7.5 Wireways, Auxiliary Gutters, Surface Raceways & Cable Trays

7.5 Wireways, Auxiliary Gutters, Surface Raceways & Cable Trays

Conduit is not the only raceway family in NEC Chapter 3. A commercial electrician routinely installs wireways beside motor control centers, auxiliary gutters at meter stacks, surface raceway in finished offices and schools, and cable tray in industrial plants. Each has its own article, its own fill rule, and its own support interval, and the numbers differ enough that mixing them up is a reliable way to lose points in the Raceways and Boxes subject area.

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Metal and Nonmetallic Wireways (Articles 376 and 378)

A wireway is a sheet-metal or flame-retardant nonmetallic trough with a hinged or removable cover, used to house and protect conductors and cable that are laid in rather than pulled.

Fill and Derating (NEC 376.22)

• Cross-sectional area: conductors may not fill more than 20 percent of the interior cross-sectional area of the wireway at any cross section.
• Adjustment factors: the bundling adjustment factors of Table 310.15(C)(1) apply only where the number of current-carrying conductors exceeds 30. Signal circuits and controller conductors between a motor and its starter that are used only for starting duty are not counted as current-carrying.

That 30-conductor allowance is the payoff of a wireway. Thirty current-carrying conductors in a conduit would be derated to 45 percent; in a wireway they carry their full Table 310.16 ampacity.

Worked example: A 6 in. by 6 in. metal wireway has an interior area of 36 in². Twenty percent of 36 in² is 7.2 in². Using the Chapter 9 Table 5 area of 0.0133 in² for 12 AWG THHN, the wireway could physically hold 541 conductors before violating the fill rule, so fill is almost never the limiting factor. The controlling constraint is the 30-conductor derating threshold and the practical need to lay conductors in neatly.

Splices and Taps (NEC 376.56)

Splices and taps are permitted in a wireway provided they are accessible through a removable cover. At any cross section where splices or taps exist, conductors, splices, and taps together may not fill more than 75 percent of the cross-sectional area at that point.

Support (NEC 376.30)

• Horizontal runs: supported at intervals not exceeding 5 feet (1.5 m), and at each end or joint unless listed for other support intervals, with no more than one joint between supports.
• Vertical runs: supported at intervals not exceeding 15 feet (4.5 m), with no more than one joint between supports.

Nonmetallic wireway (Article 378) follows the same 20 percent and 75 percent rules, but its support spacing is closer, and expansion fittings are required where the length change from temperature swing is significant, for the same reason PVC conduit needs them.

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Auxiliary Gutters (Article 366)

An auxiliary gutter looks identical to a wireway. The legal difference is purpose and length:

An auxiliary gutter supplements wiring spaces at meter centers, distribution centers, switchboards, and similar points, and it shall not extend more than 30 ft (9 m) beyond the equipment it supplements, except as permitted for elevator work in 620.35.

A trough that runs the length of a building is a wireway. A trough that gathers the conductors at a bank of meter sockets is an auxiliary gutter. The fill numbers match Article 376: 20 percent general fill (366.22), 75 percent at any cross section containing splices or taps (366.56), and the Table 310.15(C)(1) adjustment factors applying only above 30 current-carrying conductors. Where bare conductors are installed, 366.22(B) limits current to 1,000 amperes per square inch of cross section.

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Surface Metal and Nonmetallic Raceways (Articles 386 and 388)

Surface raceway is the finished-space solution: a metal or nonmetallic channel with a snap-on cover, mounted on a wall or ceiling where fishing a wall is not practical.

• Uses permitted (386.10 / 388.10): dry locations, and extension through a dry wall, floor, or partition where the length passing through is unbroken and access to the conductors is maintained on both sides.
• Uses not permitted (386.12 / 388.12): where subject to severe physical damage, where the voltage is 300 volts or more between conductors unless the metal thickness is not less than 1.0 mm (0.040 in.), in hoistways, in hazardous (classified) locations except as specifically permitted, and where concealed. Surface nonmetallic raceway additionally may not be used where ambient temperature exceeds the listing.
• Number of conductors (386.22): the number is limited by the listing of the raceway, not by the Chapter 9 percentage tables. Where the raceway is listed with a cross-sectional area and the conductors are all the same size, the manufacturer's tables govern. The adjustment factors of 310.15(C)(1) do not apply where the raceway has a cross-sectional area exceeding 4 in², there are no more than 30 current-carrying conductors, and the conductors do not fill more than 20 percent of the area.
• Grounding: surface metal raceway may qualify as an equipment grounding conductor per 250.118 when installed with listed fittings; surface nonmetallic raceway never can, so a wire-type EGC must be installed.

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Cable Trays (Article 392)

A cable tray is a support system, not a raceway in the ordinary sense, and Article 392 reflects that.

Uses (NEC 392.10)

Cable tray is permitted as a support system for services, feeders, branch circuits, communications circuits, control circuits, and signaling circuits. Permitted contents include Type MC, MI, AC, and TC cable, optical fiber cable, and other listed multiconductor assemblies. Single-conductor building wire is permitted only in industrial establishments with qualified maintenance and supervision, and only where the conductors are 1/0 AWG or larger.

Fill (NEC 392.22)

Fill is computed as a permitted sum of cable diameters or a permitted cross-sectional area, and the allowance depends on the tray width, whether the tray is ladder or ventilated versus solid bottom, and whether the cables are multiconductor or single-conductor. Unlike conduit, there is no single percentage that covers all cases; you read the applicable subsection and table in 392.22 for the tray type in front of you.

Support and Installation (NEC 392.30)

Cable trays must be supported at intervals per the manufacturer's listing, must be installed as a complete system, and each run must be completed before cable is installed. Cable tray may extend transversely through partitions and walls where the tray is continuous and unbroken.

Cable Tray as an Equipment Grounding Conductor (NEC 392.60)

This is the most testable item in Article 392. A metallic cable tray system is permitted to be used as an equipment grounding conductor only where all of the following are true:

1. The cable tray sections and fittings are identified for grounding purposes.
2. The minimum cross-sectional area of the tray conforms to Table 392.60(A) for the rating of the largest overcurrent device protecting a circuit in the tray.
3. All tray sections and fittings are legally bonded together using listed bonding jumpers or mechanical connectors.
4. The installation is in an industrial establishment where qualified persons service the system and where conditions of maintenance and supervision ensure only qualified persons do so.

Outside an industrial establishment meeting those conditions, a wire-type equipment grounding conductor must be installed in the tray. Article 392 also requires a cable tray used as an equipment grounding conductor to be legibly marked to show that fact, so an inspector can tell at a glance that the tray is carrying the grounding duty.