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Article 393

Low-Voltage Suspended Ceiling Power Distribution Systems

Part I. General

  1. Scope. This article covers the installation of low-voltage suspended ceiling power distribution systems.
  2. Definitions.

Busbar. A noninsulated conductor electrically connected to the source of supply and physically supported on an insulator providing a power rail for connection to utilization equipment, such as sensors, actuators, A/V devices, low-voltage luminaire assemblies, and similar electrical equipment.

Busbar Support. An insulator that runs the length of a section of suspended ceiling bus rail that serves to support and isolate the busbars from the suspended grid rail.

Connector. A term used to refer to an electromechanical fitting.

Connector, Load. An electromechanical connector used for power from the busbar to utilization equipment.

Connector, Pendant. An electromechanical or mechanical connector used to suspend low-voltage luminaire or utilization equipment below the grid rail and to supply power to connect from the busbar to utilization equipment.

Connector, Power Feed. An electromechanical connector used to connect the power supply to a power distribution cable, to

ARTICLE 393 — LOW-VOLTAGE SUSPENDED CEILING POWER DISTRIBUTION SYSTEMS 393.30

connect directly to the busbar, or to connect from a power distribution cable to the busbar.

Connector, Rail to Rail. An electromechanical connector used to interconnect busbars from one ceiling grid rail to another grid rail.

Grid Bus Rail. A combination of the busbar, the busbar support, and the structural suspended ceiling grid system.

Low-Voltage Suspended Ceiling Power Distribution System. A system that serves as a support for a finished ceiling surface and consists of a busbar and busbar support system to distribute power to utilization equipment supplied by a Class 2 power supply.

Power Supply. A Class 2 power supply connected between the branch-circuit power distribution system and the busbar low- voltage suspended ceiling power distribution system.

Rail. The structural support for the suspended ceiling system typically forming the ceiling grid supporting the ceiling tile and listed utilization equipment, such as sensors, actuators, A/V devices, and low-voltage luminaires and similar electrical equipment.

Reverse Polarity Protection (Backfeed Protection). A system that prevents two interconnected power supplies, connected positive to negative, from passing current from one power source into a second power source.

Suspended Ceiling Grid. A system that serves as a support for a finished ceiling surface and other utilization equipment.

393.6 Listing Requirements. Suspended ceiling power distri‐ bution systems and associated fittings shall be listed as in 393.6(A) or (B).

  1. Listed System. Low-voltage suspended ceiling distribution systems operating at 30 volts ac or less or 60 volts dc or less shall be listed as a complete system, with the utilization equip‐ ment, power supply, and fittings as part of the same identified system.
  2. Assembly of Listed Parts. A low-voltage suspended ceiling power distribution system assembled from the following parts, listed according to the appropriate function, shall be permit‐ ted:
  1. Listed low-voltage utilization equipment
  2. Listed Class 2 power supply
  3. Listed or identified fittings, including connectors and grid rails with bare conductors
  4. Listed low-voltage cables in accordance with 725.179, conductors in raceways, or other fixed wiring methods for the secondary circuit

Part II. Installation

393.10 Uses Permitted. Low-voltage suspended ceiling power distribution systems shall be permanently connected and shall be permitted as follows:

  1. For listed utilization equipment capable of operation at a maximum of 30 volts ac (42.4 volts peak) or 60 volts dc (24.8 volts peak for dc interrupted at a rate of 10 Hz to 200 Hz) and limited to Class 2 power levels in Chapter 9, Table 11(A) and Table 11(B) for lighting, control, and signaling circuits.
  2. In indoor dry locations.
  3. For residential, commercial, and industrial installations.
  4. In other spaces used for environmental air in accordance with 300.22(C), electrical equipment having a metal enclosure, or with a nonmetallic enclosure and fittings, shall be listed for use within an air-handling space and shall have adequate fire-resistant and low-smoke- producing characteristics and associated wiring material suitable for the ambient temperature.

Informational Note: One method of defining adequate fire- resistant and low-smoke producing characteristics for electrical equipment with a nonmetallic enclosure is in ANSI/ UL 2043-2008, Fire Test for Heat and Visible Smoke Release for Discrete Products and Their Accessories Installed in Air-Handling Spaces.

393.12 Uses Not Permitted. Suspended ceiling power distri‐ bution systems shall not be installed in the following:

  1. In damp or wet locations
  2. Where subject to corrosive fumes or vapors, such as stor‐ age battery rooms
  3. Where subject to physical damage
  4. In concealed locations
  5. In hazardous (classified) locations
  6. As part of a fire-rated floor-ceiling or roof-ceiling assem‐ bly, unless specifically listed as part of the assembly
  7. For lighting in general or critical patient care areas

393.14 Installation.

  1. General Requirements. Support wiring 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 is not damaged by normal building use. Such cables shall be supported by straps, staples, hangers, cable ties, or similar fittings designed and installed so as not to damage the cable.Informational Note: Suspended ceiling low-voltage power grid distribution systems should be installed by qualified persons in accordance with the manufacturer’s installation instructions.
  2. Insulated Conductors. Exposed insulated secondary circuit conductors shall be listed, of the type, and installed as described as follows:
  1. Class 2 cable supplied by a listed Class 2 power source and installed in accordance with Parts I and III of Article 725
  2. Wiring methods described in Chapter 3

393.21 Disconnecting Means.

  1. Location. A disconnecting means for the Class 2 supply to the power grid system shall be located so as to be accessible and within sight of the Class 2 power source for servicing or mainte‐ nance of the grid system.
  2. Multiwire Branch Circuits. Where connected to a multi‐ wire branch circuit, the disconnecting means shall simultane‐ ously disconnect all the supply conductors, including the grounded conductors.

