National Electrical Code Tips: Article 725, Class 1, Class 2, and Class 3 Circuits. Part 1.
Chapter 7 of the NEC starts off with a series of five Articles about onsite power generation systems: 700, 701, 702, 705, 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.
- 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) [725.2].
- When one of these cables is "abandoned," that means it's not terminated at equipment and not identified for future use with a tag. This definition has implications later in this Article. For example, the accessible portions must be removed [725.25].
- Circuit Integrity (CI) Cable is a special kind of cable with a fire rating . It's for critical circuits.
- Another type of cable to be familiar with is Power-Limited Tray Cable (PLTC). This is also a special kind of cable, and it's a factory assembly under a nometallic jacket . Don't assume your particular PLTC contains an equipment grounding conductor; some PLTCs do and some do not. Similarly, don't assume the PLTC you're ordering has four insulated conductors same as the last one; maybe it has two or maybe it has six. The PLTC designation is based on the construction of the cable, not on how many circuit conductors or (if any) grounding conductors it contains. See 725.135 and 725.154.
- These circuits don't give you a free pass on raceway fill; 300.17 still applies [725.3(A)].
- They don't give you a free pass on anything, actually. Other Articles apply, depending upon the specific installation. You'll find quite a few of these identified in 725.3; they include Articles 500 through 516, Article 392, part of Article 430, and Article 300.
- 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].
- 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].
- You must identify Class I, 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.
- You must classify as Class 1 any remote-control circuits for safety-control equipment if failure of that equipment to operate introduces a direct life or fire hazard [725.31]. Obviously, this excludes such equipment as room thermostats because they aren't safety-control equipment.