3.2 Requirements for Electrical Installations (Article 110)

Why This Matters on the Exam

Article 110 is the gateway chapter of the NEC — it sets the general requirements that apply to every installation before any article-specific rule is consulted. The Virginia Journeyman blueprint assigns 2 questions to Requirements for Electrical Installations, but Article 110 is referenced throughout the exam because it governs equipment approval, conductor material, electrical continuity, workmanship, identification of disconnects, and the working space around equipment. Knowing 110 cold is a prerequisite for every other chapter.

Equipment Approval, Listing, and Labeling (110.3)

NEC 110.3(A) lists eight factors an AHJ uses to judge equipment approval — suitability for installation, mechanical strength, wire-bending space, electrical insulation, heating effects, arcing effects, classification by type, and other factors. 110.3(B) is the load-bearing rule: equipment that is listed, labeled, or both must be installed and used in accordance with the instructions included in the listing or labeling. A listed breaker listed only for 75°C terminations cannot be used with 90°C-only conductors at full 90°C ampacity — the listing instructions control.

Voltage Ratings (110.4)

The nominal system voltage cannot exceed the equipment's voltage rating. Conductors and equipment must have voltage ratings sufficient for the applied voltage. A device rated 120/240V cannot be installed on a 277/480V system even if the current rating is adequate.

Conductor Material and Size (110.5, 110.6)

110.5 restricts conductor material to copper, aluminum, or copper-clad aluminum unless the Code provides otherwise. 110.6 requires conductor sizes to be expressed in American Wire Gauge (AWG) for sizes up to 4/0 and in circular mils (kcmil) above that. Standard AWG sizes run 18, 16, 14, 12, 10, 8, 6, 4, 2, 1, 1/0, 2/0, 3/0, 4/0 — then 250 kcmil and up. The NEC minimum size for general-purpose branch-circuit conductors is 14 AWG copper (15 A) per 240.4(D).

Continuity (110.7)

NEC 110.7 requires electrical installations to be free from short circuits and ground faults, and free from accidental contact between conductive parts and other conductive metal parts unless required or permitted by the Code. A Megger test between conductors and from each conductor to ground before energizing verifies compliance.

Identification of Disconnecting Means (110.22)

NEC 110.22(A) requires each disconnecting means to be legibly marked to indicate its purpose unless located and arranged so the purpose is evident. Revised for 2020: in other than one- or two-family dwellings, the marking must also include the identification of the circuit source that supplies the disconnecting means — the specific panel and location where the power originates, not a vague label like "Electrical Room." The marking must be durable enough to withstand the environment (weatherproof for outdoor disconnects). This rule aligns with 408.4(B), which already required similar source identification for switchboards, switchgear, and panelboards.

Workmanship (110.12)

NEC 110.12 requires electrical equipment to be installed in a neat and workmanlike manner. Specific subsections:

• 110.12(A) — Unused openings in enclosures, cabinets, and boxes must be closed with listed fittings providing equivalent protection to the enclosure wall. Knockout slugs left open are a violation.
• 110.12(B) — Integrity of electrical equipment must be maintained; protect panelboards from contamination by paint, plaster, or other construction debris.
• 110.12(C) — Added in 2020, requires exposed cables to be supported by structural components so they are not damaged by normal building use; cables through framing members must be protected per 300.4.
• 110.14(D) — Terminal connections must be tightened to the torque values indicated on equipment or in installation instructions, using an approved means (calibrated torque tool or manufacturer method).

Working Space (110.26)

NEC 110.26 governs the space around electrical equipment rated 600 V or less that may require examination, adjustment, servicing, or maintenance while energized.

Table 110.26(A)(1) — Minimum Depth of Working Space

Condition definitions:

• Condition 1 — Exposed live parts on one side and no live or grounded parts on the other side (or both sides guarded by insulation).
• Condition 2 — Exposed live parts on one side and grounded parts (including concrete, brick, or tile walls) on the other.
• Condition 3 — Exposed live parts on both sides of the working space (e.g., two panelboards facing each other across an aisle).

Width (110.26(A)(2)): at least 30 in. (762 mm) or the width of the equipment, whichever is greater; must permit doors to open at least 90 degrees.

Height (110.26(A)(3)): extends from grade/floor to 6½ ft (2.0 m) or the height of the equipment, whichever is greater. Do not confuse this working-space rule with the separate mounting-height rule in 240.24(A), which requires overcurrent devices to be readily accessible with the center of the grip of the operating handle no more than 6 ft 7 in. (2.0 m) above the floor or working platform. Article 110.26 says nothing about how high the top of the equipment may be.

Clear space (110.26(B)): the working space cannot be used for storage.

Worked Scenario

A 480Y/277V panelboard is mounted on a concrete wall opposite another 480Y/277V panelboard across a 3 ft aisle. Nominal voltage to ground = 277 V (151–600 V range). Both panels have exposed live parts facing each other = Condition 3. Table requires 4 ft (1.2 m) of depth. The 3 ft aisle is noncompliant — the aisle must be widened to 4 ft, or one panel must be guarded/relocated. The width must also be at least 30 in. (or the panel width, whichever is greater), and the height must be at least 6½ ft. This is a classic exam scenario: identify voltage to ground, identify condition, read the table.