NEC Article 110 — Requirements for Electrical Installations: The Complete Exam Guide

Article 110 is where the NEC starts. Before you pick a wiring method, size a conductor, or design a circuit, Article 110 establishes the baseline requirements that apply to all electrical installations — what equipment is approved, how it must be installed, how connections must be made, and what clearances must be maintained around electrical equipment.

These rules are abstract enough that many electricians learn them on the job without ever reading the article. On the exam, they're high-value targets precisely because they're foundational but easy to overlook.

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Article 110 Structure

The key exam-tested sections:

• 110.2 — Approval
• 110.3 — Examination, identification, installation, and use of equipment
• 110.9 — Interrupting rating
• 110.10 — Circuit impedance, short-circuit current ratings, and other characteristics
• 110.12 — Mechanical execution of work
• 110.14 — Electrical connections (temperature ratings)
• 110.26 — Spaces about electrical equipment (working clearances)
• 110.27 — Guarding of live parts
• 110.28 — Enclosure selection for specific locations (Table 110.28)

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110.2 — Approval

Conductors and equipment required or permitted by the NEC must be acceptable to the authority having jurisdiction (AHJ).

The AHJ — typically the local building inspector or fire marshal — has final say on what is acceptable. Listed and labeled equipment is presumed to comply, but the AHJ can approve non-listed equipment or reject listed equipment that's misapplied.

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110.3 — Examination, Identification, Installation, and Use of Equipment

110.3(B) — Installation and Use Per Listing

Equipment must be installed and used in accordance with any instructions included in the listing or labeling.

Exam tip: This single sentence is one of the most cited code violations in the NEC. If a piece of equipment is listed for dry locations only and installed outdoors, that's a 110.3(B) violation — regardless of whether a specific outdoor installation rule is violated. The listing instructions are incorporated into the code by reference.

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110.9 — Interrupting Rating

Every fuse, circuit breaker, switch, and other device used to interrupt current must have an interrupting rating not less than the available fault current at the point of installation.

Why this matters: The available fault current (short-circuit current) at the service can be tens of thousands of amperes. A standard residential circuit breaker has an interrupting rating of 10,000A. At a commercial service with 22,000A available fault current, a 10,000A-rated breaker would fail violently if called upon to clear a bolted fault.

Interrupting ratings are marked on the equipment. Current-limiting fuses (Class J, Class R, Class L) can have interrupting ratings of 200,000A and are used to protect downstream equipment with lower interrupting ratings.

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110.10 — Circuit Impedance and Short-Circuit Current Ratings

Equipment must have a short-circuit current rating (SCCR) that permits the circuit protective device to clear a fault without extensive damage to the components.

The distinction from 110.9: interrupting rating is for the overcurrent device itself (can it interrupt the fault current?); SCCR is for the equipment it protects (can the equipment withstand the forces during the fault, even momentarily?).

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110.12 — Mechanical Execution of Work

Electrical equipment must be installed in a neat and workmanlike manner.

Specific requirements under 110.12:

• 110.12(A) — Unused openings in boxes, raceways, and enclosures must be effectively closed (knockout seals, etc.)
• 110.12(B) — Internal parts of electrical equipment must not be damaged or contaminated by foreign materials (paint, plaster, cleaners, etc.)
• 110.12(C) — Cables and conductors must be anchored and supported; they cannot be draped or laid across pipes, ducts, or structural members without proper support

Exam tip: 110.12 is the code basis for "sloppy wiring" violations. An inspector who finds wire casually draped across ceiling grid wire or a panel with multiple open knockouts can cite 110.12 without referencing any other specific section.

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110.14 — Electrical Connections

This section governs how conductors connect to equipment — one of the most tested provisions in Article 110.

110.14(A) — Terminals

Terminals must be identified for the conductor material being used (copper, aluminum, or copper-clad aluminum). Terminals marked for aluminum or copper-aluminum service can accept both; terminals without marking are for copper only.

110.14(B) — Splices

Splices must be made with listed splicing devices (wire nuts, splice kits, compression splices) or by brazing, welding, or soldering. Splices must be mechanically and electrically secure.

110.14(C) — Temperature Limitations

This is the most exam-critical provision in Article 110.

The rule: The temperature rating associated with the ampacity of a conductor must be selected and coordinated with the lowest temperature rating of any connected termination, conductor, or device.

Two tiers:

Key point about THHN (90°C rated conductor):

THHN is physically rated at 90°C, but in most installations:

• The terminations are rated 60°C or 75°C
• Therefore the conductor must be sized using the 60°C or 75°C ampacity, not the 90°C column

When can you use the 90°C column? Only when both the conductor and the termination are rated for 90°C. Some industrial lugs are marked for 90°C — this is the exception, not the rule.

The correction/adjustment exception: You may use 90°C ampacity as the starting point for correction factor calculations (ambient temperature, conduit fill), then verify the corrected value doesn't exceed the terminal's rated temperature ampacity.

