Based on the 2017 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 503 -- Class III Hazardous Locations

  1. The definitions of Class I, Class II, and Class III locations are in Article 500.5. That's easy enough to remember. The trick there is to not simply remember that but read the definitions prior to, and occasionally during, working on a Class III location project.
  2. The general requirements (NEC Chapters 1 through 4) still apply to Hazardous locations--except as specifically modified in Chapter 5.
  3. Equipment installed in Class III locations must be able to function without getting hot enough to cause spontaneous ignition [503.5].
  4. The requirements for Class III wiring methods are in 503.10. These differ from Class I and Class II methods.
  5. The definition of "dusttight" is in Article 100.
  6. Any boxes and fittings you use in Class III, Division 1 must be dustight [503.10(A)(2)].
  7. In Class III, Division 2 locations the wiring method requirements are the same as those of Class III Division 1 locations (found in 503.10(A)], except under specific circumstances open wiring is permitted [503.10(B)].
  8. The special requirements for motors and generators are in 503.125.

  10. Luminaires have special requirements, as outlined in 503.130. Other devices, such as flexible cords, plugs, and receptacles also have special requirements. Refer to the appropriate part of 503.
  11. Per 503.30, wiring and equipment in Class III locations must be grounded and bonded as specified in Article 250, but with additional requirements. Don't proceed with this work until you have read and understood the related definitions in Article 100. Too often, people confuse grounding (connection to the earth) with bonding, leaving dangerous differences of potential in place.