9.4 Cabinets, Panelboards, Wireways, Cable Trays & Busways

9.4 Cabinets, Panelboards, Wireways, Cable Trays & Busways

ICC 703's Wiring Methods and Materials content area (26% of the exam) and Prov's matching area (17 of 80 questions) reach beyond conduit and cable into the enclosures and larger raceway systems that tie an electrical system together. This section covers Article 312 (cabinets and cutout boxes), Article 408 (panelboards), Article 376 (metal wireways), Article 366 (auxiliary gutters), Article 392 (cable trays), and Article 368 (busways).

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1. Cabinets and Cutout Boxes (Article 312)

• Damp and wet locations (312.2): Surface-mounted enclosures must be placed or equipped to keep moisture out and mounted with at least 1/4 inch (6 mm) of airspace between the enclosure and the wall. Raceway or cable entries above the level of uninsulated live parts need fittings listed for wet locations.
• Position in walls (312.3): In walls of concrete, tile, or other noncombustible material, a flush cabinet may be set back no more than 1/4 inch (6 mm) from the finished surface. In walls of wood or other combustible material, the cabinet must be flush with the finished surface or project from it.
• Repairing gaps (312.4): Plaster, drywall, or plasterboard around the edge of a cabinet must be repaired so there are no gaps or open spaces larger than 1/8 inch (3 mm).
• Unused openings (312.5(A)): Openings through which conductors enter must be closed in an approved manner, and unused openings must be closed with protection substantially equal to the wall of the enclosure.
• Securing cables (312.5(C)): Each cable must be secured to the cabinet. An exception allows nonmetallic-sheathed cables to enter the top of a surface-mounted enclosure through a single nonflexible raceway 18 inches to 10 feet long, under conditions that include a fitting on each end, a sealed raceway end, and cable sheaths extending at least 1/4 inch into the enclosure.
• Wire-bending space (312.6): Table 312.6(A) sets the minimum bending space at terminals when the conductor does not enter or leave through the wall opposite its terminal; Table 312.6(B) applies when it does.
• Feed-through conductors and splices (312.8(A)): Enclosures for switches or overcurrent devices may contain conductors feeding through, spliced, or tapping off to other enclosures only if the conductors at any cross section fill no more than 40% of the wiring space, conductors plus splices and taps fill no more than 75%, and a warning label identifies the closest disconnecting means for any feed-through conductors.

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2. Panelboards (Article 408)

Every service and feeder ends at a panelboard, so Article 408 appears often on the exam.

• Circuit directory (408.4(A)): Every circuit and circuit modification must be legibly identified as to its clear, evident, and specific purpose or use. The identification cannot depend on transient conditions of occupancy (such as "Bob's office"), and spare positions with unused overcurrent devices must be described as spares.
• Source marking (408.4(B)): In other than one- and two-family dwellings, panelboards supplied by feeders must be permanently marked with the device or equipment where their power originates.
• High leg (408.3(E) and (F)): On a 3-phase, 4-wire delta system, the conductor with the higher voltage to ground (about 208 volts on a 120/240-volt delta) must be connected to the B phase, identified by orange or other effective marking (110.15), and the panel must be field-marked "CAUTION ___ PHASE HAS ___ VOLTS TO GROUND."
• Overcurrent protection (408.36): Each panelboard must be protected by an overcurrent device with a rating not greater than the panelboard rating, located within or ahead of the panelboard. An exception covers a panelboard used as service equipment with multiple disconnecting means under 230.71.
• Back-fed devices (408.36(D)): Plug-in breakers that are back-fed to terminate field-installed supply conductors must be secured by an additional fastener that requires more than a pull to release them.
• Number of devices (408.54): A panelboard must have physical means that prevent installing more overcurrent devices than it was designed, rated, and listed for.
• Neutral terminations (408.41): Each grounded conductor must terminate in its own terminal, not shared with another conductor. The only exception is for the grounded conductors of circuits with parallel conductors, on a terminal identified for more than one conductor.
• Equipment grounding (408.40): Equipment grounding conductors land on a terminal bar bonded to the metal enclosure. They connect to the grounded-conductor bar only where 250.142 permits it, such as at service equipment.
• Orientation (408.43): Panelboards must not be installed in the face-up position.

