National Electrical Code Tips: Article 725, Class 2 and Class 3 Circuits

Chapter 7 of the NEC starts off with a series of six Articles about onsite power generation systems: 700, 701, 702, 705, 706.and 720. Notice that these are not perfectly sequential. Missing are 702, 703, 704, 706, and 707. And there's no 709. We jump from there across the teens to Article 720. It makes sense that Articles 720 (under 50V) and Article 725 would be more or less sequential in the NEC. Why there's a four Article gap between them is a bit mysterious, expecially when you consider that the next one in sequence (Article 727) is about instrumentation tray cable. Because Article 727 is about a type of cable, it seems to be more a fit in Chapter 3 than in Chapter 7.

To correctly apply Article 725, you'll have to "knuckle down" and spend some time understanding the definitions. People really hate to read these, but skipping over them in this Article is a huge mistake.

1. Obviously, you need to know the difference between Class 1 (portion of the circuit on the load side of the OCPD or power-limited supply), Class 2 (portion of the circuit on the load side of a Class 2 source), and Class 3 (portion of the circuit on the load side of a Class 3 power source) [100]. With the 2023 revision, Class 1 circuits were removed from Article 725. This resulted in extensive reorganization of Article 725.
   
2. These circuits don't give you a free pass on raceway fill; 300.17 still applies [725.3(A)].
   
3. As with any installation in a ceiling space or behind panels, you must arrange wires and cables such that they don't impede access to electrical equipment [725.21].
   
4. The mechanical execution of work requirements [110.12] apply to all installations, but Article 725 provides an additional paragraph particular to these systems [725.24].
   
5. You must identify Class 2 and 3 circuits at terminal and junction locations, in a manner that prevents unintentional interference with other circuits during testing and servicing [725.30]. Several manufacturers make labeling systems that you can use to efficiently satisfy this requirement.
   
6. The requirements for Class 2 and Class 3 circuits are in Part II of Article 725 [725.60 - 725.144].
   
7. 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. The rules for separating the conductors of these power supplies from various other types of conductors are in 725.136.
   
8. 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, and not at all from 2020 to 2023).
   
9. 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) [725.139].
   
10. Class 2 or Class 3 circuit conductors can't be strapped, taped, or attached by any means to the exterior of any racway. 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.