National Electrical Code ExplanationsBased on the 2014 NEC
National Electrical Code Tips: Article 725, Class 1, Class 2, and Class 3 Circuits. Part 4.
- The requirements for Class 2 and Class 3 circuits are in Part III of Article 725 [725.121 - 725.154. Much of the text is new with the 2014 NEC.
- You can use one of five types of power source for a Class 2 or Class 3 circuit [725.121(A)]. But choose one that's big enough for the application; you cannot interconnect output connections (unless the supplies involved are listed for such use) [725.121(B)].
- On the supply side, treat the power source as you would any other load; apply the relevant portions of Chapters 1 - 4 [725.127]. But on the load side, comply with the requirements of 725.130(A) or 725.130(B). Which way is better? That really depends upon your specific application. Review both sets of requirements, and then decide based on your engineering goals and cost constraints.
- 725.133 is a very long section, and it's entirely new with the 2015 NEC. It takes up slightly more than two and a half pages. For the most part, it tells you which cable types you can use with a given application.
- The rules for separating the conductors of these power supplies from various other types of conductors are in 725.136. This section is slightly more than two pages long, but is intact from the 2011 version (no changes with the 2015 revision).
- You can install conductors of different circuits in the same cable, enclosure, cable tray, or raceway. New with the 2014 NEC is permission to install them in the same cable routing assembly. You'll find a laundry list of requirements in 725.139, which changed only slightly from the 2011 NEC with the 2014 revision.
- What if you want to extend these conductors beyond one building? Generally, it's better to simply install a separate power supply in the other building; power loss at this distribution voltage is one reason why. But there are functional exceptions to this generalization. For example, the guard shack is 15 feet away from the building in which this power supply is located and you want information from the guard shack fed back to the main system that this power supply powers. It's not the case that you have the guard shack on its own monitoring system, which you cordone off to that space. So you extend beyond the one building to the other.
If these conductors are subject to accidental contact with conductors operating at over 300V (to ground), or is exposed to lightning (on interbuilding circuits), you have to apply specific requirements from Article 820 (if coaxial conductors) or Article 800 (if anything other than coax) [725.141].
- Class 2 or Class 3 circuit conductors can't be strapped, taped, or attached by any means to the exterior of any racway [725.143]. This type of "support" is generally prohibited by 300.11(B). For some reason, there are installers who think this doesn't apply to equipment grounding conductors (huge, huge mistake), PA system wires, and low-voltage conductors from Class 2 or Class 3 circuits. It very much does. Rather than waste manhours cable-tying these conductors to raceway just to save a few bucks on separate raceway you'll have to install later anyhow, install the correct raceway from the beginning.
- Table 725.154 takes up a couple of pages. It tells you which listed Class 2, Class 3, and PLTC cables you can use in specific types of applications.
- Due to layout limitations, Figure 725.154(A) appears on the page following the page in which Part IV starts. But it is a part III figure. It provides the cable substitution hierarchy for cable substitutions permitted under 725.154(A).
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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