8.6 Cabinets, Cutout Boxes, and Panelboards

Two Articles, One Assembly

When you install a load center you are installing two code objects at once. The metal box in the wall is a cabinet governed by Article 312. The bussed assembly of breakers inside it is a panelboard governed by Article 408. Exam items routinely put a 312 fact and a 408 fact in the same set of distractors, so keep the two articles separate in your tabs.

A quick vocabulary check from Article 100:

• Cabinet — an enclosure designed for surface or flush mounting, provided with a frame in which a swinging door is hung.
• Cutout box — a surface-mounted enclosure with a swinging door secured directly to the wall of the box.
• Panelboard — a single panel or group of panel units designed to be placed in a cabinet or cutout box, accessible only from the front.
• Switchboard — a large single panel or assembly accessible from the front and the rear, not intended to be installed in a cabinet.

That last distinction is worth a mark on its own: if a candidate is told the assembly is accessible from front and rear, it is a switchboard, not a panelboard.

Article 312 — The Enclosure

312.2 Damp and wet locations

In damp or wet locations, cabinets and cutout boxes must be weatherproof, and enclosures installed in wet locations must be mounted with at least 1/4 in. of air space between the enclosure and the mounting surface. Nonmetallic enclosures in wet locations must have provisions for drainage. The purpose is to keep the back of the box from sitting in a film of water against a masonry wall.

312.3 Position in the wall

• In walls of concrete, tile, or other noncombustible material, cabinets must be installed so the front edge is set back no more than 1/4 in. from the finished surface.
• In walls of combustible material — wood framing and wood paneling — the front edge must be flush with or project from the finished surface. A recessed cabinet in a wood wall leaves an exposed combustible edge next to arcing parts.

312.4 Repairing gaps

Surfaces that are broken or incomplete around the cabinet must be repaired so there are no gaps or open spaces greater than 1/8 in. at the edge of the cabinet. This is the drywall-cut-too-big rule, and it parallels 314.21 for boxes.

312.5(C) Cables entering the enclosure

Each cable entering a cabinet, cutout box, or meter socket enclosure must be secured to the enclosure. Candidates remember that much and then miss the exception, which is narrower than most people think. Exception No. 1 permits cables with entirely nonmetallic sheaths to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 18 in. and not more than 10 ft long, provided all of the following are met:

1. Each cable is fastened within 12 in., measured along the sheath, of the outer end of the raceway.
2. The raceway extends directly above the enclosure and does not penetrate a structural ceiling.
3. A fitting is provided at each end of the raceway to protect the cable from abrasion, and the fittings remain accessible.
4. The raceway is sealed or plugged at the outer end so the enclosure cannot be accessed through it.
5. The cable sheath is continuous through the raceway and extends not less than 1/4 in. into the enclosure beyond the fitting.
6. The raceway is fastened at its outer end and at other points per the applicable article.
7. Cable fill does not exceed what Chapter 9, Table 1 permits for a complete raceway system.

This is the "nipple full of Romex above the load center" detail. It applies only to surface-mounted enclosures entered from the top — it is not general permission to skip connectors, and it does not apply to a flush-mounted panel. Independently of 312.5(C), the 2023 NEC added 334.19, which requires the nonmetallic sheath to extend at least 1/4 in. past any cable clamp or entry into any enclosure.

312.6 Wire-bending space

Table 312.6(A) gives the minimum wire-bending space at terminals where the conductor does not enter or leave through the wall opposite the terminal. Table 312.6(B) gives the larger dimension required where the conductor does enter or leave the enclosure through the wall directly opposite its terminal, because the conductor has to make a full bend. Bigger conductor, bigger number — the tables are indexed on wire size and the number of conductors per terminal.

312.8 Gutter fill

The wiring space (the gutter) of a cabinet or cutout box may be used for conductors feeding through, splices, and taps, under two ceilings:

The 2023 NEC also added 312.8(B), which permits power-monitoring and energy-management equipment to be installed inside the enclosure where it is listed for the purpose, does not occupy more than the permitted fill, and does not require the door to be modified.

Article 408 — The Panelboard

408.3 Arrangement and phase order

Conductors and busbars must be located to avoid physical damage and be arranged in an orderly sequence. 408.3(E) fixes the phase arrangement on 3-phase buses as A, B, C from front to back, top to bottom, left to right, viewed from the front.

On a 4-wire delta system, the conductor with the higher voltage to ground — 208 V on a 120/240 V high-leg delta — must terminate on phase B at the panelboard (408.3(E)). Three related rules travel with it:

• 110.15 requires that conductor to be durably and permanently marked by an outer finish that is orange or by other effective means, at each point of connection where the grounded conductor is present.
• 408.3(F)(1) requires the panelboard or switchboard to be field-marked with a caution that phase B carries 208 V to ground.
• The ANSI standard for meter equipment puts the high leg on the C phase of a meter socket. That is the contradiction candidates trip on: inside a panelboard the NEC says B, and 408.3(E) only permits C where the meter is in the same section of the switchboard or panelboard.

Before 1975 the high leg terminated on C in panelboards, so replacing equipment in an older building without tracing the existing termination can put 120 V loads on 208 V.

408.4 Field identification

• 408.4(A) Circuit directory — every circuit and circuit modification must be legibly identified as to its clear, evident, and specific purpose. The identification must include enough detail to distinguish it from all others, and it may not depend on transient conditions of occupancy. "Bedroom" is acceptable; "Johnson's room" is not, and "lights" alone is not. The directory goes on the face or inside the door of a panelboard, and inside doors or on the exposed interior of a switchboard.
• 408.4(B) Source of supply — every panelboard and switchboard supplied by a feeder must be permanently marked to identify the device or equipment where the supply originates.

408.30 and 408.36 Ratings and protection

408.30 requires a panelboard to have a rating not less than the minimum feeder capacity required by Article 220 for the load computed.

408.36 is the headline rule: each panelboard shall be protected on the supply side by an overcurrent protective device having a rating not greater than that of the panelboard. Two familiar exceptions:

• Individual protection is not required for a panelboard used as service equipment with multiple disconnects under 230.71.
• Individual protection is not required where the panelboard is protected on its supply side by two main circuit breakers or two sets of fuses having a combined rating not greater than the panelboard rating.

408.36(B) adds that where a panelboard is supplied from a transformer under 240.21(C), the panelboard's overcurrent protection must be on the secondary side — it may sit in a separate enclosure ahead of the panelboard or inside the panelboard itself. 408.37 carries the wet-location rule back to 312.2: a panelboard cabinet in a wet location must be weatherproof and mounted with 1/4 in. of air space.

408.36(D) requires plug-in overcurrent devices or plug-in main lug assemblies that are back-fed and used to terminate field-installed ungrounded supply conductors to be secured in place by an additional fastener that requires more than a pull to release. This is the retaining clip on an interconnected solar breaker.

408.41 Grounded conductor terminations

Each grounded conductor must terminate within the panelboard in an individual terminal that is not also used for another conductor. The lone exception covers grounded conductors of circuits with parallel conductors, which may land in one terminal if it is identified for more than one conductor. Equipment grounding conductors are not restricted this way, which is why a neutral bar may legitimately show one wire per hole while the ground bar shows two.

408.55 Wire-bending space

Wire-bending space at the top and bottom of a panelboard is taken from Table 312.6(B), using the largest conductor entering or leaving at that end.