NEC (National Electrical Code) Article 725 covers remote-control, signaling, and power-limited circuits that are not integral to a device, appliance, or utilization equipment. These are control circuits. Possible circuits include burglar alarms, access control, and computer systems.

Circuits covered in NEC 725 are Class 2 and Class 3 power-limited circuits. Class 1 circuits have been moved to NEC article 724 as they are no longer considered power-limited as of the 2023 edition of the National Electric Code. Here is what you need to know about everything covered in this Article, including cables:

Class 2 and Class 3 Power-Limited and Remote-Control Circuits

• Class 2 Circuits:
• Class 2 circuits operate at up to 30 volts and a power limit of 100 VA. Their goal is to protect from electric shock.
• Typical applications include thermostats, intercoms, automated lighting, and control circuits where power demands are minimal.
• Due to their low power and voltage levels, these circuits pose minimal risk for shock or fire hazards, so NEC permits more relaxed installation methods, like running cables directly along structural elements without the need for extensive conduit protection.
• Class 3 Circuits:
• Class 3 circuits share similarities with Class 2 but allow slightly higher voltage and current levels. Class 3 circuits are allowed to operate up to 150 volts. They are used in more demanding control applications like HVAC or specific industrial settings.
• These circuits require more robust insulation and need additional containment if installed in high-traffic or mechanically vulnerable locations (public corridors, rooms with industrial equipment, etc). In these cases, metal conduits, raceways, or cable trays can be used.

Cable types, ratings, and markings

• CL2 (Class 2 Cable):
• Primarily used for low-voltage general control circuits in building automation, residential controls, and intercoms. The voltage is up to 30 volts.
• CL3 (Class 3 Cable):
• Designed for applications requiring slightly higher power than Class 2 circuits, commonly seen in commercial and industrial environments for HVAC controls, security systems, and lighting controls. The voltage is up to 300 volts.

Specialized Cable Types

• Power-Limited Tray Cable (PLTC):
• A robust, durable cable type suited for industrial settings requiring tray installations. PLTC cables offer flexibility for direct burial or open installation, with appropriate additional ratings. ITC cables are not used in power-limited circuits; they are limited to instrumentation circuits.
• Fire Alarm and Security System Cables (FPL, PLFA):
• Used for power-limited fire alarm systems and signaling applications, these cables are designed to remain operational under fire conditions, ensuring safety-critical systems continue functioning.
• CMP (Plenum-Rated) and CL2P: In applications where cables run through plenums or air-handling spaces, plenum-rated cables like CMP or CL2P are required to reduce smoke and toxic emissions during a fire.
• Markings and ratings:
• Cables must be marked with specific voltage, temperature, and environmental ratings. This marking ensures they suit the intended installation environment (e.g., indoor, outdoor, damp locations).
• Temperature ratings indicate the maximum operating conditions the cable insulation can handle, which is especially relevant for installations with high ambient temperatures.

Wiring methods and installation practices

• Conduit and raceway requirements:
• In many cases, PLTC and CL2/CL3 cables can be run outside of the conduit as long as they are supported and protected. However, the NEC mandates conduit or raceway installation in environments where cables could be subject to mechanical damage, such as in high-traffic areas or industrial sites.
• When installed in plenums (air handling spaces), cables must have a plenum rating (marked CMP or CL2P) to comply with fire safety standards.
• If installed in a raceway, the BICSI Cabling Installation Manual recommends:
• Limiting raceway runs to 100 feet.
• Allowing no more than two 90-degree bends
• Ensuring a maximum pull force of 25 pounds for Category 5 cable and 100 pounds for optical fiber.
• Separation requirements:
• To prevent interference and potential hazards, NEC Article 725 requires that power-limited circuits remain separate from higher-voltage circuits unless they are in separate conduits or have a grounded barrier.
• Separation is critical in cases where electrical noise could interfere with signal transmission, such as in data and communication systems that often share pathways with control circuits.
• Grounding:
• Unlike higher-power circuits, Class 2 and Class 3 circuits do not require a dedicated grounding conductor due to lower voltage.
• In environments with high electromagnetic interference (EMI), such as industrial facilities, grounding is recommended to reduce noise and improve signal quality. Shielded cables, if used, should be grounded at one end to avoid creating ground loops that could introduce noise.

Special Provisions for Applications

• Fire Alarm Systems:
• Class 2 and Class 3 circuits used in fire alarm and safety signaling applications must meet specific insulation and installation requirements to ensure continued functionality during emergencies.
• For fire-resistant installations, Fire Alarm (FPL) and Power-Limited Fire Alarm (PLFA) cables are recommended, especially when circuits connect smoke detectors, alarms, or other critical equipment. These cables are designed to withstand heat and maintain integrity in fire conditions.

Installation Conditions and Environmental Concerns

• Hazardous locations:
• For installations in hazardous environments (e.g., areas with flammable gases, vapors, or dust), Article 725 mandates using cables rated for such locations, such as UL-rated cables for Hazardous locations.
• Proper enclosures or explosion-proof fittings are required to prevent ignition sources from interacting with hazardous materials, reducing the risk of fire or explosion.
• Outdoor and damp locations:
• When installed outdoors or in damp locations, Class 2 and Class 3 power-limited circuits must use cables with moisture-resistant insulation and UV protection if exposed to sunlight.
• NEC specifies that outdoor installations should use conduits or raceways for mechanical protection or to secure the cable along building structures, especially in environments where physical damage or exposure to the elements is likely.