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National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 240 -- Overcurrent Protection

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

  1. General philosophy: Providing overcurrent protection isn't just a matter of selecting a fuse or breaker from a table. When current exceeds the rating of conductors or equipment—due to overload, short circuit, or ground fault—you have overcurrent. This is what you must protect conductors and equipment and equipment against, using Overcurrent Protection Devices (OCPDs).
  2. General background: Article 240, consisting of seven parts, provides the requirements for selecting and installing OCPDs.
  3. General concept of equipment protection. To protect equipment, an OCPD opens when it detects a short-circuit or ground fault.
  4. General concept of circuit protection. To protect a circuit, an OCPD opens when current reaches a value that will cause an excessive temperature rise in conductors.
  5. Tables 310.15 through 310.21 provide the ampacities you must use for selecting conductors for a given load. Please note the exceptions.
  6. Motor overload protection and motor circuit protection are not the same thing. See the table at the beginning of Article 430. To size motor branch-circuit conductors, see Table 310.16, 430.22, and Table 430.150. To size motor branch-circuit protection, see 240.6(A), 430.52(C), and Table 430.150.
  7. Install an OCPD in series with each ungrounded conductor [240.20].
  8. Location, location, location. Locate OCPDs to prevent exposure to physical damage [110.27(B)]--but do so in a way that leaves them readily accessible [240.24].
  9. For fuse requirements, see 240.50 through 240.61.
  10. For breaker requirements, see 240.80 through 240.86.

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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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