393.30 Securing and Supporting.

  1. Attached to Building Structure. A suspended ceiling low- voltage power distribution system shall be secured to the mounting surface of the building structure by hanging wires, screws, or bolts in accordance with the installation and opera‐ tion instructions. Mounting hardware, such as screws or bolts,393.30 ARTICLE 394 — CONCEALED KNOB-AND-TUBE WIRING
    shall be either packaged with the suspended ceiling low-voltage lighting power distribution system, or the installation instruc‐ tions shall specify the types of mounting fasteners to be used.
  2. Attachment of Power Grid Rails. The individual power grid rails shall be mechanically secured to the overall ceiling grid assembly.

393.40 Connectors and Enclosures.

  1. Connectors. Connections to busbar grid rails, cables, and conductors shall be made with listed insulating devices, and these connections shall be accessible after installation. A soldered connection shall be made mechanically secure before being soldered. Other means of securing leads, such as push-on terminals and spade-type connectors, shall provide a secure mechanical connection. The following connectors shall be permitted to be used as connection or interconnection devices:
    1. Load connectors shall be used for power from the busbar to listed utilization equipment.
    2. A pendant connector shall be permitted to suspend low- voltage luminaires or utilization equipment below the grid rail and to supply power from the busbar to the utilization equipment.
    3. A power feed connector shall be permitted to connect the power supply directly to a power distribution cable and to the busbar.
    4. Rail-to-rail connectors shall be permitted to interconnect busbars from one ceiling grid rail to another grid rail.
    Informational Note: For quick-connect terminals, see UL 310, Standard for Electrical Quick-Connect, and for mechanical splicing devices, see UL 486A, Standard for Wire Connectors and Soldering Lugs for Use with Copper Conductors, and 486B, Standard for Wire Connectors.
  2. Enclosures. Where made in a wall, connections shall be installed in an enclosure in accordance with Parts I, II, and III of Article 314.

393.45 Overcurrent and Reverse Polarity (Backfeed) Protec‐ tion.

  1. Overcurrent Protection. The listed Class 2 power supply or transformer primary shall be protected at not greater than 20 amperes.
  2. Interconnection of Power Sources. Listed Class 2 sources shall not have the output connections paralleled or otherwise interconnected unless listed for such interconnection.
  3. Reverse Polarity (Backfeed) Protection of Direct-Current Systems. A suspended ceiling low-voltage power distribution system shall be permitted to have reverse polarity (backfeed) protection of dc circuits by one of the following means:
  1. If the power supply is provided as part of the system, the power supply is provided with reverse polarity (backfeed) protection; or
  2. If the power supply is not provided as part of the system, reverse polarity or backfeed protection can be provided as part of the grid rail busbar or as a part of the power feed connector.
  1. Splices. A busbar splice shall be provided with insula‐ tion and mechanical protection equivalent to that of the grid rail busbars involved.
  2. Connections. Connections in busbar grid rails, cables, and conductors shall be made with listed insulating devices and

be accessible after installation. Where made in a wall, connec‐ tions shall be installed in an enclosure in accordance with Parts I, II, and III of Article 314, as applicable.

393.60 Grounding.

  1. Grounding of Supply Side of Class 2 Power Source. The supply side of the Class 2 power source shall be connected to an equipment grounding conductor in accordance with the applicable requirements in Part IV of Article 250.
  2. Grounding of Load Side of Class 2 Power Source. Class 2 load side circuits for suspended ceiling low-voltage power grid distribution systems shall not be grounded.

Part III. Construction Specifications

393.104 Sizes and Types of Conductors.

  1. Load Side Utilization Conductor Size. Current-carrying conductors for load side utilization equipment shall be copper and shall be 18 AWG minimum.Exception: Conductors of a size smaller than 18 AWG, but not smaller than 24 AWG, shall be permitted to be used for Class 2 circuits. Where used, these conductors shall be installed using a Chapter 3 wiring method, shall be totally enclosed, shall not be subject to movement or strain, and shall comply with the ampacity requirements in Table 522.22.
  2. Power Feed Bus Rail Conductor Size. The power feed bus rail shall be 16 AWG minimum or equivalent. For a busbar with a circular cross section, the diameter shall be 1.29 mm (0.051 in.) minimum, and, for other than circular busbars, the area shall be 1.32 mm2 (0.002 in.2) minimum.

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