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National Electrical Code Articles and Information

Based on the 2020 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]. For the installer, this means the equuipment needs to be listed and/or labeled as suitable for Class III locations. You should not be trying to make this determination yourself. Note also that you can defeat this listing or labeling by failing to follow instructions specifically for Class III for that equipment. These often have to do with ventilation, protection from fiber accumulation, and location away from heat sources. Other factors may include loading, especially with transformers.
  4. The requirements for Class III wiring methods are in 503.10. These differ from Class I and Class II methods. They are more lenient, because Class III deals with larger molecules or particles.
  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)]. Pay extra attention to entryways, for example a box cannot have any unused openings that are not sealed. With fittings, ensure you are using the grommet or other part that keeps dust out.
  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. There are some things you can do to exceed these, in the interests of further reducing the likelihood of ignitng fibers. For example:
    A. Use a high-efficiency motor; it is simply built better and will last longer. But it also runs cooler.
    B. Eliminate voltage imbalance issues at their source and also put motors on their own transformers/panels as much as is practical.
    C. Install vibration monitoring for early detection of bearing problems.
    D. Choose a higher temperature-rated insulation motor.
    E. Install a shield and ducting such that the motor has less exposure to flying fibers.
    F. Install power correction at the motor, either with a capacitor (for across the line starting) or (preferably) with a variable speed drive or soft starter that has power factor correction.

  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. Most of these requirements are minimal, but they still need to be understood and applied.
  11. Per 503.30, wiring and equipment in Class III locations must be bonded as specified in Article 250, but with additional requirements. Note that prior to the 2022 revision, the word "grounding" was also included; its exclusion is a welcome improvement. 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. Essentially, you never ground on the load side. Instead, create a low-impedance path back to the source. See 250.118 for a list and description of acceptable bonding conductors (you won't see "dirt" or "earth" on that list).