Article 408 covers the requirements for switchboards and panelboards that control power and lighting circuits (Fig. 1). These rules address the equipment that forms the core of a premises electrical system. For that reason, you should invest the time it takes to become familiar with them.
What’s the difference between a panelboard and a switchboard?
- A panelboard is an assembly with buses and overcurrent protective devices (OCPDs) designed to be placed in a cabinet or enclosure [Art. 100].
- A switchboard 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 [Art. 100].
Bus arrangement
Panelboards supplied by a 4-wire, delta-connected, 3-phase (high-leg) system must have the high-leg conductor (which operates at 208V to ground) terminate to the “B” phase of the panelboard [Sec. 408.3(E)(1)]. Such a panel or switchboard must have a label that is legibly and permanently field-marked to state the phase and voltage [Sec. 408.3(F)(1)].
On a 4-wire, delta-connected, 3-phase system, where the midpoint of one phase winding of the secondary is grounded, the conductor with the resulting 208V to ground (high-leg) must be durably and permanently marked by an outer finish (insulation) that is orange in color or other effective means. Such identification must be at each point where a connection is made if the neutral conductor is present [Sec. 110.15].
The ANSI standard for meter equipment requires the high-leg conductor (208V to neutral) to terminate on the “C” (right) phase of the meter socket enclosure. This is because the demand meter needs 120V, which it gets from the “B” phase.
When replacing equipment in existing facilities that contain a high-leg conductor, double-check that you replace the high-leg conductor in its original phase position. Failure to re-terminate the high-leg per the existing installation can result in 120V circuits being inadvertently connected to the 208V high-leg with disastrous results.
The minimum wire bending space at terminals provided in switchboards and panelboards must comply with Sec. 312.6 [Sec. 408.3(G)].
Circuit directory and description
Provide a legible and permanent description on a circuit directory. Per Sec. 408.4(A)(1) through (6), the circuit description must be:
(1) Located at each circuit breaker in a switchboard.
(2) Located on the face, inside, or in an approved location adjacent to the panel door.
(3) Clear and specific as to the purpose or use of each circuit, including spare positions for unused OCPDs.
(4) Have a degree of detail and clarity that is unlikely to produce confusion between circuits.
(5) The circuit description must not depend on transient conditions of occupancy such as “dad’s office.”
(6) Be clear in explaining abbreviations and symbols when used.
Switchboards and panelboards supplied by a feeder, in other than one- and two-family dwelling units, must be marked as follows [Sec. 408.4(B)]:
(1) The identification and physical location where the power supply originates (Fig. 2).
(2) A permanent label that withstands the environment involved per Sec. 110.22(A).
(3) A method that is not handwritten.
Additional general rules
Just before stating the requirements for replacement panelboards, Part I contains three short sections — each of which provides a general rule:
- If raceways enter a switchboard, floor-standing panelboard, or similar enclosure, the raceways (including end fittings) cannot rise more than 3 in. above the bottom of the enclosure [Sec. 408.5].
- Switchboards and panelboards must have a short-circuit current rating not less than the available fault current on the line side of the equipment. In other than one- and two-dwelling units, the available fault current and the date the calculation was performed must be marked on the enclosure. The marking must be sufficiently durable to withstand the environment involved [Sec. 408.6].
- Unused openings for circuit breakers must be closed using identified closures (or other approved means) which provide protection substantially equivalent to the wall of the enclosure [Sec. 408.7].
Replacement panelboards can be installed in existing enclosures:
- If listed for the enclosure by either catalog number or dimensional information, the panelboard can be used with its short-circuit current rating [Sec. 408.9(A)].
- If not listed for the enclosure — and the available fault current is greater than 10,000A — the completed work must be field labeled. If the available fault current is 10,000A or less, the replacement panelboard shall be identified for the application. Any previously applied listing marks on the cabinet that pertain to the panelboard shall be removed [Sec. 408.9(B)].
