NEC Requirements for Switchgear and Switchboards

How the NEC applies

The NEC is a model code adopted, sometimes with amendments, by states and municipalities, and enforced by the authority having jurisdiction (AHJ). Which edition applies, and with what local amendments, is decided by the jurisdiction rather than the designer or manufacturer, and the AHJ's interpretation controls.

Equipment construction is covered by the product standards (UL 891 for switchboards, UL 1558 for low-voltage power circuit breaker switchgear), and the NEC generally requires that equipment be listed and installed in accordance with its listing and instructions. Applicable listings, standards and design requirements depend on equipment type, configuration, project specifications and jurisdiction.

Article 408: switchboards, switchgear and panelboards

Article 408 is the home article for this equipment. It covers the support and arrangement of busbars and conductors, including the standard phase arrangement (A, B, C from front to back, top to bottom or left to right as viewed from the front). It requires barriers in service equipment so that no uninsulated, ungrounded service busbar or terminal is exposed to inadvertent contact while load terminations are worked on, and it addresses clearances, wire bending space, grounding and bonding of the enclosure, and circuit directory and field marking requirements.

Working space, egress and dedicated space: 110.26

Section 110.26 governs the space around equipment likely to require examination, adjustment, servicing or maintenance while energized. Working space depth depends on the voltage to ground and on what faces the equipment (insulated surfaces, grounded surfaces or exposed live parts), and width and height are also specified. Rear-accessible switchgear requires the same working space at the rear.

Large equipment brings more: above stated ampere ratings and widths, the code calls for entrance and egress provisions such as a second entrance and personnel doors that open in the direction of egress with panic hardware. Dedicated equipment space above and below the equipment must be kept clear of foreign systems such as piping and ductwork, and the working space must be illuminated.

Fault current, arc-flash marking and arc energy reduction

Section 110.24 requires service equipment in other than dwelling units to be field marked with the maximum available fault current and the date of the calculation, updated when system changes affect it. Section 110.16 requires an arc-flash hazard warning label on switchgear, switchboards and similar equipment likely to be worked on while energized, and for larger service equipment calls for additional information such as nominal voltage, available fault current, clearing time and date. Label content generally comes from the arc-flash study performed under NFPA 70E.

Section 240.87 addresses arc energy reduction for circuit breakers rated or adjustable to 1,200 A or higher. One of the listed methods must be provided so that clearing time is reduced when an arcing fault occurs or while maintenance is under way, and documentation must show the method is set to operate below the available arcing current.

• Zone-selective interlocking (ZSI)
• Differential relaying
• An energy-reducing maintenance switch with a local status indicator
• An energy-reducing active arc-flash mitigation system
• An instantaneous trip setting or instantaneous override below the available arcing current
• An approved equivalent means

Services, standby systems and special occupancies

Article 230 covers services and service equipment: the number and grouping of service disconnects, marking of equipment as suitable for use as service equipment, and ground-fault protection of equipment for solidly grounded wye services above 150 V to ground and 1,000 A or more. Requirements for arranging multiple service disconnects have changed between editions, so check the service section against the adopted one.

Articles 700, 701 and 702 cover emergency, legally required standby and optional standby systems. Emergency and legally required systems carry requirements for transfer equipment, separation from normal wiring, testing and selective coordination of overcurrent devices, which directly influences whether switchgear with short-time delay capability or switchboards are appropriate. Article 517 defines the essential electrical system for health care facilities, with its branches and transfer requirements. Article 495 covers equipment over 1,000 V, including medium-voltage switchgear; it was renumbered from Article 490, so older specifications may cite the earlier number.

A design checklist

Before releasing an equipment specification or room layout, confirm the following against the adopted edition and the AHJ. They are the items that most often force late changes when discovered at inspection rather than at design.

• Adopted NEC edition and local amendments identified and documented
• Working space, rear access, egress and dedicated space per 110.26 shown on the layout
• Available fault current calculated and the 110.24 marking planned
• Arc-flash labels per 110.16 sourced from the study, including the additional service equipment data
• 240.87 method selected for every breaker at 1,200 A or above, with documentation of settings
• Selective coordination and transfer requirements from Articles 700, 701 and 517 reflected in the equipment class and settings