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. This runs many pages and includes a big table, a small table, and two illustrations.
- 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) for Class I power sources or 725.130(B) for Class II and Class III power sources.
- 725.133 is a very long section. 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 about a full page in size.
- You can install conductors of different circuits in the same cable, enclosure, cable tray, or raceway. As of the 2014 NEC, you can 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. They have not changed much since then (not at all from 2014 to 2107, and only a little from 2017 to 2020)..
- 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 full page. It tells you which listed Class 2, Class 3, and PLTC cables you can use in specific types of applications.
- Figure 725.154(A) provides the cable substitution hierarchy for cable substitutions permitted under 725.154(A).