construction code books

Home | Search | About us                  Bookmark and Share

 
nec training

National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 300 -- Wiring Methods

by Mark Lamendola

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 300 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

The language throughout NEC Article 300 misplaces the word "only," but the meaning where this is misused is still clear if you interpolate just a bit.

  1. NEC Article 300.3 addresses conductors and enclosures. (B) requires all conductors of the same circuit to be in the same wireway. One reason for this is the basic physics involved when the electromagnetic fields of conductors interact. Many other reasons make this requirement of great practical value.

    An exception to it does exist, and there are sometimes practical reasons for taking advantage of that exception. (C) allows conductors at or below 600V to be mixed in the same enclosure, cable, or raceway regardless of their voltage. That mixing is safe from the standpoint of the NEC, but the more of it you do the higher your risk of misoperation and other problems. Good engineering practice demands separating wiring systems as much as is practical. Thus, you would run 5V signal wires in one wireway, 120V control wires in another, and 480V power in yet anotheróeven though you might terminate them all in one control cabinet. Even inside the cabinet, you want to route and bundle the wires so as to maintain the maximum separation that is reasonably attainable. Motor drive power and output wiring deserves extra attention in this regard.
     
  2. NEC Article 300.4 addresses protection against physical damage. Many folks who run nonmetallic-sheathed cables (e.g., Romex) donít consider adding protection. In residential applications, this is usually unnecessary, but a hole drilled off-center could easily leave the wiring susceptible to puncture from a nail or screw driven to support shelving, cabinets, or other wall-mounted objects.
     
  3. NEC Table 300.5 provides the minimum cover requirements for buried cable of 0 to 600V.
     
  4. NEC Article 300.8. Raceways or cable trays containing electrical conductors cannot contain elements of other systemsóno water pipes, gas pipes, or any other non-electrical system elements can run in those electrical wireways. The intent of the NEC Article 300.8 also means, for example, running Romex through an A/C duct is a Code violation.
     
  5. NEC Article 300.11 addresses the issues of securing and supporting. Cables and raceways must have their own supportóindependent of other systems. Their supporting structures cannot be piggybacked onto other supports. For example, you canít hang conduit from ceiling grids, but you can clamp to the I-beams or rafters to hang rod and strut specifically for the conduit.

    (C) prohibits using wireways to support other wireways, cables, or non-electrical equipment. Thus, using cable ties to secure the wiring for that new PA system to conduit is a Code violation. A chief concern of NEC Article 300.11 is that electrical wireways be independent. They may share a supportófor example, you can strap multiple conduits to a strut suspended by two rods. But, you cannot then strap a strut to those conduits and hang a secondary set of rods to support another set of conduit or anything else.
     
  6. NEC Article 300.12 requires mechanical continuity of raceways and cable sheaths.
     
  7. NEC Article 300.13 requires mechanical and electrical continuity for conductors in raceways. In other words, you cannot have a splice in a raceway (but you can have it in a box or conduit body that has an accessible cover). NEC Article 300.5 does allow splices in direct-buried conductors. Thatís because you can use instruments to locate the splices and you can excavate to get to them. However, itís much more difficult to do maintenance and inspection on conductors that are in raceways.
     
  8. NEC Article 300.15 explains the exceptions noted in our comments in the preceding item, and it addresses similar issues in 13 subheadings.
     
  9. NEC Article 300.19 and NEC Table 300.19(A) provide specifics on conductor and raceway supports.
     
  10. NEC Article 300.20 requires conductors to be grouped together to reduce heating (this takes advantage of magnetic field interaction and cancellation). It contains two exceptions. (B) prescribes a technique few people know about. In fact, when this appeared in EC&M Magazine, many readers thought it was a hoax. It is not. The technique involves cutting cooling slots in the holes through which a single conductor passes. This item has an exception and an FPN. It is worth becoming familiar with.

 

Don't take your electrical exam twice

Learn more about: Electrical Calculations | Electrical Theory | Grounding | Harmonics | Motors | Power Quality
 

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.

 

Master the NEC | Solve Harmonics | Become a Motor Maintenance Guru

Codebookcity is a subsidiary of Mindconnection, LLC. If you have any questions, comments, or concerns, please write to sales @ mindconnection.com. We do want your business.

 

We support engineering and the construction trades. Based in Kansas City, we also participate locally. Here are the meetings of the IEEE Kansas City Section and Society Chapters: