Why NEC Definitions Matter
The National Electrical Code is a legal document that is adopted into law by state and local jurisdictions. Like any legal document, it uses words in precise, defined ways that may differ from common usage. A word like "accessible" has a specific technical meaning in the NEC that affects how equipment must be installed. Misunderstanding these definitions leads to code violations, inspection failures, and — more seriously — unsafe installations.
Most fundamental NEC definitions appear in Article 100, though some terms are defined within specific articles where they apply. This glossary covers 50 of the most important and frequently referenced terms, organized alphabetically with NEC references and plain-language explanations.
A
- Accessible (as applied to equipment) — Article 100
- Capable of being reached quickly for operation, renewal, or inspection without requiring those to whom ready access is requisite to use tools, to climb, or to remove obstacles. Plain language: a panel door that swings open is accessible; a panel bolted shut with hex bolts is not. This definition applies to service disconnects, GFCI devices, junction boxes, and other equipment that must be accessible for operation.
- Accessible (as applied to wiring methods) — Article 100
- Capable of being removed or exposed without damaging the building structure or finish, or not permanently closed in by the structure or finish of the building. Plain language: wire in conduit is accessible because the conduit can be de-pulled; wire embedded in concrete is not. This is why NEC 300.15 requires all splices to be in accessible junction boxes.
- Ampacity — Article 100
- The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Ampacity depends on conductor size, insulation type, installation method (in conduit, in free air, in raceway), ambient temperature, and the number of current-carrying conductors bundled together. NEC Table 310.12 (for residential wiring) and Table 310.16 (for conduit wiring) list ampacity values. When conductors are bundled, ampacity derating factors from NEC 310.15(C) apply.
- Appliance — Article 100
- Utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions such as clothes washing, air conditioning, food mixing, deep frying, and so on. Plain language: a washer, refrigerator, microwave, and dishwasher are appliances; a general-purpose motor is not.
- Arc-Fault Circuit Interrupter (AFCI) — Article 100
- A device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected. AFCIs detect the high-frequency electrical signature of arcing from loose connections, damaged insulation, or cord damage that standard breakers cannot detect. Required by NEC 210.12 for most circuits in dwelling units.
B
- Bonding — Article 100
- The act of connecting together metal parts to form an electrically conductive path that ensures electrical continuity and has the capacity to safely conduct any current likely to be imposed. The purpose of bonding is to prevent voltage differences between metal parts that could cause shock or damage. Bonding connects metal parts together; grounding connects that bonded system to the earth. The two concepts are related but distinct.
- Bonding Jumper — Article 100
- A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected. Main bonding jumpers (in the service panel) connect the neutral bus to the panel enclosure. Equipment bonding jumpers connect individual equipment housings to the equipment grounding system.
- Branch Circuit — Article 100
- The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s). In plain language: the wiring from a circuit breaker to the outlets it serves is a branch circuit. The branch circuit begins at the breaker, not at the panel bus. NEC Article 210 governs branch circuit requirements.
- Branch Circuit, Appliance — Article 100
- A branch circuit that supplies energy to one or more outlets to which appliances are to be connected and that has no permanently connected luminaires (light fixtures) that are not a part of an appliance. Kitchen small appliance circuits (two required by NEC 210.52(B)) are appliance branch circuits.
- Branch Circuit, General-Purpose — Article 100
- A branch circuit that supplies two or more receptacles or outlets for lighting and appliances. The typical bedroom or living room circuit with multiple outlets is a general-purpose branch circuit.
- Branch Circuit, Individual — Article 100
- A branch circuit that supplies only one utilization equipment. A dedicated circuit for a dishwasher, garbage disposal, or microwave is an individual branch circuit. NEC 210.23 governs what loads an individual branch circuit can serve.
C
- Circuit Breaker — Article 100
- A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating. A circuit breaker provides two functions: manual switching and automatic overcurrent protection.
