During the past couple of months, I have had questions about whether a chase nipple, a raceway (conduit) nipple or a hub can be used to enter into panelboards, load centers, switchboards, switchgear, motor control centers and substations as transitions from cable trays into enclosures. Is this an acceptable way to transition to termination or do these installations violate the National Electrical Code (NEC)? Are special bonding and grounding requirements necessary, and is there an issue with the cable tray not being mechanically connected to the electrical equipment? Is there a difference between a cabinet for a panelboard and an enclosure for a switchboard, switchgear or a motor control center? Electricians and electrical inspectors often discuss these questions, and repair or reinstallation are often major issues with costly results.
Let’s look at the requirements for panelboards and load centers, and then later, we can look at switchboards, switchgear and motor control centers. Most identify the interior of the panelboard as the entire panel, which it is not. Panelboards and load centers are made up of a cabinet, as covered by Article 312, that houses the busbar assembly, the neutral bus and the equipment grounding bus, all of which are covered by Part I and Part III of Article 408.
The interior of the panelboard is the point of termination of the conductors on the overcurrent protective devices as well as the neutral and any equipment grounding conductors. The cabinet encloses the busbar, neutral bar and equipment grounding bar. A panelboard is usually manufactured as a discrete assembly with the cabinet ordered separately based on the location of supply with adequate spacing provided at the top, the bottom or the sides, depending on the method of entry into the cabinet with raceways and conductors or cables. A load center is manufactured as an assembly with the interior, a 4-inch-deep cabinet and a cover, while a panelboard cabinet is 6 inches deep and is manufactured separately from the interior.
The definition of a switchboard in Article 100 is “a large single panel, frame, or assembly of panels on which are mounted on the face, back, or both, switches, overcurrent and other protective devices, buses, and usually instruments. These assemblies are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets.”
A switchboard is similar in construction to a motor control center and switchgear. These enclosures are manufactured and listed as an overall assembly, including the enclosure, and are usually designed to be mounted on the floor with access into the bottom and to the interior of the enclosure by doors, removable covers or both.
To answer the first question of whether a chase nipple, a conduit nipple or a hub can be installed in a cabinet, conductors entering a cabinet must comply with 312.5(A) where openings through which conductors enter into the cabinet must be closed in an approved manner. In addition, where cables are installed into the cabinet, 312.5(C) requires each cable to be secured to the cabinet.
There is an exception to 312.5(C) permitting applying to jacketed cables with nonmetallic sheaths usually applied to Type NM cable installations. The requirement in 312.5(C) only applies to a cabinet and does not apply to switchboards, switchgear and motor control centers.
Section 300.3(A) requires any single conductors to be installed as part of a recognized wiring method in Chapter 3 of the NEC. Since Article 392 is a recognized wiring method in Chapter 3 and 392.30(B) permits cables and conductors to be secured and supported by the cable tray system transverse members, so supports will be provided to prevent stress on cables where entering raceways from the cable tray and individual conductors can be installed from the cable tray to a raceway, a conduit nipple or a chase nipple as long as the distance is not greater than 6 feet. This would not be permitted for a cabinet, but is acceptable for other enclosures, such as switchboards, etc.
Where cable trays are mechanically discontinuous, as permitted in 392.18(A), a bonding jumper, sized in accordance with 250.102, is required to connect the two sections of the cable tray or between the cable tray and the electrical equipment in accordance with 250.96. Even where bonding is provided between the cable tray and electrical equipment, equipment grounding must be established between the electrical equipment, the cable tray and the source of power so that a low impedance path is established to provide an effective ground-fault current path. Clearly, based on the references in the NEC in this written article, there should be no question on how to install these systems in accordance with the Code.