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National Electrical Code Explanations
Based on the 2008 NEC
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.
- 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
- General background: Article 240, consisting of
seven parts, provides the requirements for selecting and installing
- General concept of equipment protection. To
protect equipment, an OCPD opens when it detects a short-circuit or
- 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.
- Tables 310.15 through 310.21 provide the ampacities you must
use for selecting conductors for a given load. Please note the
- 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.
- Install an OCPD in series with each ungrounded
- 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].
- For fuse requirements, see 240.50 through
- For breaker requirements, see 240.80 through
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How the NEC is arranged
- The first four Chapters of the NEC apply to all installations.
- Article 90 precedes Chapter One, and establishes the authority of the NEC.
- Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
- Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
- Chapter 8 provides the requirements for communications systems.
- Chapter 9 provides tables.
- The appendices provide mostly reference information.
- 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, what 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 application, 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.
Here are a couple more NEC resources:
Cafe Code Site |
Joe Tedesco's National Electrical Code
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