- Concealed — Article 100
- Rendered inaccessible by the structure or finish of the building. Plain language: wiring inside walls or above ceilings is concealed. The distinction between concealed and exposed wiring determines which wiring methods are permitted in various locations.
- Continuous Load — Article 100
- A load where the maximum current is expected to continue for 3 hours or more. This definition matters for conductor and overcurrent device sizing because NEC 210.19(A) and 215.2(A) require that continuous loads be sized at 125% — the conductors and overcurrent devices must be rated for 125% of the continuous load current. Lighting systems, HVAC equipment, and data center UPS systems are common continuous loads.
D
- Dead Front — Article 100
- Without live parts exposed to a person on the operating side of the equipment. A panelboard with a dead-front cover has no energized parts accessible when the cover is in place — the bus bars and connections are behind barriers. This is the standard design for all modern residential and commercial panelboards.
- Demand Factor — Article 100
- The ratio of the maximum demand of a system, or part of a system, to the total connected load of a system, or part of a system, under consideration. A demand factor less than 1.0 (100%) recognizes that not all connected loads operate simultaneously at full load. NEC Article 220 uses demand factors to reduce the calculated service load below the sum of all connected loads.
- Device — Article 100
- A unit of an electrical system, other than a conductor, that carries or controls electric energy as its principal function. Switches, receptacles, and circuit breakers are devices. Light bulbs and motors are not devices — they are utilization equipment or luminaires. The distinction matters because device boxes (for switches and receptacles) have different box fill rules than fixture boxes.
E
- Energized — Article 100
- Electrically connected to, or is, a source of voltage. A conductor connected to a voltage source is energized even if no current is flowing. Safety rules for working on electrical equipment depend on whether conductors are energized — OSHA and NFPA 70E (the electrical safety standard) require specific precautions for work on or near energized conductors.
- Equipment — Article 100
- A general term including fittings, devices, appliances, luminaires, apparatus, machinery, and the like used as a part of, or in connection with, an electrical installation. Essentially everything that is not a conductor is "equipment" in NEC terminology.
- Equipment Grounding Conductor (EGC) — Article 100
- The conductive path(s) that provide a ground fault current path and connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. The EGC is the safety ground — the green or bare wire in a cable or raceway that connects the frames of outlets, appliances, and equipment to the grounding system. When a fault occurs, the EGC provides the path for fault current to flow back to the source and operate the overcurrent device.
- Exposed (as applied to live parts) — Article 100
- Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to parts that are not suitably guarded, isolated, or insulated. Exposed live parts create arc flash and shock hazards and are generally prohibited in accessible locations without appropriate guarding per NEC 110.27.
F — G
- Feeder — Article 100
- All circuit conductors between the service equipment (or the source of a separately derived system or other power supply source) and the final branch-circuit overcurrent device. Feeders carry power from the main service panel to sub-panels, from a transformer to a switchboard, or from a generator to distribution panels. Feeder sizing is governed by NEC Article 215.
- Ground — Article 100
- The earth. A "ground" is a physical connection to the earth — made through a grounding electrode system (ground rods, water pipes, concrete-encased electrodes, etc.). "Ground" is not interchangeable with "neutral" in NEC terminology, though both conductors may occupy the same physical wire in some configurations.
- Ground Fault — Article 100
- An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. A ground fault is what a GFCI protects against in personnel protection applications and what GFPE protects against in equipment protection applications.
- Grounded Conductor — Article 100
- A system or circuit conductor that is intentionally grounded. The grounded conductor is the white (or gray) neutral wire in a standard single-phase AC system. It is intentionally connected to ground (earth) at the service entrance. It carries return current in normal operation. The NEC distinguishes grounded conductors from grounding conductors and from equipment grounding conductors.
- Grounding Electrode — Article 100
- A conducting object through which a direct connection to earth is established. Grounding electrodes include ground rods, ground plates, buried metal water pipes, concrete-encased electrodes (Ufer grounds), ground rings, and structural metal frames of buildings. NEC 250.52 lists all acceptable grounding electrodes.
