NEC Article 100 — Definitions That Drive Compliance

Why Article 100 Matters

Every rule in the National Electrical Code (NEC) is built on the definitions found in Article 100. If you misread a term — even slightly — you can misapply a code rule, fail an inspection, or create a safety hazard. Journeyman licensing exams test Article 100 heavily because inspectors and examiners know that a worker who commands the vocabulary commands the code. Make these definitions automatic.

What Article 100 Covers

Article 100 contains definitions of terms used in two or more other NEC articles. Terms used in only one article are defined within that article instead. The definitions here are not suggestions — they are the legal meaning of each word wherever it appears in the code.

The Supply-Side Chain: From Utility to Load

Understanding how power flows through a building starts with three terms that inspectors check on every job.

Service is the conductors and equipment that deliver energy from the utility to the premises wiring system. The conductors that run from the utility transformer to your meter base and into the service equipment are service conductors. They operate under utility rules and limited NEC jurisdiction until they reach the service point.

Feeder means all circuit conductors between the service equipment (or other power source) and the final branch-circuit overcurrent device. Think of a feeder as the trunk of the tree — it supplies multiple branch circuits from a panelboard, subpanel, or switchboard.

Branch Circuit is the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s). This is where most of your day-to-day wiring work happens — receptacles, luminaires, and appliances hang off branch circuits.

Remember: service → feeder → branch circuit. Power flows down this chain. Overcurrent protection must be coordinated at each level.

Branch Circuit Variations

Not every branch circuit is the same. The NEC defines several types you must distinguish:

• Individual Branch Circuit: supplies only one piece of utilization equipment.
• General-Purpose Branch Circuit: supplies two or more receptacles or outlets for lighting and appliances.
• Appliance Branch Circuit: supplies one or more outlets to which appliance loads will be connected; has no permanently connected lighting fixtures that are not part of an appliance.
• Multiwire Branch Circuit: a branch circuit that consists of two or more ungrounded conductors with a voltage between them, plus a grounded conductor (neutral) with equal voltage from each ungrounded conductor to the grounded conductor. Multiwire branch circuits must be handled carefully — improper disconnection of the neutral while the circuit is energized can double voltage on connected equipment.

Conductors and Their Roles

The NEC defines conductors by function, not just by color.

Grounded Conductor: a system or circuit conductor that is intentionally connected to ground (earth). In most systems this is the neutral. It carries return current under normal operation.

Grounding Conductor (Equipment Grounding Conductor — EGC): the conductive path installed to connect normally non-current-carrying metal parts of equipment to the system grounded conductor, the grounding electrode conductor, or both. The EGC does not carry current under normal operation; it provides a fault-current path to operate overcurrent devices.

Identified (as applied to a conductor): recognizable as suitable for a specific purpose, function, or use. In practice, the grounded conductor must be identified by white or gray insulation or by white markings at terminations. An "identified" conductor is not the same as an "identifying" color — read the article that uses the term.

Ungrounded Conductor: a conductor that has no intentional connection to ground and that carries current to the load. These are your "hot" wires.

Equipment and Locations

Utilization Equipment: equipment that uses electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes. A motor, a luminaire, and a water heater are all utilization equipment.

Outlet: a point on the wiring system at which current is taken to supply utilization equipment. A receptacle is a type of outlet, but so is a luminaire connection point. Not every outlet has a receptacle.

Receptacle: a contact device installed at the outlet for the connection of an attachment plug. The cord-and-plug connection to a lamp or appliance plugs into a receptacle.

Exposed (as applied to wiring methods): on or attached to the surface, or behind panels designed to allow access. Knowing whether wiring is "exposed" or "concealed" drives which installation rules apply.

Voltage Terms

Voltage, Nominal: a nominal value assigned to a circuit or system for the purpose of designating its voltage class. The actual voltage may vary. A system called 120/240 V may measure slightly different in practice — the nominal label is what the code rules reference.

Voltage to Ground: for grounded circuits, the voltage between the given conductor and the point of grounding of the circuit. For ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit. This definition affects which clearance and insulation rules apply.

Safety Reminders

Studying definitions is a low-voltage classroom activity, but the concepts connect directly to live-work hazards. Multiwire branch circuit neutrals that are opened under load can energize equipment at twice the expected voltage — always treat every conductor as energized until proven otherwise with a tested meter. When you move from the classroom to the field, apply lockout/tagout (LOTO) procedures before touching any conductor, wear appropriate PPE (safety glasses, voltage-rated gloves for the system voltage), and verify absence of voltage with a properly rated tester. Correct identification of conductors — grounded, grounding, ungrounded — before energizing is not just a code matter; it is how workers go home safely.

Putting It Together

Every time you encounter an NEC rule, ask yourself: which Article 100 terms appear in this rule? Define them precisely before you apply the rule. This habit will serve you through your journeyman exam, field inspections, and your entire career.