5.7 Cable Trays, Wireways, Busways, Gutters and Surface Raceways

Metal Wireways — Article 376

A wireway is a sheet-metal trough with a hinged or removable cover, used for housing and protecting conductors and cable where the conductors are laid in place after the wireway is installed.

376.12 Uses Not Permitted. Where subject to severe physical damage or corrosive vapor, and in any hazardous (classified) location except as permitted by other articles.

376.22 Number of Conductors and Ampacity.

• (A) Cross-Sectional Areas of Wireway. The sum of the cross-sectional areas of all contained conductors at any cross section shall not exceed 20 percent of the interior cross-sectional area of the wireway.
• (B) Adjustment Factors. The adjustment factors in 310.15(C)(1) shall be applied only where the number of current-carrying conductors, including neutrals classified as current-carrying under 310.15(E), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter, used only for starting duty, shall not be considered current-carrying.

The 30-conductor threshold is a large relaxation compared with the 3-conductor threshold for raceways, and it is a common exam item.

376.23(A) Deflected Insulated Conductors. Where insulated conductors are deflected within a wireway, either at the ends or where conduits enter or leave the wireway, or where the direction of the wireway is deflected more than 30 degrees, dimensions corresponding to 312.6(A) shall apply.

376.56 Splices, Taps, and Power Distribution Blocks. Splices and taps are permitted within a wireway provided they are accessible. The conductors, including splices and taps, shall not fill the wireway to more than 75 percent of its area at that point.

376.30 Securing and Supporting. Wireways shall be supported where run horizontally at each end and at intervals not exceeding 5 feet, or for listed wireways at intervals not exceeding 10 feet. Vertical runs shall be securely supported at intervals not exceeding 15 feet with not more than one joint between supports.

Nonmetallic Wireways (Article 378) have parallel rules with the same 20 percent fill, but their adjustment threshold and expansion-fitting requirements differ — 378.30 requires support at intervals not exceeding 3 feet unless listed for other spacing, and 378.44 requires expansion fittings where the expected length change exceeds 0.25 inch.

Auxiliary Gutters — Article 366

An auxiliary gutter supplements wiring spaces at meter centers, distribution centers, switchboards, and similar points. It looks like a wireway but has a different purpose and a length restriction: 366.12(1) prohibits an auxiliary gutter from containing conductors extending more than 30 feet (9 m) beyond the equipment it supplements, with an exception for elevators.

366.22(A) Sheet Metal Auxiliary Gutters. Conductors shall not fill more than 20 percent of the interior cross-sectional area. Where the number of current-carrying conductors exceeds 30, the adjustment factors of 310.15(C)(1) apply. Conductors for signaling circuits or controller conductors between a motor and its starter used only for starting duty are not counted.

366.23(A) Ampacity of Bare Conductors. Where bare conductors are installed in an auxiliary gutter, the current shall not exceed 1000 amperes per square inch of cross section of the conductor.

366.30 requires gutters to be supported throughout their length at intervals not exceeding 5 feet for sheet metal, or per the listing for nonmetallic.

Cable Trays — Article 392

A cable tray is a support system, not a raceway. This distinction drives several rules.

392.10 Uses Permitted. Cable tray may support MI, MC, MV, AC, NM, TC, ITC, PLTC, optical fiber cables, and other listed assemblies. In industrial establishments only where conditions of maintenance and supervision ensure that only qualified persons service the installation, single conductors 1/0 AWG and larger may be installed in ladder, ventilated trough, or ventilated channel-type cable tray.

392.12 Uses Not Permitted. In hoistways; where subject to severe physical damage.

392.18(A) Complete System. Cable trays shall be installed as a complete system, with field bends and modifications made so the electrical continuity and cable support of the system are maintained.

392.20 Cable and Conductor Installation. (C) Multiconductor cables rated 2000 V or less shall be permitted to be installed in the same cable tray. (D) Cables rated over 2000 V shall not be installed in the same tray with cables rated 2000 V or less unless separated by a solid fixed barrier or the higher-voltage cables are Type MC.

