National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 501 -- Class I Locations

1. Read Article 500 before reading Article 501. Why? Because Article 500 provides the basis for interpreting and correctly applying Articles 501 - 516. For one thing, you will find the definitions for those Articles in Article 500. So, do not work with Article 501 until you have read and understood Article 500.
    
2. Class I locations are those in which flammable gases or vapors are (or may be) present in sufficient quantities to produce explosive or ignitible mixtures [501.1].
    
3. Class locations are further broken down into Division 1 (normal operations) and Division 2 (abnormal operations). That is, point #2 above applies in normal or abnormal conditions.
    
4. You must use Division 1 wiring methods when combustibles are present under normal operations [501.10(A)].
    
5. You must use Division 2 wiring methods when combustibles are present under  abnormal operations [501.10(B)]. However, you can use Division 1 wiring methods in a Division 2 location.
    
6. Seal requirements for Class I locations [501.15(A)] are highly detailed and far more extensive than those for Class II locations [502.15(B)]. Do not confuse the two. Go back to the definitions in Article 500 if you are unsure.
    
7. Any electrical parts that operate at more than 30V can't be exposed, but this drops to 15V under wet conditions. Further, you must apply the appropriate protection technique from 500.7(E), (F), or (G) to these parts [501.25].
    
8. The grounding and bonding requirements for Class I locations are in 501.30. If you ground where you should, instead, bond, you will create a difference of potential that violates 501.30 and will pose a threat to people and property. To avoid catastrophic consequences, read the definitions of grounding and bonding in Article 100, and take some time to study Article 250, Part V. Basically, you bond (not ground) on the load side. A ground path (the earth, see Article 100) is a very high resistance. A bond path, being a copper conductor, is a very low resistance. Grounding does not, therefore, create an equipotential plane but bonding does. Simple thing, it's Ohm's Law.
    
9. Surge protection is a good idea for Class I locations, for obvious reasons. But it must comply with 501.35, and the rules are different for Division 1 versus Division 2.
    
10. Any luminaire used in a Class I, Division 1 location must be identified as a complete assembly for Class I, Division 1 locations [501.130(A)(1)]. Any luminaire used in a Class I, Division 2 location must meet some conditions that the typical person in the field can't possibly conform to with any certainty, or it must conform to the requirements for a Class I, Division 1 location [501.130(B)(1)].