- Grounding Electrode Conductor (GEC) — Article 100
- A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system. The GEC runs from the service neutral bar (or the main bonding jumper connection) to the grounding electrode system. Its size is determined by the size of the service conductors per NEC Table 250.66.
H — L
- Identified (as applied to equipment) — Article 100
- Recognizable as suitable for the specific purpose, function, use, environment, application, and so forth, where described in a particular Code requirement. A wire identified for use as a grounded conductor (neutral) has white or gray insulation; an identified receptacle is one with the correct ampere rating and configuration for the application.
- In Sight From — Article 100
- Where this Code specifies that one piece of equipment shall be "in sight from," "within sight from," or "within sight," and so forth, of another piece of equipment, the specified equipment is to be visible and not more than 50 feet (15 m) distant from the other. This definition governs motor disconnecting means (NEC 430.102) and HVAC equipment disconnects (NEC 440.14).
- Interrupting Rating — Article 100
- The highest current at rated voltage that a device is identified to interrupt under standard test conditions. A circuit breaker must have an interrupting rating equal to or greater than the available fault current at the point where it is installed. Installing a breaker with an insufficient interrupting rating creates a catastrophic failure risk during a bolted fault.
- Labeled — Article 100
- Equipment or materials to which has been attached a label, symbol, or other identifying mark of an organization that is acceptable to the authority having jurisdiction and concerned with product evaluation, that maintains periodic inspection of production of labeled equipment or materials, and by whose labeling the manufacturer indicates compliance with appropriate standards or performance in a specified manner. "Labeled" equipment has been tested by a recognized third-party organization (like UL, CSA, or ETL) and bears their mark.
- Listed — Article 100
- Equipment, materials, or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned with evaluation of products or services, that maintains periodic inspection of production of listed equipment or materials or periodic evaluation of services, and whose listing states that either the equipment, material, or services meets appropriate designated standards or has been tested and found suitable for a specified purpose. "Listed" is the broader term; "labeled" refers specifically to the physical label on the equipment.
M — O
- Multioutlet Assembly — Article 100
- A type of surface or flush raceway designed to hold conductors and receptacles, assembled in the field or at the factory. The strip of outlets on a kitchen countertop or above a workbench is a multioutlet assembly. NEC Article 380 governs multioutlet assemblies.
- Neutral Conductor — Article 100 (added in 2011 NEC)
- The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. The neutral conductor is the grounded conductor in a grounded system. In a single-phase, 3-wire system (typical residential service), the neutral carries the unbalanced current between the two hot legs.
- Nonautomatic — Article 100
- Action requiring personal intervention for its control. A manually operated switch or disconnect is nonautomatic. A circuit breaker that trips on overload without human intervention is automatic in its protection function, but nonautomatic in its manual switching function.
- Outlet — Article 100
- A point on the wiring system at which current is taken to supply utilization equipment. An outlet is a broader concept than a receptacle — it includes any point where a device or equipment is connected to the wiring system. A ceiling box with a light fixture installed is an outlet; a duplex receptacle is also an outlet. NEC 210.52 specifies where outlets are required in dwelling units.
- Overcurrent — Article 100
- Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault. Overcurrent protective devices (circuit breakers and fuses) are calibrated to open the circuit under overcurrent conditions before damage occurs to the conductors or connected equipment.
- Overload — Article 100
- Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. An overload is distinguished from a short circuit or ground fault by being a lesser magnitude — a motor drawing 120% of its rated current is overloaded; a direct short circuit draws many times rated current.
P — R
- Panelboard — Article 100
- A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front. The "breaker box" in a home is a panelboard.
- Premises Wiring System — Article 100
- Interior and exterior wiring, including power, lighting, control, and signal circuit wiring, together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed. The premises wiring system is everything on the building owner's side of the service — it excludes the utility service and metering equipment.
