National Electrical Code ExplanationsBased on the 2014 NEC
National Electrical Code Tips: Article 770, Optical Fiber Cables and Raceways, Part 2
Understanding the listing requirements of fire alarm circuit cables can help you make sense of the cable alphabet soup. Here are some highlights from Part IV of Article 770.
- There's plenty of "expansion room" built into Article 770. Part I ends with Section 770.26, and Part II begins with 770.47. The CMPs must have been expecting much more new material in future revisions, because Part II ends with 770.49 and Part III begins with 770.93.
- Part II of Article 770 provides the requirements for cables outside and entering buildings. Just about all of it is new with the 2012 NEC.
- Conductive fiber-optic cables must be separated from other cables. The specifics are in 770.47. Note that two exceptions exist.
- You can use unlisted outside plant optical fiber cables, and you can install them in building spaces. But they can't go in risers, environmental air ducts, environmental air plenums, or any spaces used for environmental air. In short, don't mix them with environmental air. And you're limited to 50ft of length [770.48(A)].
- If you install unlisted outside plant optical fiber cables in building spaces and those cables are nonconductive, you must install them in one of four specific types of raceway. Those are IMC, RMC, PVC, and EMT.
- If you use metallic conduit for these systems, it must be bonded to the EGC [770.49]. There is no such requirement for metallic tubing, but what do you think is the intent for the conduit? Don't choose EMT to get out of the bonding requirement. Choose EMT for other reasons, and bond it anyhow. It's also a good idea to bond around the joints (a bonding jumper across each connector); note that EMT manufacturers and other experts say a properly assembled EMT system does not need this but you'll find it done in high-reliability installations anyhow.
Gee, we're at Part III already. As noted, we jump from 770.49 (which, incidentally, was new with the 2012 NEC) to 770.93 (which, incidentally, was already in the previous revision of the NEC so was not new with the 2012 NEC). This is a short Section that is titled "Protection". But it is really about bonding issues with the non-current carrying metallic members of optical fiber cables.
- Gee again, now we're at Part IV already. And it starts with Section 100. It almost ends there too. 770.100 takes up almost a page and a half, and then we get to 770.16 which is fairly short. This last section is about grounding and bonding of entrance cables at mobile homes. 770.100, we are left to assume, is for all other types of buildings. It simply says "buildings" not "buildings other than mobile homes" or "structures other than mobile homes." This illustrates the need to read an entire Part rather than just glam onto something that you first see and assume it applies your application. Other text in that same Part may apply to your specific application.
- Part IV is titled "Grounding Methods" but you need to be careful with this concept. If you just drive a ground rod and connect your metallic entrance cable to it you will have dangerous differences of potential. You want to bond that to the electrical service ground (or grounding system) via an Equipment Grouding (bonding) Conductor. This may seem like a silly dicing of words, but keep in mind that soil has far greater resistance than wire does. So don't try to use the dirt to connect everything together. Use EGCs for that purpose.
- Don't forget that metallic raceway, if properly instsalled, is an approved form of EGC. You will find these listed in Article 250 (250.118, to be exact), but you'll also find them named in the Part IV text.
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Learn more about:
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.