Understanding NEC Article 724: Class 1 Power-Limited Circuits and Class 1 Remote-Control and Signaling Circuits

NEC Article 724 outlines the requirements for Class 1 power-limited circuits and Class 1 remote-control and signaling circuits. These systems are essential for operating motor controllers, lighting control systems, security devices, and other low-voltage circuits in buildings.

Scope of Article 724

This article applies to:

• Class 1 circuits, including power-limited circuits for remote control and signaling.
• Circuits extending from power supplies to connected equipment, excluding integral device wiring.

Informational Notes:

• Systems like nurse call circuits, access control, and some low-voltage lighting controls may fall under Article 724.
• Wiring for information technology equipment rooms constructed per NFPA 75 is governed by Article 645, not Article 724.

Key Requirements for Class 1 Circuits

1. Voltage and Power Limitations (724.40)
   • Circuits are limited to 30 volts and 1000 volt-amperes (VA).
   • Transformers supplying these circuits must comply with Article 450.
2. Circuit Protection (724.43)
   • Overcurrent devices must match conductor ampacity.
   • Specific limits apply:
     • 7 amperes for 18 AWG conductors.
     • 10 amperes for 16 AWG conductors.
3. Wiring Methods (724.46–724.48)
   • Circuits must comply with general wiring methods in Chapter 3.
   • Cables must be installed neatly and securely to avoid damage.
   • Power-supply conductors and Class 1 circuits may share raceways if functionally associated.

Fire Safety and Environment

1. Fire Spread Prevention (724.3(B))
   • Installations must limit fire spread or smoke production.
   • Plenum-rated cables are required for ducts or air-handling spaces.
2. Hazardous Locations (724.3(D))
   • Class 1 circuits are prohibited in hazardous locations unless permitted by specific articles.

Special Considerations

1. Identification and Access (724.30, 724.21)
   • Circuits must be labeled to prevent interference during testing or servicing.
   • Wiring must not block access to panels or equipment.
2. Circuit Integrity (CI) Cables (724.3(G))
   • Vertical installations of CI cables must comply with fire-rated support and separation methods.
3. Aerial Circuits (724.52)
   • Circuits extending between buildings must follow Article 225 standards for aerial installations.

Applications

Article 724 supports systems such as:

• Motor Controls: Elevators, conveyors, and shunt-trip circuits for breakers.
• Security Systems: Access controls and alarms.
• Lighting Controls: Dimming systems and remote switches.
• Low-Voltage Systems: Specialty control circuits for building automation.

Conclusion

NEC Article 724 ensures the safe installation of Class 1 circuits by addressing voltage limits, overcurrent protection, fire safety, and wiring methods. These standards protect personnel and property while supporting critical control and signaling functions.