Understanding NEC Article 725: Class 2 and Class 3 Power-Limited Circuits

NEC Article 725 establishes requirements for the installation and maintenance of Class 2 and Class 3 power-limited circuits. These circuits are commonly used in building automation, security systems, access control, and communication networks.

Scope of Article 725

This article applies to:

• Power-limited remote-control and signaling circuits.
• Systems not integral to a device or utilization equipment.

Key Features:

• Class 2 circuits prioritize safety by limiting voltage and power.
• Class 3 circuits allow higher voltage or power for specialized applications.

Key Requirements for Class 2 and Class 3 Circuits

1. Power Source Limitations (725.60)
   • Must use listed Class 2 or Class 3 transformers, power supplies, or other approved devices.
   • Batteries under 30 volts may qualify as Class 2 power sources if listed.
2. Circuit Identification (725.30)
   • Terminal and junction locations must clearly identify circuit classifications to prevent interference during servicing or testing.
3. Separation of Circuits (725.136)
   • Class 2 and Class 3 conductors must remain separate from electric light, power circuits, and Class 1 circuits unless protected by barriers, raceways, or other approved methods.
   • Cables sharing enclosures with power circuits must comply with specific insulation and separation standards.
4. Fire Safety (725.3)
   • Installation methods must limit fire spread and smoke production.
   • Cables used in air ducts and plenums require fire-resistant ratings per 300.22.
5. Wiring Methods (725.130)
   • Class 2 and Class 3 circuits may use cables like CL2, CL3, or CL2R for general installations.
   • Separation from other systems and wiring practices must comply with Article 722 standards.

Bundling of Cables and Power over Ethernet (PoE) (725.144)

• Cables transmitting both power and data (e.g., PoE systems) must comply with ampacity limits in Table 725.144.
• Elevated cable temperatures due to current must not degrade data transmission performance.
• Special markings (e.g., CL2-LP) identify cables designed for higher power applications.

Applications of Article 725

• Building Automation: Low-voltage controls for HVAC, lighting, and access systems.
• Security Systems: Cameras, alarms, and access control.
• Communication Networks: PoE systems for phones and Wi-Fi routers.
• Specialty Circuits: Intrusion detection and signaling circuits in industrial settings.

Conclusion

NEC Article 725 ensures the safe use and integration of Class 2 and Class 3 circuits. Its provisions for separation, fire safety, and circuit identification protect both the integrity of low-voltage systems and the safety of users and facilities.