Worked example: A 225-ampere, 208Y/120-volt main-lugs-only panelboard is fed from a switchboard. A 200-ampere feeder breaker complies with 408.36 because 200 A does not exceed the panel's 225-A rating. A 250-ampere feeder breaker would not comply unless the panelboard had its own main breaker rated 225 A or less.

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3. Metal Wireways (Article 376) and Auxiliary Gutters (Article 366)

A metal wireway is a sheet-metal trough with hinged or removable covers that houses and protects conductors laid in place after the wireway is installed. An auxiliary gutter supplements the wiring space at meter centers, distribution centers, switchboards, and similar equipment.

When a wireway is used as a pull box for conductors 4 AWG and larger, 376.23(B) applies the pull-box distances of 314.28(A)(1) and (A)(2): 8 times the trade size for straight pulls and 6 times for angle pulls.

Worked example: A 6 in. × 6 in. wireway has an interior area of 36 square inches. The 20% fill limit is $36 \times 0.20 = 7.2$ square inches. A 3/0 AWG THHN conductor has an area of 0.2679 square inch (Chapter 9, Table 5), so $7.2 \div 0.2679 = 26.9$, and no more than 26 such conductors are allowed. Because 26 is not more than 30, no ampacity adjustment is needed. At a splice point, conductors plus splices may fill up to $36 \times 0.75 = 27$ square inches.

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4. Cable Trays (Article 392)

A cable tray is a support system, not a raceway. Wiring methods permitted in it are listed in Table 392.10(A) and include Types MC, MI, TC, ITC, PLTC, and multiconductor SE and UF cables. In industrial establishments where only qualified persons service the system, single conductors 1/0 AWG and larger are also permitted (392.10(B)).

• Tray as an equipment grounding conductor (392.60(A)): Metal cable trays may serve as equipment grounding conductors only where continuous maintenance and supervision ensure qualified persons service the system and the tray meets 392.60, including the minimum metal cross section in Table 392.60(A). Metal trays that carry only non-power conductors must still be electrically continuous through approved connections or bonding jumpers.
• Fill (392.22): Tables 392.22(A) and (B) set the allowable fill. For example, in a ladder or ventilated trough tray containing only multiconductor cables 4/0 AWG and larger, the sum of the cable diameters may not exceed the inside width of the tray, and the cables are installed in a single layer.
• Ampacity (392.80(A)): For multiconductor cables in a tray, the 310.15(C)(1) adjustment factors apply only to the number of current-carrying conductors in each cable, not to the total in the tray.

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5. Busways (Article 368)

• Where installed (368.10): Busways must be located in the open and visible. They may be installed behind access panels only under the conditions in 368.10(B), such as no overcurrent devices other than those for individual luminaires.
• Not permitted (368.12): Where subject to severe physical damage or corrosive vapors, in hoistways, in hazardous (classified) locations unless identified for the use, and outdoors or in wet or damp locations unless identified for the use.
• Support (368.30): At intervals not exceeding 5 feet, unless otherwise designed and marked.
• Reduction in ampacity (368.17(B)): Overcurrent protection is required where a busway is reduced in ampacity. In industrial establishments only, it may be omitted where the smaller busway is no more than 50 feet long, has an ampacity of at least one-third of the rating or setting of the overcurrent device next back on the line, and is free from contact with combustible material.
• Taps (368.17(C)): Plug-in devices that tap feeders or branch circuits from a busway contain the overcurrent devices for those circuits and must be externally operable circuit breakers or fusible switches. Where they are mounted out of reach, a rope, chain, or stick must be provided to operate them from the floor.