392.22 Number of Multiconductor Cables Rated 2000 Volts or Less in Cable Trays. The permitted fill depends on the tray type and whether the cables are all 4/0 or larger, all smaller than 4/0, or mixed, with formulas given in 392.22(A)(1) through (A)(4). Ladder and ventilated trough trays are permitted a greater fill than solid-bottom trays.

392.30(A) Cable Trays as Support. Cable trays shall be supported at intervals in accordance with the installation instructions.

392.60(B) Cable Tray as an Equipment Grounding Conductor. Metallic cable trays shall be permitted to be used as equipment grounding conductors only in industrial establishments where qualified persons service the installation, the tray sections and fittings are identified for grounding purposes, and the minimum cross-sectional area of the tray complies with Table 392.60(A).

392.80 Ampacity of Conductors. For multiconductor cables in a tray, the ampacity is generally per 310.14 for cables with three or fewer current-carrying conductors, and adjustment factors apply as specified. Where multiconductor cables are installed in a cable tray with a solid cover more than 6 feet in length, the ampacity shall not exceed 95 percent of the allowable ampacities in the applicable table — and for single conductors, 75 percent applies to covered trays exceeding 6 feet. Read 392.80 carefully; the percentages differ between the multiconductor and single-conductor subsections.

Busways — Article 368

A busway is a grounded metal enclosure containing factory-mounted bare or insulated conductors, usually copper or aluminum bars, rods, or tubes.

368.10 Uses Permitted. Busways shall be permitted to be installed where they are located in the open and visible, except as permitted in 368.10(C) for behind panels where access is provided and specified conditions are met.

368.12 Uses Not Permitted. Where subject to severe physical damage or corrosive vapors; in hoistways; in any hazardous location unless specifically approved; outdoors or in wet or damp locations unless identified for such use; where exposed to corrosive vapors.

368.17(A) Rating of Overcurrent Protection — Feeders. A busway shall be protected against overcurrent in accordance with the allowable current rating of the busway.

368.17(B) Reduction in Ampacity Size of Busway. Overcurrent protection shall be required where busways are reduced in ampacity. Exception: For industrial establishments only, omission of overcurrent protection is permitted at points where busways are reduced in ampacity, provided the length of the busway having the smaller ampacity does not exceed 50 feet (15 m) and has an ampacity at least equal to one third the rating or setting of the overcurrent device next back on the line, and provided such busway is free from contact with combustible material.

368.17(C) Feeder or Branch Circuits. Where a busway is used as a feeder, devices or plug-in connections for tapping off feeder or branch circuits from the busway shall contain the overcurrent devices required for the protection of the feeder or branch circuits. An exception permits omission where the tapped conductors are not more than 50 feet long, are enclosed in metal raceway, and are of adequate ampacity.

368.30 Support. Busways shall be securely supported at intervals not exceeding 5 feet (1.5 m) unless otherwise designed and marked.

Surface Raceways — Articles 386 and 388

Surface Metal Raceway (386) and Surface Nonmetallic Raceway (388) are used for exposed work in dry locations.

386.12 / 388.12 Uses Not Permitted. Where subject to severe physical damage unless approved for that purpose; where the voltage is 300 volts or more between conductors unless the metal has a thickness of not less than 1.02 mm (0.040 in.); where subject to corrosive vapors; in hoistways; in hazardous locations except as permitted. Surface nonmetallic raceway is additionally prohibited where subject to ambient temperatures exceeding those for which it is listed.

386.22 / 388.22 Number of Conductors. The number of conductors shall not be greater than the number for which the raceway is designed, and the cross-sectional area shall not exceed that for which the raceway is listed. The 310.15(C)(1) adjustment factors do not apply where all of the following conditions are met: the cross-sectional area of the raceway exceeds 4 square inches, the current-carrying conductors do not exceed 30 in number, and the sum of the cross-sectional areas of all conductors does not exceed 20 percent of the interior cross-sectional area of the raceway.

386.56 / 388.56 Splices and Taps. Splices and taps shall be permitted in surface raceways having a removable cover that is accessible after installation. The conductors, including splices and taps, shall not fill the raceway to more than 75 percent of its area at that point.