12.5 Class 1, 2 and 3 Circuits, Fire Alarm, Optical Fiber and Communications

Article 725 — Class 1, Class 2, and Class 3 Circuits

725.1 Scope. Article 725 covers remote-control, signaling, and power-limited circuits that are not an integral part of a device or appliance.

725.3 lists the other articles that apply, including 300.7 for temperature-differential sealing, 300.21 for spread of fire or products of combustion, 300.22 for ducts and plenums, and 300.11 for support.

The Three Classes

725.41(A) Class 1 Power-Limited Circuits. These circuits shall be supplied by a source that has a rated output of not more than 30 volts and 1000 volt-amperes.

725.43 Class 1 Circuit Overcurrent Protection. Overcurrent protection for conductors 14 AWG and larger shall be provided in accordance with the conductor ampacity, without applying the ampacity adjustment and correction factors of 310.15 to the ampacity calculation. Overcurrent protection shall not exceed 7 amperes for 18 AWG conductors and 10 amperes for 16 AWG.

725.46 Class 1 Circuit Wiring Methods. Class 1 circuits shall be installed in accordance with Part I of Article 300 and with the wiring methods from the appropriate articles in Chapter 3 — that is, Class 1 circuits are wired essentially like power circuits.

725.48(B) Class 1 Circuits with Power Supply Circuits. Class 1 circuits and power supply circuits shall be permitted to occupy the same cable, enclosure, or raceway only where the equipment powered is functionally associated.

725.121 Power Sources for Class 2 and Class 3 Circuits. The power source for a Class 2 or Class 3 circuit shall be one of the following: a listed Class 2 or Class 3 transformer; a listed Class 2 or Class 3 power supply; other listed equipment marked to identify the Class 2 or Class 3 power source; listed information technology equipment limited power source; a listed audio/video information and communication technology equipment limited power source; a listed audio amplifier; or a dry cell battery that provides a Class 2 power source.

Chapter 9, Tables 11(A) and 11(B) define the class limits. For an inherently limited Class 2 alternating-current source at over 20 through 30 volts, the maximum power source is 100 volt-amperes; for a Class 3 source at over 30 through 100 volts, the maximum is 100 volt-amperes with additional current limits. Look them up rather than memorizing; the tables are dense and the exam is open book.

Separation — 725.136

725.136(A) General. Cables and conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, non-power-limited fire alarm circuits, and medium-power network-powered broadband communications circuits, unless permitted by 725.136(B) through (I).

The listed exceptions include:

• (B) Separated by a barrier;
• (C) In raceways within enclosures, where the electric light and power conductors are routed to maintain a minimum of 6 mm (0.25 in.) separation;
• (D) Associated systems within enclosures where introduced solely to connect the equipment, with a minimum 6 mm (0.25 in.) separation maintained;
• (I) Other Applications. Cables and conductors of Class 2 and Class 3 circuits shall be separated by at least 50 mm (2 in.) from conductors of any electric light, power, Class 1, non-power-limited fire alarm, or medium-power network-powered broadband communications circuits, unless one of the specified conditions is met — including that the power conductors are in a raceway or metal-sheathed cable, or the Class 2/3 conductors are in a raceway or metal-sheathed cable, or the Class 2/3 conductors are permanently separated by a continuous and firmly fixed nonconductor.

The 2-inch separation rule in 725.136(I) is the number to remember for open wiring.

Power over Ethernet — 725.144

New and expanded in the 2020 edition. 725.144 Transmission of Power and Data. Where transmitting power and data, Class 2 and Class 3 cables shall be installed in accordance with the section, and Table 725.144 limits the current per conductor based on conductor size, cable temperature rating, and the number of cables in a bundle.

The underlying concern is thermal. A large bundle of small-conductor cables each carrying meaningful current has no way to shed heat from the interior of the bundle. Table 725.144 accordingly reduces the permitted per-conductor current as bundle size grows — for example, from a much higher value for a small bundle down to a fraction of that for bundles of 192 cables.

725.144(B) requires that where Type CL2, CL3, or listed communications cables transmit power and data and the current in any conductor exceeds the values in Table 725.144, the conductors shall be sized and installed per the ampacity requirements of that table.

Article 760 — Fire Alarm Systems

760.1 Scope. Article 760 covers the installation of wiring and equipment of fire alarm systems, including all circuits controlled and powered by the fire alarm system.

760.24 Mechanical Execution of Work. Fire alarm circuits shall be installed in a neat and workmanlike manner. Cables and conductors installed exposed on the surface of ceilings and sidewalls shall be supported by the building structure in such a manner that the cable will not be damaged by normal building use. Such cables shall be secured by hardware including straps, staples, cable ties, hangers, or similar fittings designed and installed so as not to damage the cable. The installation shall also comply with 300.4(D) and 300.11. Cable ties shall be listed as having low smoke and heat release properties where installed in plenums.

760.30 Fire Alarm Circuit Identification. Fire alarm circuits shall be identified at terminal and junction locations in a manner that helps to prevent unintentional signals on fire alarm system circuits during testing and servicing of other systems.

