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National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 110 -- Installation Requirements

by Mark Lamendola

Based on the 2020 NEC

Our remarks in are parentheses. Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 NEC Article 110 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

  1. 110.3. Examination, Identification, and Use of Equipment. This section gives 8 requirements for examination in part (A). In (B), it says "Listed or Labeled equipment shall be installed and used in accordance with any instructions included in the Listing or Labeling." In other words, use the product as intended. Unauthorized modifications void the Listing and expose the modifier to civil, and potentially criminal, litigation and liability.
  2. 110.12. Mechanical Execution of Work. "Electrical equipment shall be installed in a neat and workmanlike manner." Why is the NEC so vague on this? Well, it’s hard to quantify and describe something like that. The Code is basically giving the Authority Having Jurisdiction (AHJ) the power to reject work that doesn’t meet industry standards. It’s a situation where "everyone knows the rules," and the AHJ can make sure those rules get followed. One reason contractors back such language is they can use this rule to level the competitive playing field against "fly by night outfits" that do sloppy work. The costs associated with work that isn’t "neat and workmanlike" are enormous—a small investment in upfront costs saves the customer big money over the life of the equipment. One requirement many people often overlook is that of filling unused openings in enclosures—the omission of which is potentially lethal.
  3. 110.13. Mounting and Cooling of Equipment. This requirement is always in dispute, it seems. Cramming equipment into an overcrowded arrangement to maximize revenue per square foot sounds like a really good idea until that equipment starts failing left and right, or the whole place just burns down.
  4. 110.18. Arcing parts. This is one of several code requirements that rule out using an electrical equipment room as a storage area for combustible materials.
  5. 110.23. Current Transformers. "Unused current transformers associated with potentially energized circuits shall be short-circuited." Leaving the leads to dangle is an invitation for disaster. A testing firm will always insist on leaving these shorted—this is why.
  6. 110.26. Spaces about electrical equipment (600V or less). Most people wrongly assume working clearances (depth of working space) are three feet. Under some circumstances, the NEC requires them to be more. Beyond simply safety, good engineering or maintenance practices may require more still. Don’t assume just because you have 2 feet and 10 inches you are "good enough" or if you have 3 feet 2 inches you must fill in that 2 inches so you don’t exceed the Code. Also, the Code has minimums for the width and height of working spaces. These widths are outdated, as they are based on an earlier era of less "girth endowed" workers. Use common sense, with safety as your goal.
  7. 110.27. Guarding of live parts. Various methods exist, including construction of a mezzanine level, restricted access to an area (via locked door), and so on. It is more than just ensuring the factory covers are in place. When desiging or installing guard systems, think more in terms of idiot protection than code compliance. That is, anticipate human error rather than merely seek to check off the box for code compliance.
  8. 110.54 (A). Grounded and Bonded. This requires "effective grounding," which is defined in Article 100. It does not include the common practice of connecting to a ground rod that is not bonded to the grounding system. Always be thinking of the actual current path. Remember, electricity is always trying to get back to its source (thus making a circuit). If you provide a low resistance (metallic) path, much less current will flow through the other paths available. Electricity does not take "the path of least resistance," it takes all paths before it in inverse relationship to their impedance (Kirckoff's Law of Parallel Circuits).
  9. 110.54(B). Equipment Grounding Conductors. This requires you to run a grounding conductor "with circuit conductors inside the metal raceway or inside the multiconductor cable jacket."
  10. 110.58. Disconnecting Means. You must have a disconnect within sight of each transformer or motor. This allows a person to lock out the transformer or motor for maintenance, or to shut it off quickly in case of misoperation.


How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.


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