly hazardous. Thus, areas housing Group E dusts shall be Division 1.
2. Group F: Combustible carbonaceous dusts; e.g., coal, carbon black, charcoal, and coke dusts.
3. Group G: Combustible dusts not included in Groups E or F; e.g., flour, grain, wood, and plastic dusts.
Per NFPA 70, Class III locations are hazardous because of the presence of ignitable fibers or flyings that create a fire hazard. Class III locations are further separated into Divisions 1 or 2. Generally, Class III, Division 1 locations handle, manufacture and use ignitable fibers or flyings. Class III, Division 2 locations store and handle ignitable fibers or flyings, outside of manufacturing processes. Class III locations do not contain material groups.
In conclusion, to prevent a fire or explosion, hazardous materials should be considered while specifying electrical installations. Design documentation should electrically classify hazardous portions of a building to determine appropriate safeguards for electrical equipment and wiring. When properly protected, a hazardous location is not necessarily more dangerous than any other area.
Written By: Mark R. Richards, PE
† MESG ≡ Maximum Experimental Safe Gap
‡ MIC ratio ≡ Minimum Ignitable Fuel ratio
1 National Electric Code (NFPA 70-2017). Quincy: National Fire Protection Association, 2016. Pages 350-352. Print.