760.35 Fire Alarm Circuit Requirements. Fire alarm circuits shall comply with Parts I and II for non-power-limited fire alarm (NPLFA) circuits and Parts I and III for power-limited fire alarm (PLFA) circuits.

760.41 NPLFA Circuit Power Source Requirements. (A) The power source of non-power-limited fire alarm circuits shall comply with Chapters 1 through 4, and the output voltage shall be not more than 600 volts, nominal. (B) Branch Circuit. The branch circuit supplying the fire alarm equipment shall supply no other loads. The branch circuit shall not be supplied through ground-fault circuit interrupters or arc-fault circuit interrupters.

The prohibition on GFCI and AFCI protection is deliberate and testable: an unwanted trip would disable the fire alarm system.

760.43 NPLFA Circuit Overcurrent Protection. Overcurrent protection for conductors 14 AWG and larger shall be provided in accordance with the conductor ampacity without applying the adjustment and correction factors of 310.15. Overcurrent protection shall not exceed 7 amperes for 18 AWG and 10 amperes for 16 AWG.

760.121 Power Sources for PLFA Circuits. The power source for a power-limited fire alarm circuit shall be one of the following: a listed PLFA or Class 3 transformer; a listed PLFA or Class 3 power supply; listed equipment marked to identify the PLFA power source; or a dry cell battery listed and provided in the equipment. The branch circuit supplying the fire alarm equipment shall supply no other loads and shall not be supplied through GFCI or AFCI devices.

760.124 Marking. PLFA cables shall be marked in accordance with Table 760.154(A), using types FPLP for plenum, FPLR for riser, and FPL for general purpose.

760.136 contains the separation rules paralleling 725.136, requiring PLFA cables to be separated by at least 2 inches from conductors of electric light, power, Class 1, and NPLFA circuits, with the same set of exceptions.

760.139 governs installation of conductors of different PLFA circuits, Class 2, Class 3, and communications circuits in the same cable, enclosure, cable tray, or raceway, generally permitting it where all the conductors are insulated for the maximum voltage of any conductor present.

Article 770 — Optical Fiber Cables

770.2 Definitions. Conductive optical fiber cable contains non-current-carrying conductive members such as metallic strength members and metallic vapor barriers. Nonconductive optical fiber cable contains no metallic members and no other electrically conductive materials. Composite optical fiber cable contains optical fibers and current-carrying electrical conductors.

770.24 Mechanical Execution of Work. Optical fiber cables shall be installed in a neat and workmanlike manner, supported by the building structure so they will not be damaged by normal building use, and secured by hardware designed and installed so as not to damage the cable.

770.93 Grounding or Interruption of Non-Current-Carrying Metallic Members of Optical Fiber Cables. Where exposed to lightning or accidental contact with power conductors, the non-current-carrying metallic members shall be grounded as close as practicable to the point of entrance, or shall be interrupted as close as practicable to the point of entrance by an insulating joint or equivalent device.

770.133 Installation of Optical Fibers and Electrical Conductors. (A) With conductors for electric light, power, Class 1, NPLFA, or medium-power network-powered broadband — nonconductive optical fiber cables shall not be permitted to occupy the same cable, cable tray, or raceway with those conductors unless the functions are associated and the nonconductive optical fiber cable is installed in factory- or field-assembled control cables or the conductors are installed in accordance with the specified conditions.

Chapter 8 — Communications Systems

The 2020 NEC restructured Chapter 8 substantially. Article 800 was retitled "General Requirements for Communications Systems" and now contains the common requirements that apply across the chapter, while the individual articles retain their subject-specific rules:

Remember 90.3: Chapter 8 is not subject to the requirements of Chapters 1 through 7 except where those requirements are specifically referenced. A Chapter 3 requirement does not automatically apply to a communications cable.

800.24 Mechanical Execution of Work. Communications circuits and equipment shall be installed in a neat and workmanlike manner, with exposed cables supported by the building structure so they are not damaged by normal building use, and secured by listed hardware designed and installed so as not to damage the cable. Cable ties in plenums shall be listed as having low smoke and heat release properties.

800.100 Bonding Conductor and Equipment Grounding Conductor. The bonding conductor or grounding electrode conductor shall be copper or other corrosion-resistant conductive material, shall be insulated, covered, or bare, and shall be not smaller than 14 AWG, with a current-carrying capacity approximately equal to that of the grounded metallic sheath member and protected conductors. The bonding conductor shall be as short as practicable, and in one- and two-family dwellings shall not exceed 6 m (20 ft) in length.

800.100(B) Electrode. The bonding conductor or grounding electrode conductor shall be connected to the nearest accessible location on the intersystem bonding termination as required by 250.94, or where an intersystem bonding termination is not available, to specified alternatives.

800.53 Separation from Lightning Conductors. Where practicable, a separation of at least 1.8 m (6 ft) shall be maintained between communications cables on buildings and lightning conductors.

805.90 Protective Devices requires a listed primary protector on each circuit run partly or entirely in aerial wire or aerial cable not confined within a block, and on each circuit not so exposed but running into the building and exposed to accidental contact with power conductors over 300 volts to ground.