Definitions and general rules

The code is a legal document, and like any legal document it depends on precise words. "Bonding" and "grounding" mean different things; a "feeder" is not a "branch circuit"; "readily accessible" is a stricter requirement than "accessible". Many disagreements with inspectors, and many wrong answers on exams, come from reading a word in its everyday sense instead of its code definition. This chapter covers the definitions every electrician needs, then the general rules that apply to every installation: approval and marking, workmanship and connections, identification of conductors, locations and enclosures, and working space.

Where the definitions are

In the NEC, definitions are in Article 100. Since the 2023 edition, almost all definitions have been gathered there, with the article each one mainly applies to shown in brackets. When a rule seems ambiguous, the first step is to check whether its key words are defined.

Essential definitions

Circuits: service, feeder and branch circuit

• Service: the conductors and equipment that deliver energy from the serving utility to the wiring system of the premises. Service equipment (usually the main breaker or disconnect) is the main control and means of cutoff.
• Feeder: the conductors between the service equipment (or another source, such as a transformer or generator) and the final branch-circuit overcurrent device. A feeder from the main panel to a subpanel is the common example.
• Branch circuit: the conductors between the final overcurrent device and the outlets it supplies. A 15 A receptacle circuit in a bedroom is a branch circuit.

Different rules apply to each, so identifying which one you are working on is the first step in sizing conductors and protection.

Grounding and bonding

• Grounded (grounding): connected to ground or to a conductive body that extends the ground connection. The grounding electrode conductor (GEC) connects the system to the grounding electrode.
• Bonded (bonding): connected to establish electrical continuity and conductivity. The equipment grounding conductor (EGC) is the conductive path that connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor, the grounding electrode conductor, or both.
• Grounded conductor: the system or circuit conductor that is intentionally grounded, usually the neutral (the neutral conductor is the one connected to the neutral point of a system). It is a current-carrying conductor, identified by white or gray insulation, and must not be confused with the equipment grounding conductor.

The distinction matters on every job. The neutral carries normal load current back to the source. The bonding or equipment grounding conductor carries current only during a fault, to trip the breaker. Connecting them together anywhere other than the service (or a separately derived source) puts normal current on metal parts, a common and dangerous error (Lesson 9).

Load and location terms

• Continuous load: a load where the maximum current is expected to continue for three hours or more. Water heaters, space heating, many lighting loads and most commercial loads are continuous.
• Ampacity: the current a conductor can carry continuously under its conditions of use without exceeding its temperature rating (Lesson 4).
• Dwelling unit: one or more rooms used as a housekeeping unit by one or more persons, with space for eating, living and sleeping, and permanent provisions for cooking and sanitation.
• Dry, damp and wet locations: a dry location is not normally subject to moisture; a damp location is partly protected but subject to moderate moisture (such as a covered porch or some basements); a wet location is exposed to weather, water or saturation (outdoors exposed, underground, in concrete in contact with earth). Wiring methods and equipment must suit the location.
• Readily accessible: capable of being reached quickly for operation, renewal or inspection without having to climb over or remove obstacles or resort to portable ladders or tools. "Accessible" alone is less demanding: removing a panel may be needed.
• Qualified person: one who has the skills and knowledge related to the construction and operation of the electrical equipment and installation and has received safety training to recognize and avoid the hazards involved.

General rules for every installation

Approval, marking and instructions

Equipment must be approved (110.2) and installed and used according to its listing and labeling and the manufacturer's instructions (110.3(B)). Equipment must be marked with the manufacturer and its ratings (110.21). Installing equipment contrary to its listing, for example using a breaker in a panelboard it is not listed for, is a code violation.

Workmanship and connections

Work must be installed in a neat and workmanlike manner (110.12). Connections are the weakest points of any installation and cause many electrical fires. Terminals must be identified for the conductor material (copper, aluminum or both, marked for example "AL/CU"), the number of conductors per terminal must be within its listing, and terminals must be tightened to the torque specified by the manufacturer using a calibrated torque tool where a value is given (110.14(D)). Terminal temperature ratings, usually 60 °C or 75 °C, limit the conductor ampacity that can be used (110.14(C) and Lesson 4).

Loose connections heat up under load: the resistance of a poor joint turns current into heat, which oxidizes the contact, which raises resistance further. A connection that is fine on day one can fail years later. Torque specifications exist precisely to prevent this.

Identification of conductors

Conductor colors are not a matter of preference:

• The grounded (neutral) conductor must be white or gray, or have three continuous white or gray stripes (200.6). White or gray insulation must not be used for other conductors, except where permitted with re-identification, such as a white conductor in a cable used as a switch loop and re-marked (200.7).
• Equipment grounding conductors must be bare, or have green insulation, or green with one or more yellow stripes (250.119).
• Ungrounded (hot) conductors may be any other color. By widespread convention (not code), black, red and blue are used for 120/208 V systems and brown, orange and yellow for 277/480 V. On a 4-wire delta system with a "high leg" (208 V to neutral), the high leg must be marked orange (110.15).

Locations and enclosures

Equipment and enclosures must suit their environment. Enclosures are rated by type numbers that indicate their protection against dust, water, ice and corrosion. Common types include:

A panelboard installed outdoors in a Type 1 enclosure, or a Type 3R enclosure in a food plant washed down daily with hoses, is a violation even if every wire inside is correct.

Working space

Equipment that may need examination, adjustment, servicing or maintenance while energized, such as panelboards, switchboards and motor control centers, must have clear working space in front of it so that a worker can do the job and escape quickly if something goes wrong. The space must be kept clear; storage in front of a panel is one of the most common violations found in existing buildings.

The NEC (110.26) requires a depth of working space of at least 0.9 m (3 ft) for 0–150 V to ground, and 0.9, 1.0 or 1.2 m (3, 3.5 or 4 ft) for 151–1000 V to ground depending on what is opposite (Conditions 1, 2 and 3). The space must be at least 762 mm (30 in) wide or the width of the equipment, at least 2.0 m (6.5 ft) high, and kept clear of storage. Equipment likely to be worked on while energized must carry an arc flash warning label (110.16).

Protection of live parts and interrupting ratings

Live parts must be enclosed or guarded so that people cannot accidentally contact them. Unused openings in panels and boxes must be closed with approved fillers; missing knockouts and breaker filler plates are frequent findings. Every overcurrent device must also have an interrupting rating at least equal to the fault current available where it is installed; a residential breaker rated 10 kA installed where 22 kA is available could fail violently during a fault. ELEC-320 returns to fault current and coordination; for now, remember that equipment ratings include more than just amperes and volts.

The general rules of this lesson are applied to branch circuits in Lesson 3 and to devices and panelboards in Lessons 7 and 8.