To understand how and when to apply NEC Art. 725*, you must first understand how the NEC classifies remote-control, signal, and power-limited circuits. Each classification addresses different energy levels.
Class 1 circuits. Class 1 remote-control and signaling circuits typically operate at 120V, but the NEC permits them to operate at up to 600V [725.21(B)]. You must install these circuits within a wiring method listed in Chapter 3 of the NEC, which includes raceways, cables, and enclosures for splices and terminations [725.25].Remote-control circuit. These circuits, which control other circuits through relays or equivalent devices , are commonly used to operate motor controllers in moving equipment, mechanical processes, elevators, and conveyors.Signaling circuit. These circuits energize signaling equipment  and are typically used to operate a bell or horn, or to illuminate a light in a control panel.Power-limited circuit. These circuits are supplied by a source with a rated output of not more than 30V and 100VA [725.21(A)].Class 2 circuits. Class 2 circuits typically include wiring for low-energy (100VA or less), low-voltage (under 30V) loads such as low-voltage lighting, thermostats, PLCs, security systems, and limited-energy voice, intercom, sound, and public address systems. You can also use them for twisted-pair or coaxial local area networks (LAN) [725.41(A)(4)].
Class 2 circuits protect against electrical fires by limiting the power to 100VA for circuits that operate at 30V or less, and 0.5VA for circuits between 30V and 150V. You protect against electric shock by limiting the current of the circuit to 5mA or less for circuits between 30V and 150V [Chapter 9, Table 11].
You can wire Class 2 circuits with Class 2 cable or any of its substitutes permitted by Table 725.61(A), depending on the condition of use.
Class 3 circuits. Use Class 3 circuits when the power demand for circuits over 30V exceeds 0.5VA, but is not more than 100VA [Chapter 9, Table 11]. We often use Class 3 signaling circuits for security systems and public address systems; voice, intercom, and sound systems; and some nurse call systems.
Higher levels of voltage and current are permitted for Class 3 circuits (in contrast to Class 2 circuits). To prevent an electric shock hazard, the wiring must be rated no less than 300V [725.71(E) and (F)]. Wiring methods that meet this requirement include PLTC Cable, Class 3 Cable, or any of its permitted substitutions listed in Table 725.61(A), depending on the condition of use.
*All Code references come from the 2002 NEC.