Terms that are used in more than one article of the NEC are defined in Article 100. The dot two section of each article is usually reserved for definitions of terms that are used only in that particular article. These are the guidelines found in the NEC Style Manual but they have not always been adhered to. The latest edition of the code has made efforts to correct this oversight and once again we find definitions that have been relocated to their appropriate places. Chapter 5 covers special occupancies. Article 500 defines hazardous locations. 500.5 in particular tells us that a Class I location is hazardous because of the presence of flammable gases or vapors. A Class II location is hazardous because of the presence of combustible dust and a Class III location is hazardous due to the presence of easily ignitable fibers or flyings. The definition of combustible dust is one of the definitions that have been relocated to Article 100.

Combustible dust is defined as dust particles that are 500 microns or smaller and present a fire or explosion hazard when dispersed and ignited in air. Sugar is a great example of a combustible dust. It will not burn or explode when in a pile but when it is dispersed in the air it will explode. Sprinkle a small amount of sugar or powder coffee creamer over a fire and see the sparks fly.

Another very minor change made in article 500 is the title of section 500.5(A). This section is now titled General, which is more appropriate than the previous title that simply duplicated the section header title. You may also notice that the word could is highlighted in gray. The previous edition of the code used the word may. Either word seems to suffice in this case but the code-making panelists decided to restructure the sentence to read that hazardous locations shall be classified depending on the properties of the flammable gas, flammable liquid produced vapor, combustible liquid produced vapors, combustible dusts, or fibers and flyings that could be present, and the likelihood that a flammable or combustible concentration or quantity is present. Each room, section, or area shall be considered individually in determining its classification. Where pyrophoric materials are the only materials used or handled, these locations are outside the scope of this article.

The last sentence of this section answers a commonly asked question. What is the classification of an area where fireworks or other explosive material is stored? The National Electrical Code does not cover areas where explosives are stored. NFPA 1124 is one document that deals with this topic and should be referenced when dealing with materials of this nature.

Table 500.8(D)(2) has been removed for the 2017 code. This table was previously used to provide temperature limitations for Class II locations. These temperature ratings are no longer used to judge equipment, which renders this former table useless.

Article 500 provides the classifications for hazardous locations. Class I is used for flammable gases and vapors, Class II for combustible dust, and Class III for easily ignitable fibers or flyings. Each of these areas can also be broken down into either division one or two locations. A Division 1 location is a location where the hazard is normally present. A Division 2 location is a location where the hazard is present but normally contained. An accident would have to occur to make the area extremely hazardous.

For example, the area inside of a spray paint booth is a Class I, Division 1 location. It is a Class I location because flammable gases and vapors will be coming out of the paint gun. It is a Division 1 location because in normal operating conditions the hazard is present. You don’t want to strike a match inside of a paint booth while someone is painting.

The area within 20 feet of a gasoline pump is a Class I, Division 2 location. It is a Class I location because flammable gases and vapors are present. It is a Division 2 location because the gases and vapors are normally contained. An accident or a spill must occur before the hazard is abundant. You still would not want to strike a match if you are 20 feet from a gas pump but there is a chance that the vapors would not ignite in that case. Division 1 locations are more hazardous than Division 2 locations.

Article 501 covers Class I locations and provides rules for the wiring methods allowed and the manner in which they must be installed. Threaded metal conduit and specific cable types have previously been the required wiring methods for Class I locations. The 2017 code has added an allowance for threadless fittings used in Class I, Division 2 locations only when used with rigid metal or intermediate metal conduit. Threadless or compression fittings are still not allowed in Class I, Division 1 locations. Again, Division 1 locations are more hazardous than Division 2 locations and extra precautions must be taken in those areas.

Some cable types are also allowed to be used as a wiring method in Class I locations. MI or mineral insulated, metal-sheathed cable is an example of one.

Also clarified for the 2017 code is section 501.15(D)(1). Seals for cables entering enclosures shall be installed within 18 inches of the enclosure or as required by the enclosure marking. Only explosionproof unions, couplings, reducers, elbows, and capped elbows that are not larger than the trade size of the enclosure entry shall be permitted between the sealing fitting and the enclosure. This new text was added to clear up confusion as to whether or not fittings could be installed between sealing fittings and equipment when cables are used in Class I locations. The code now clearly states that fittings can be used if they are suitable for the location.

Sometimes changes present problems that could not have been thought of previously. Technological advances are not just being made in the electrical field but in other areas as well. Cars, trucks, and buses are not dependent on gasoline only for power any longer. Hydrogen, propane, and natural gas are all available as fuel for internal combustion engines. These different types of gases each create a unique set of hazards. For example, LPG or liquefied propane gas is heavier than air and will settle at the floor area whereas hydrogen and natural gas are lighter than air and will rise to the ceiling area. Where these gases migrate to help us determine where the primary hazardous location is.

Added to the 2017 code are 2 new tables that cover this dilemma in commercial garages. Table 511.3(C) is used for classifying hazardous locations in commercial garages when heavier than air fuel is used. This table tells us that in a major repair garage where heavier than air fuel is used the entire space within a pit or subfloor that is not ventilated is a Class I, Division 1 location but if proper ventilation is provided the area becomes a Class 1, Division 2 location.

Table 511.3(D) is used for classifying hazardous locations in commercial garages when lighter than air fuel is used. On this table we find that the ceiling area becomes a hazardous location. In a major repair garage where lighter than air fuel is used, the area within 18 inches of the ceiling is a Class I, Division 2 location but when proper ventilation is provided the same area becomes unclassified. As with all portions of the code be sure to choose the correct table for the conditions present.

Also changed for commercial garages is the new section 511.8. Underground wiring shall be installed in rigid metal conduit or intermediate metal conduit. Rigid or IMC must be used for all underground wiring in a commercial garage. The exception does allow PVC, RTRC, and HDPE conduit when buried not less than two feet deep. If you do choose to use nonmetallic conduit as allowed by the exception, you must convert to rigid or IMC for the last two feet of the underground run. This change now makes this rule the same as what is required for motor fuel dispensing facilities in section 514.8.