National Electrical Code ExplanationsBased on the 2014 NEC
National Electrical Code Tips: Article 720, Circuits and Equipment Operating at Less Than 50 Volts
Chapter 7 of the NEC starts off with a series of five Articles about onsite power generation systems: 700, 701, 702, 705, and 720. Notice that these are not perfectly sequential. Missing are 702, 703, 704, 706, and 707. And there's no 709. We jump from there across the teens to Article 720. It's unclear what might go in this reserved space in the future, but it's there if we need it!
Article 720 has only 8 subsections.
Let's address some
highlights of the Article 720 requirements:
- As stated in the name of this Article, it applies to circuits operating at under 50V. That includes ac and dc circuits [720.1].
- But it might not apply to the under 50V circuit you're working on. Always look at the list of "Other Articles" in 720.2 if your circuit is under 50V. Anything covered by these references doesn't have to comply with Article 720.
- If your circuit just happens to be in a Hazardous Location and it's covered by Article 720 (e.g., not excluded by 720.2), then both Article 720 and the relevant Chapter 5 Article(s) apply [720.3].
- Conductors can't be smaller than 12 AWG (copper or the equivalent AWG in aluminum) [720.4].
- But if the conductors are for appliance circuits (supplying more than one appliance), they can't be smaller than 10 AWG (copper or the equivalent AWG in aluminum) [720.4].
- You must use standard lampholders [720.4]. They must have a rating of at least 600W.
- Any receptacles you use must have a rating of at least 15A [720.6].
- But if the receptacles are in kitchens, laundries, and other areas where portable equipment is likely to be used, they must be at least 20A.
- Storage batteries must comply with 480 through 480.5 and 480.8 through 480.10 [720.9]. This requirement seems unnecessary, as Article 480 is about storage batteries. So whether you have storage batteries under 50V or over 50V, Article 480 applies.
- We find two other unneccary requirements in 720.11. These appear in Chapters 2 and 3, and thus apply to all installations. The NEC repeats these in individual chapters, for reasons that are not clear.
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, 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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