Exam tip: The most common 110.14(C) exam question: "A 100A circuit uses THHN conductors. What ampacity column of Table 310.16 applies at the panelboard terminals?" Answer: 75°C (equipment > 100A... wait — at exactly 100A, it's ≤100A, so 60°C applies unless the equipment is specifically marked for 75°C). Modern panels are typically marked for 75°C — read the question carefully.

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110.26 — Spaces About Electrical Equipment

This section is consistently one of the highest-tested provisions in the entire NEC — particularly Table 110.26(A)(1) and the width/height requirements.

110.26(A)(1) — Depth of Working Space

The minimum depth of working space (measured from the front of the equipment) is found in Table 110.26(A)(1).

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

Conditions:

• Condition 1: Exposed live parts on one side; no live or grounded parts on the other side (or insulated)
• Condition 2: Exposed live parts on one side; grounded parts (concrete wall, grounded metal) on the other side
• Condition 3: Exposed live parts on both sides of the working space

Exam tip: Condition 2 (live parts vs. a concrete block wall) at 480V = 3.5 feet. This exact scenario appears frequently. Condition 3 at 480V = 4 feet. Memorize the 151–600V row — that's the commercial/industrial voltage range tested most.

110.26(A)(2) — Width of Working Space

The working space must be at least 30 inches wide or the width of the equipment — whichever is greater.

A 42-inch-wide panelboard requires 42 inches of clear width. A 24-inch panelboard still requires the minimum 30 inches.

110.26(A)(3) — Height of Working Space

Working space must extend from the floor (or platform) to a minimum height of 6.5 feet (6 feet 6 inches) or the height of the equipment — whichever is greater.

A 7-foot-tall switchboard requires 7 feet of clear height.

110.26(B) — Clear Spaces — No Storage

Working space required by 110.26 must not be used for storage.

Stacking boxes, equipment, or supplies in front of a panelboard violates 110.26(B) regardless of the clearance depth. The space must be kept clear at all times.

110.26(C) — Entrance to Working Space

For large equipment (1,200A or more, over 6 feet wide) at voltages above 150V to ground, there must be one entrance at each end of the working space unless sufficient clearance exists (8 feet or more) for unobstructed exit from either end.

110.26(D) — Illumination

Illumination must be provided for all working spaces about service equipment, switchboards, switchgear, panelboards, and motor control centers — even in dwelling units.

110.26(E) — Dedicated Equipment Space

Above the equipment and up to 25 feet (or the structural ceiling, whichever is lower), the space must be dedicated to electrical equipment only.

Plumbing, HVAC ducts, and other non-electrical equipment must not be installed above panelboards where leakage could damage the electrical equipment.

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110.27 — Guarding of Live Parts

Live parts of electrical equipment operating at 50 volts or more must be guarded against accidental contact by:

• Approved enclosures
• Location in a room, vault, or similar space accessible only to qualified persons
• Suitable permanent, substantial partitions or screens with access only to qualified persons
• Location on a suitable balcony, gallery, or platform elevated and arranged to exclude unqualified persons

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110.28 — Enclosure Selection

Table 110.28 lists enclosure types and the environmental conditions they are designed for.

Key enclosure types:

• Type 1 — General-purpose indoor; protects against contact with enclosed equipment, limited dust
• Type 3R — Outdoor; protects against falling rain, sleet, snow, ice formation
• Type 4 — Watertight; protects against windblown dust and rain, hose-directed water
• Type 4X — Corrosion-resistant; same as Type 4 plus corrosion resistance (stainless steel, fiberglass)
• Type 12 — Industrial; protects against dust, falling dirt, dripping non-corrosive liquids
• Type 6 — Submersible; designed for occasional submersion

Exam tip: Outdoor panels use Type 3R minimum. Wash-down environments (food processing, car washes) require Type 4 or 4X. A Type 4X in a corrosive environment vs. Type 4 — the X adds corrosion resistance.

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Key Article 110 Numbers to Remember

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Common Exam Mistakes on Article 110

1. Using the 90°C ampacity at standard terminals — THHN is rated 90°C, but most terminals are 60°C or 75°C; always match ampacity column to terminal rating
2. Forgetting the 30-inch minimum width — the depth gets all the attention; don't forget width must be at least 30 inches regardless of equipment size
3. Misidentifying the working space condition — a concrete block wall is grounded, making it Condition 2 (not Condition 1); this changes the required depth
4. Allowing storage in front of panels — 110.26(B) prohibits storage in working space; this is one of the most common field violations
5. Forgetting the dedicated space above panels — piping and ductwork above panelboards violate 110.26(E) if they could leak onto electrical equipment

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Ready to Practice Article 110 Questions?

GetLicenseReady includes Article 110 questions covering temperature ratings, working space clearances, and equipment approval — the scenarios most likely to appear on your exam.

Related reading: NEC Article 210 — Branch Circuits | NEC Article 240 — Overcurrent Protection | NEC Table 310.16 — Wire Sizing