- Raceway — Article 100
- An enclosed channel of metallic or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. A raceway provides mechanical protection for conductors and may also serve as the equipment grounding path in metallic raceways. Types of raceways include: rigid metal conduit (RMC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), rigid PVC conduit, flexible metal conduit, cable trays, and wireways.
- Readily Accessible — Article 100
- Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders and so forth. "Readily accessible" is more restrictive than "accessible" — a meter on the side of a building is accessible but may not be readily accessible if it requires a ladder. Service disconnects must be readily accessible per NEC 230.70.
- Receptacle — Article 100
- A contact device installed at the outlet for the connection of an attachment plug, or for the direct connection of electrical utilization equipment designed to mate with such a contact device. A receptacle is a specific type of outlet. The most common types are 15A/125V duplex receptacles (NEMA 5-15R) and 20A/125V single receptacles (NEMA 5-20R).
S — Z
- Service — Article 100
- The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served. The "service" includes the service conductors (drop or lateral), the service entrance conductors, the metering equipment, and the service disconnect. It does not include the utility's distribution lines or transformers.
- Service Conductors — Article 100
- The conductors from the service point to the service disconnecting means. These are the conductors between where the utility's responsibility ends (the service point — typically the meter or weatherhead connection) and the main disconnect. Service conductors are governed by NEC Article 230.
- Service Drop — Article 100
- The overhead conductors between the utility electric supply system and the service point. These are the wires running from the utility pole to the weatherhead on the building. They are generally owned and maintained by the utility, but the attachment point hardware and service mast on the building are typically the building owner's responsibility.
- Service Equipment — Article 100
- The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply. The main panelboard or meter-main combination is the service equipment.
- Service Lateral — Article 100
- The underground conductors between the utility electric supply system and the service point. Underground service conductors, typically installed by the utility in conduit or as direct-buried cable from the transformer to the meter base.
- Short Circuit — Article 100
- A low impedance path, created by a fault or inadvertent connection, that allows current to flow by a path other than the intended load path. A short circuit is different from an overload — it is characterized by extremely high fault current (often thousands of amperes) that must be interrupted quickly to prevent catastrophic damage. Overcurrent devices must be rated to interrupt the available short circuit current at their location.
- Splice — (NEC 110.14)
- A connection of two or more conductors. All splices must be made with listed splicing devices and must be enclosed in an accessible junction box per NEC 300.15. The splice itself must be mechanically and electrically secure without relying on the insulation tape for mechanical strength.
- Switchboard — Article 100
- A large single panel, frame, or assembly of panels on which are mounted switches, overcurrent and other protective devices, buses, and usually instruments. Switchboards are governed by NEC Article 408. They are typically larger than panelboards and are designed for industrial and commercial applications where many distribution circuits originate.
- Utilization Equipment — Article 100
- Equipment that utilizes electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes. Motors, heaters, appliances, luminaires, and electronic equipment are all utilization equipment. Distinguishing utilization equipment from wiring equipment (conductors, boxes, conduit) matters for product listing, load calculations, and circuit requirements.
- Voltage (of a Circuit) — Article 100
- The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned. The NEC distinguishes between nominal voltage (the standard label, e.g., 120V or 240V) and actual voltage, which may vary ±5-10% in real systems. Code requirements that reference voltage thresholds (such as GFCI requirements for "125V through 250V") use nominal voltage.
- Voltage to Ground — Article 100
- For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit. This definition is critical for applying NEC clearance, protection, and insulation requirements — a 208Y/120V system has a voltage to ground of 120V from each phase conductor, while a 480V delta system has a voltage to ground of 277V (or higher, in an ungrounded delta).
- Wiring Method
- The type of cable or conduit system used to route conductors through a building. Common wiring methods include NM cable (Romex), AC cable (BX), MC cable (metal clad), rigid metal conduit (RMC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), and rigid PVC conduit. Each wiring method is governed by its own NEC article (Articles 320–398) and is permitted or prohibited based on the occupancy type, location, and environmental conditions.