“Field Labeled” means “equipment or materials which have a label, symbol, or other identifying mark of a field evaluation body (FEB) indicating the equipment or materials were evaluated and found to comply with the requirements described in the accompanying field evaluation report” [Art. 100].
“Field Evaluation Body (FEB)” is an organization (or part of an organization) that performs field evaluations of electrical equipment and materials.
Switchboard clearances
For other than a totally enclosed switchboard, a space of at least 3 ft must be provided between the top of the switchboard and any combustible ceiling unless a noncombustible shield is provided between the switchboard and the ceiling [Sec. 408.18(A)]. Clearances around switchboards must comply with Sec. 110.26 [Sec. 408.18(B)].
Each section of equipment that requires rear or side access to make field connections must be marked by the manufacturer on the front of the equipment. Section openings requiring rear or side access must comply with the workspace and access to workspace requirements of Sec. 110.26 [Sec. 408.18(C)].
Panelboards
The panelboard rating must not be less than the minimum feeder capacity as determined by load calculations per Art. 220, as applicable [Sec. 408.30].
Section 408.36 Overcurrent Protection
Panelboards must be provided with OCPDs within or at any point on the supply side of the panelboard with a rating not greater than the panelboard [Sec. 408.36] (Fig. 3).
When a panelboard is supplied from a transformer, as permitted in Sec. 240.21(C), the OCPDs for the panelboard can be in an enclosure ahead of the panelboard or within the panelboard [Sec. 408.36(B)].
Plug-in circuit breakers that are back-fed must be secured in place by a fastener that requires other than a pull to release the breaker from the panelboard [Sec. 408.36(D)]. This fastener prevents the circuit breaker from being accidentally removed from the panelboard while energized, which would expose someone to dangerous voltage.
Circuit breakers marked “Line” and “Load” must be installed per listing or labeling instructions [Sec. 110.3(B)]. These types of devices are therefore not permitted to be back-fed.
Cabinets for panelboards installed in damp or wet locations must be weatherproof per Sec. 312.2 [Sec. 408.37].
Panelboards must be mounted in cabinets, cutout boxes, or identified enclosures and must have dead-front covers [Sec. 408.38].
Where equipment grounding conductors of the wire type enter a panelboard metal cabinet, they must terminate to a grounding terminal bar within the panelboard metal cabinet frame [Sec. 408.40].
Equipment grounding conductors cannot terminate on the neutral terminal bar except as permitted by Sec. 250.142(D) for services and Sec. 250.30(A) for separately derived systems.
Many panelboards are rated for use as service disconnects, which means they are supplied with a main bonding jumper [Sec. 250.28]. This screw or strap is not permitted to be installed except when the panelboard is used for a service disconnect [Sec. 250.24(B)] or separately derived system [Sec. 250.30(A)(1)].
Each neutral conductor within a panelboard must terminate in an individual terminal [Sec. 408.41].
Exception: Neutral conductors run in parallel are permitted to terminate to a single terminal if the terminal is identified for more than one conductor [Sec. 110.14(A)].
If two neutral conductors are connected to the same terminal and someone removes one of them, the other neutral conductor might unintentionally be removed as well. If that happens to the neutral conductor of a multiwire circuit, it can result in excessive line-to-neutral voltage for one of the circuits, plus undervoltage for the other.
Do not install panelboards in the face-up or face-down position [Sec. 408.43].
Final thoughts
The Code provides the minimum requirements for the practical safeguarding of persons and property. At each phase of the project, think beyond those requirements. Panel layout, conductor labeling considerations, OCPD labeling considerations, conductor routing, positioning of any terminal strips, and working space between objects within the switchboard or panelboard (which differs from “spaces about equipment” per Sec. 110.26) all play a role in how safe the installation will be for maintenance purposes. A small change might make only a small difference in personnel safety. Rack up a few of those, and you can make a big difference in personnel safety.
