Overview of Article 110
Article 110 establishes the general requirements that govern all electrical installations. These rules apply universally regardless of the specific occupancy, equipment type, or wiring method involved. Think of Article 110 as the ground rules that every electrician, engineer, and inspector must follow before any other chapter-specific rules come into play.
Exam Tip: Article 110 questions appear frequently because the requirements apply to virtually every installation scenario. Pay special attention to working space dimensions, which are among the most commonly tested topics.
Equipment Examination and Approval (110.2 - 110.3)
Examination (110.2)
Before any equipment is connected, it must be acceptable to the authority having jurisdiction (AHJ). The AHJ evaluates equipment based on suitability for the installation, mechanical strength, wire-bending space, electrical insulation, heating effects, arcing effects, and other factors contributing to safe operation.
Installation and Use (110.3)
Listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling. This is a critical requirement: ignoring manufacturer installation instructions is a code violation regardless of what the rest of the NEC says.
- Equipment must be installed in a neat and workmanlike manner.
- Unused openings in boxes, raceways, and enclosures must be effectively closed.
- Internal parts of electrical equipment must not be damaged during installation.
Electrical Connections (110.14)
Proper electrical connections are essential for safety and performance. Article 110.14 addresses three key areas:
Terminal Connections
- Conductors must be connected using pressure connectors (including set-screw type), solder lugs, or splices to flexible leads.
- Connection terminals for more than one conductor must be identified for that purpose.
Temperature Limitations
Equipment termination provisions are rated for specific conductor temperatures:
- Equipment rated 100A or less: Use 60 degrees C ampacity unless the equipment is listed and identified for higher-temperature conductors.
- Equipment rated over 100A: Use 75 degrees C ampacity unless the equipment is listed for higher temperatures.
Key Point: Even if you install 90 degrees C rated wire, you must typically derate to the termination temperature rating of the equipment, not the conductor rating.
Splices
Conductors must be spliced using devices identified for the purpose or by brazing, welding, or soldering with a fusible metal or alloy. Splices must be covered with insulation equivalent to that of the conductors.
Working Space Requirements (110.26)
This section specifies the minimum working space around electrical equipment operating at 600 volts or less. These requirements exist to protect workers who must operate, maintain, or inspect the equipment.
Depth of Working Space
Working space depth is measured from the exposed live parts or the front of the enclosure, whichever is closer:
- Condition 1 (exposed live parts on one side, no live or grounded parts on the other): 3 feet for 0-150V, 3 feet for 151-600V.
- Condition 2 (exposed live parts on one side, grounded parts on the other): 3 feet for 0-150V, 3.5 feet for 151-600V.
- Condition 3 (exposed live parts on both sides): 3 feet for 0-150V, 4 feet for 151-600V.
Width and Height
- The working space must be at least 30 inches wide or the width of the equipment, whichever is greater.
- The workspace must have a minimum headroom of 6.5 feet (6 feet 6 inches).
Illumination and Access
- Adequate illumination must be provided for all working spaces around service equipment, switchboards, switchgear, panelboards, and motor control centers installed indoors.
- At least one entrance of sufficient area must provide access to and egress from the working space. Equipment rated 1200A or more and over 6 feet wide requires two entrance/egress points.
Arc-Flash Hazard Warnings (110.16)
Switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures, and motor control centers in other than dwelling units that are likely to require examination, adjustment, servicing, or maintenance while energized must be field-marked to warn qualified persons of potential electric arc-flash hazards.
Mounting and Cooling (110.13, 110.13(B))
- Equipment must be firmly secured to the surface on which it is mounted.
- Sufficient space must be provided around equipment to permit adequate ventilation and maintenance.
Understanding Article 110 provides the foundation for evaluating every electrical installation you encounter on the exam and in the field.