Quick Reference Table — 50 NEC Terms
| Term | Primary NEC Reference | Key Point |
|---|---|---|
| Accessible (equipment) | Article 100 | Can be reached without tools or obstacles |
| Accessible (wiring) | Article 100 | Can be removed without damaging building |
| AFCI | Article 100, 210.12 | Detects arcing faults; required in dwellings |
| Ampacity | Article 100, 310 | Maximum continuous current for conductor |
| Appliance | Article 100 | Standardized utilization equipment |
| Bonding | Article 100, 250 | Connecting metal parts for conductivity |
| Branch Circuit | Article 100, 210 | Breaker to outlet(s) |
| Circuit Breaker | Article 100, 240 | Switching + automatic overcurrent protection |
| Continuous Load | Article 100 | 3+ hour maximum current; size at 125% |
| Dead Front | Article 100 | No live parts exposed on operating side |
| Demand Factor | Article 100, 220 | Max demand ÷ total connected load |
| Device | Article 100 | Switches, receptacles, breakers |
| EGC | Article 100, 250 | Safety ground path; green or bare wire |
| Energized | Article 100 | Connected to a voltage source |
| Equipment | Article 100 | Everything except conductors |
| Exposed (live parts) | Article 100 | Can be inadvertently touched |
| Feeder | Article 100, 215 | Service panel to sub-panel |
| GEC | Article 100, 250 | Connects neutral/bonding to earth |
| Ground | Article 100 | The earth; physical earth connection |
| Ground Fault | Article 100 | Unintended path to earth |
| Grounded Conductor | Article 100 | White/gray wire; intentionally grounded |
| Grounding Electrode | Article 100, 250.52 | Ground rods, water pipes, Ufer ground |
| Identified | Article 100 | Suitable and recognizable for specific use |
| In Sight From | Article 100 | Visible and within 50 feet |
| Interrupting Rating | Article 100 | Max fault current device can clear |
| Labeled | Article 100 | Third-party listing mark present |
| Listed | Article 100 | In a recognized third-party list |
| Neutral Conductor | Article 100 | Connected to neutral point; carries unbalanced current |
| Nonautomatic | Article 100 | Requires human action |
| Outlet | Article 100 | Any point current is taken for equipment |
| Overcurrent | Article 100, 240 | Overload, short circuit, or ground fault |
| Overload | Article 100 | Excess current over time; less severe than short |
| Panelboard | Article 100, 408 | Breaker box; front-access only |
| Premises Wiring System | Article 100 | All wiring on building owner's side |
| Raceway | Article 100 | Enclosed channel for conductors |
| Readily Accessible | Article 100 | No climbing or obstacles required |
| Receptacle | Article 100, 406 | Plug connection device |
| Service | Article 100, 230 | Utility to premises wiring system |
| Service Conductors | Article 100, 230 | Service point to main disconnect |
| Service Drop | Article 100, 230 | Overhead utility conductors to building |
| Service Equipment | Article 100, 230 | Main panel or meter-main |
| Service Lateral | Article 100, 230 | Underground utility conductors |
| Short Circuit | Article 100 | Low-impedance fault; high current |
| Switchboard | Article 100, 408 | Large distribution panel; front and rear access |
| Utilization Equipment | Article 100 | Motors, appliances, luminaires |
| Voltage (Circuit) | Article 100 | Greatest RMS potential difference |
| Voltage to Ground | Article 100 | Voltage from conductor to grounded point |
| Wiring Method | Articles 320–398 | Cable or conduit system type |
| Multioutlet Assembly | Article 100, 380 | Surface raceway with built-in outlets |
| Bonding Jumper | Article 100, 250 | Conductor joining metal parts |
| Splice | 110.14, 300.15 | Conductor connection; must be in accessible box |