The Basics of Switchboards, Switchgear, and Panelboards

Panelboards

The NEC defines a panelboard as: “A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front” [NEC 100]. These can be divided into load centers and panelboards, both of which are often referred to as “Panels.”

Switchgear

Load centers are commonly used in residential and small commercial applications. Since nearly every house in America has one, they are the least expensive way to accommodate circuit breakers. The breakers are usually less expensive because they are mass-produced and simply plug into the load center bus. Load centers are primarily intended for applications up to 240V and are normally only rated up to 225A. At those ratings, they are shallow enough to recess into a 2×4 stud wall and narrow enough to fit between studs on 16″ centers.

With larger surface mount enclosures, one manufacturer offers load centers rated up to 600A. Also, voltages up to 277V can be obtained but are not common. Both load centers and panelboards are mounted in cabinets “. . . provided with a frame, mat, or trim in which a swinging door or doors are or can be hung” [NEC 100]. As required in NEC 408.38, they also have dead fronts, meaning that there are no “. . . live parts exposed to a person on the operating side of the equipment” [NEC 100]. Normally, panelboards are used for voltages up to 600V, but higher voltage ratings are also available. Panelboards can be rated for up to 1,200A. Smaller panelboards can accommodate plug-in or bolt-on breakers. Larger panelboards only use bolt-on breakers and can have standard thermal-magnetic trip or electronic trip breakers with adjustable settings. Panelboards are deeper than load centers. The wall into which a recessed panelboard is mounted must be constructed using 2×6 studs. Panelboards rated 600A and higher are deeper and surface mounted on the wall.

Switchboards

Switchboards are defined in the NEC as “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 and front and are not intended to be installed in cabinets” [NEC 100].

Switchboards are similar to panelboards in that they are normally rated for up to 600V, they can handle higher fault currents than panelboards and load centers. They are floor mounted and are deeper than panelboards, usually starting at 18” deep. Since switchboards are larger and more expensive than panelboards, they are seldom used for bus ratings less than 1,200A and can be rated up to 5,000A. Both bolt-on breakers and draw-out breakers can be installed inside a switchboard lineup. Often, just access to the front is required, but rear and side access may also be required.

Switchgear

The NEC identifies switchgear as: “An assembly completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is provided by doors, removable covers, or both.”

Switchgear is the largest of the three. It can be rated for up to 38 kV and can have current ratings up to 6,000A. Normally draw-out breakers are used, and therefore, access to the front and rear of the gear is required. Switchgear is tested to a different UL standard than panelboards and switchboards. Because the breakers in the switchgear are each in their own compartment, the gear is rated to withstand a short circuit condition for up to 30 cycles. Panelboards and switchboards are only rated to withstand a short circuit condition for up to 3 cycles.

Switchgear often utilizes draw-out breakers. These breakers can be detached from the bus and removed for maintenance or replacement without shutting down the main or affecting the other breakers in the gear lineup. With moving parts, a draw-out breaker does need to be maintained regularly to ensure that the mechanisms are adequately lubricated and will function properly when required. Working with the breaker energized also requires particular attention to potential arc fault dangers, and personal protective equipment must be utilized.

When One Type of Gear is Preferred Over the Others

Factors that drive decisions about which type of gear to use include economics, space constraints, utility service requirements, the ability to shut down the facility, and the electrical system size (voltage and current ratings).

• Economics often drives decisions regarding which type of equipment to use. If the loads are few and small, a load center may do the job. Since specialty enclosures can get very expensive, the smallest feasible panelboard is normally used if the environment requires such an enclosure.
• Required space for switchboards or switchgear frequently becomes an issue, especially in a leased facility where square footage equals income to the owner. Wherever possible, panelboards are used to minimize wall and floor space needed for the electrical equipment.
• In order to meet the utility’s service requirements or save space inside, the main service switchgear is often mounted outside the building. This alleviates the need for a Current Transformer (C/T) enclosure for the service entrance, as a section of the gear can accommodate the utility C/Ts and meter.
• Shutting down the electrical system for maintenance may not be economically feasible in industrial or critical operation facilities. Therefore, switchgear with draw-out breakers is used.
• Depending on the facility’s power requirements, switchboards or switchgear may be needed for the main distribution equipment. However, panelboards are used wherever possible throughout the building for the economic and space considerations noted above.

Conclusion

With options for panelboards, switchboards, and switchgear, the electrical system designer has a robust palette of options to provide the required overcurrent protection for conductors throughout the facility. There are always workable solutions based on whatever factors are in play at a particular facility.