National Electrical Code ExplanationsBased on the 2014 NEC
National Electrical Code Tips: Article 725, Class 1, Class 2, and Class 3 Circuits. Part 3.
- What are the size and use requirements for Class 1 circuit conductors? You can use sizes as small as 16AWG and 18AWG, if the loads they supply don't exceeed the ampacities given in 402.5 and if you install them in a raceway, approved enclosure, or listed cable [725.49(A)]. If you use conductors larger than 16AWG, they can't supply loads greater than the ampacities given in 310.15. If you use flexible cords, they must comply with Article 400.
- What are the insulation requirements for Class 1 circuit conductors? It must be rated for at least the system voltage and at least 600V. If the conductors are larger than 16AWG, they must comply with Article 310. For conductors 18AWG and 16AWG, you can use one of the types of insulation listed in 725.49(B). You can use other types and thicknesses, but only if the conductors are actually listed for Class 1 circuit use.
- As you might expect, there's a limit to how many Class 1 circuit conductors you can put in a raceway. If the raceway has only Class 1 circuit conductors, you can determine this limit per 300.17. You can use the ampacity adjustment factors of 310.15(B), but only if the conductors carry continuous loads in excess of 10% of the ampacity of each conductor [725.51(A)]. Note the use of aggregate ("the conductors") and individuation ("each conductor"), else you will not understand and correctly apply this requirement.
- You can run power-supply conductors and Class 1 circuit conductors in the same raceway, if you meet the requirements of 725.48 and you determine the number of conductors per 300.17 [725.51(B)].
- If you have the arrangement just discussed in Point 4, you must apply the ampacity factors of 310.15(B)(3)(a). But there are two caveats, which we discuss in the next two Points.
- One caveat for apply the ampacity factors of 310.15(B)(3)(a) to power-supply conductors and Class 1 circuit conductors in the same raceway is you must apply those to all conductors where the Class 1 circuit conductors carry continous loads in excess of 10% of the ampacity of each conductor and you have more than three conductors in total [725.51(B)(1)].
- Another caveat for apply the ampacity factors of 310.15(B)(3)(a) to power-supply conductors and Class 1 circuit conductors in the same raceway is you must apply them to the power-supply conductors only, if the Class 1 circuit conductors do NOT carry continuous loads in excess of 10% of the ampacity of each conductor and you have more than three power supply conductors [725.51(B)(2)].
- Cable tray is often a good way to run Class 1 circuit conductors. Just make sure you comply with 392.22 and 392.80(A) if you go that route [725.51(C)].
- If you have any Class 1 circuit(s) extending aerially beyond one building, ensure it complies with Article 225.
- Part III of Article 725 covers the requirements for Class 2 and Class 3 circuits. Huge swaths of this are new with the 2014 NEC. Note that we jump from 725.52 as the end of Part II to 725.121 as the start of Part III.
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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, 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.
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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.