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 770.44 (in the 2014 NEC, it began with 770.47; 770.44 was introduced with the 2017 NEC). 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 (no change there in 2017 or 2020).
- Part II of Article 770 provides the requirements for cables outside and entering buildings. Of course, if it's entering a building it would necessarily be outside unless it is entering from within another building that shares a common wall. So basically, this is about outdoor cables.
- 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. This requirement should be revised to say metallic raceway, to avoid confusion. If you look at 770.48(B), you will see EMT listed as one of the raceway types you can use. Two of the other three (IMC and RMC) are metallic, the third (PVC) is not.
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 770.106 which is fairly short. This last section is titled, "Grounding and Bonding of Entrance Cables at Mobile Homes. 770.100, we are left to assume, is for all other types of buildings. It is titled, "Entrance Cable Bonding and Grounding" not "Entrance Cable Bonding and Grounding for Buildings 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. In this particular case, better organization would avoid confusion. For example,
could start with 770.100(A) and the text of 770.100 before that could be moved to 770.706(B) as "All other types of buildings". The existing 770.706 text would go under 770.706(A), and 770.706 would be titled "Grounding and Bonding of Entrance Cables". Simple solution....
- 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 bonding jumpers and EGCs for that purpose.
- Don't forget that metallic raceway, if properly installed, is an approved form of EGC. You will find these listed in Article 250 (250.118, to be exact).