2.4 Raceways and Under-Floor Cable Support Systems

Raceways and Under-Floor Cable Support Systems

Not every horizontal route can use open J-hooks or overhead tray. Finished walls, historic buildings, open offices, and data centers often call for raceways (enclosed channels fastened to surfaces or built into the floor) and under-floor support systems beneath access floors. The INSTF blueprint lists both as separate pathway tasks, and each has its own installation rules.

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Surface and Perimeter Raceways

A surface raceway is an enclosed metallic or nonmetallic channel fastened to the face of a wall, column, or ceiling. Perimeter raceway is a larger multi-channel version that runs along the walls of a room at desk height or along the base, carrying power and telecommunications together.

Installation practice

1. Plan the route and capacity. Use the manufacturer's fill table for the raceway and the cables you will install, and leave room for growth. For fiber, the corner fittings, not the cross-section, usually limit capacity.
2. Use radius-controlled fittings. Inside corners, outside corners, flat elbows, and tees must be the manufacturer's bend-radius fittings (commonly 1 to 2 inches of radius). Square electrical fittings violate the 25 mm (1 in) at-rest radius of two- and four-fiber cable.
3. Fasten securely. Mount the base with screws or anchors at the manufacturer's spacing and near the end of every section. Adhesive alone is not an acceptable support for permanent work.
4. Separate power and telecommunications. In multi-channel raceway, keep power conductors and telecommunications cables in separate compartments divided by a barrier. Under NEC 770.133, nonconductive optical fiber cable may share a raceway with power conductors, but it may not occupy the same box or enclosure as power terminations unless the functions are associated or another listed exception applies, so outlets go in their own compartments.
5. Bond metallic raceway. Continuous metallic raceway is bonded to the telecommunications bonding system, and it serves as an equipment grounding path when it carries power circuits.
6. Place the cable, then the covers. Lay fiber into the base without pulling it tight around corners, then snap on the covers so they do not pinch the cable.

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Under-Floor Pathways

Access (raised) floors

Data centers and some offices use access floors: removable tiles on pedestals with an open cavity below. Cabling under the floor must be supported, organized, and kept out of the airflow path:

• Use tray, not the slab. Mount ventilated cable tray or wire basket on pedestal brackets or floor stands rather than laying cable loose on the slab. ANSI/TIA-942-B limits cable depth in under-floor tray to 150 mm (6 in) and requires at least 20 mm (0.75 in) between the underside of the floor tiles and the top of the tray and cabling at full calculated capacity, so cables can exit the pathway and tiles can be lifted. Some owner standards require 50 mm (2 in).
• Keep cooling paths open. Route trays in the aisles reserved for cabling and keep them clear of perforated tiles and cooling paths so cable does not dam the air.
• Separate power and data. Use separate aisles for power and telecommunications where possible; where they must cross, cross at right angles with vertical separation.
• Bond the tray system to the telecommunications bonding infrastructure per ANSI/TIA-607.
• Check the air use. If the space under the floor is used for environmental air, cable listings must suit it. In an information technology equipment room that meets NEC Article 645, the NEC permits several listed cable types under the raised floor; outside Article 645, the space follows the plenum rules of NEC 770.113.

Underfloor duct and cellular floor systems

Older office buildings embed underfloor duct (rectangular metal ducts in the slab, fed from header ducts) or cellular floor (metal deck whose cells serve as raceways). Outlets are made at preset inserts or at afterset inserts drilled into the duct.

• Locate the duct from the building drawings or with a locator before drilling an afterset insert.
• Fiber must turn up through the insert into the floor box without violating its bend radius, so use fiber-rated floor boxes with enough depth.
• Keep power and telecommunications cells separate. The NEC articles for underfloor raceways (Article 390) and cellular metal floor raceways (Article 374) govern the electrical parts.

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Under-Floor Installation Sequence

1. Survey the floor: tile layout, pedestal grid, perforated tiles, existing trays, and obstructions.
2. Mount the tray or basket on pedestal brackets at the planned elevation, keeping the clearance to the tiles.
3. Bond the tray sections together and to the secondary bonding busbar (SBB).
4. Lay or pull the cables into the tray, keeping fiber on top of or beside heavy copper bundles, never underneath them.
5. Dress cables with hook-and-loop fasteners and leave service slack at the rack entry points.
6. Replace tiles carefully. A cut tile edge, or a tile dropped onto a cable, can crush fibers; use grommets or brush seals where cables pass through tiles.