Article 100 definitions that decide answers

Ray Ochoa has a saying he repeats to every apprentice who argues about a rule: 'Read the definition first.' Half the arguments he has seen on job sites, and a good share of the questions candidates miss on exams, come down to a word that the Code defines more narrowly or more broadly than everyday speech. Is a closet with a sink a bathroom? Is a panel behind a locked door readily accessible? Is a load that runs from 8 a.m. to 11 a.m. continuous? Each of those questions has a definite answer, and the answer is in Article 100.

How definitions work in the NEC

Article 100 is the dictionary of the Code. Its definitions apply wherever the defined term is used, unless an article gives a different meaning for its own purposes. When a defined term appears in a rule, the rule means exactly what the definition says, not what the term means on the street. Exam writers know this, and many questions are built so that the everyday meaning leads to the wrong answer.

In older editions, definitions were split between Article 100, which held terms used in two or more articles, and the second section of individual articles, such as 680.2 for swimming pools or 690.2 for solar photovoltaic systems, which held terms used only in that article. The 2023 edition gathered almost all definitions into Article 100, and many entries now note in parentheses the article or articles where the term is used. If your exam is on 2023 or 2026, there is one place to look. If it is on 2020 or earlier, remember to check the .2 section of the article as well.

Definitions are listed alphabetically, and a defined term often has several related entries that sit together, such as the family of terms beginning with grounding or service. When a question uses a phrase you are not sure about, turning to Article 100 and checking costs twenty seconds. Guessing costs a point.

Access words: accessible, readily accessible and in sight from

Several rules depend on whether equipment can be reached easily. The Code distinguishes two levels. Equipment that is accessible can be approached closely without someone having to remove or damage building finishes, though it may be behind a door or a removable panel. Equipment that is readily accessible can be reached quickly for operation, renewal or inspection without climbing over or removing obstacles, without needing a portable ladder and without needing tools, unless the tools are part of an approved arrangement. A separate definition of accessible applies to wiring methods: they are accessible if they can be reached without damaging the building structure or finish, and concealed if they are hidden by the structure.

The term in sight from, also written within sight from or within sight, is one of the most tested definitions on master exams. One piece of equipment is in sight from another if it is visible from it and not more than 50 feet away. Both conditions must be met. A disconnect 30 feet away but hidden behind a wall is not in sight; a disconnect in plain view but 60 feet away is not in sight either. Motor and air-conditioning disconnect rules, covered in MELU-105, depend on this definition.

Approval words: approved, listed, labeled and identified

Four terms describe how equipment is judged acceptable, and exam questions often turn on the differences between them. Approved means acceptable to the authority having jurisdiction. It is the inspector's decision. Listed means the equipment appears on a list published by an organization acceptable to the AHJ, such as a nationally recognized testing laboratory, which has evaluated it and checks production. Labeled means the equipment carries the label, symbol or mark of such an organization. Identified means recognizable as suitable for a specific purpose, function, use, environment or application, often shown by listing or labeling.

These terms connect to one of the most important rules in the Code, 110.3, covered in lesson 4. It requires listed or labeled equipment to be installed and used according to the instructions included in its listing or labeling. Many exam answers depend on the phrase installed in accordance with the manufacturer's instructions, and the authority for that phrase is 110.3(B), not a general idea of good workmanship.

Circuit words: branch circuit, feeder and service

The Code divides the wiring of a building into three systems, and every rule about sizing and protection depends on knowing which one you are dealing with. A branch circuit is the wiring between the final overcurrent device protecting the circuit and the outlets. A feeder is the wiring between the service equipment, or the source of a separately derived system or other supply, and the final branch-circuit overcurrent device. The service is the conductors and equipment that deliver energy from the serving utility to the wiring system of the premises.

Several related terms locate the boundaries. The service point is where the utility's conductors meet the premises wiring, and the NEC applies on the premises side. Service conductors run from the service point to the service disconnecting means. Branch circuits come in several defined types: an individual branch circuit supplies only one piece of equipment; a general-purpose branch circuit supplies two or more outlets for lighting and appliances; an appliance branch circuit supplies outlets intended for appliances with no permanent lighting; and a multiwire branch circuit has two or more ungrounded conductors with a voltage between them and a grounded conductor with equal voltage to each.

The grounding family

No group of definitions causes more confusion than the terms built on ground. They sound alike, and the everyday words neutral and ground are used loosely in the field. The Code uses precise terms, and exam questions often depend on telling them apart.

Two common errors are worth naming. First, grounded and grounding are different words: the grounded conductor normally carries current, while an equipment grounding conductor carries current only during a fault. Second, the neutral and the grounded conductor are not always the same thing. In a corner-grounded delta system, one phase conductor is grounded and there is no neutral. On a 120/240-volt single-phase system the grounded conductor is also the neutral. When a question asks about the neutral, check that the system actually has one.

Load words: continuous load, overload and overcurrent

A continuous load is one where the maximum current is expected to continue for three hours or more. The definition is the basis of one of the most used sizing rules in the Code: for most branch circuits and feeders, the overcurrent device must be rated for the noncontinuous load plus 125 out of each 100 parts of the continuous load, which works out to loading a standard breaker to no more than four-fifths of its rating with continuous loads. Store lighting, office lighting, many heating loads and electric vehicle chargers are treated as continuous.

# Continuous load check for a branch circuit (illustrative values)
breaker_amps = 20
continuous_amps = 15.2      # e.g. store lighting that runs all day
noncontinuous_amps = 0.0

required_rating = noncontinuous_amps + 1.25 * continuous_amps
print(f"Required overcurrent device rating: {required_rating:.1f} A")
print("20 A breaker adequate?", breaker_amps >= required_rating)
print(f"Most continuous load on a {breaker_amps} A breaker: {breaker_amps / 1.25:.1f} A")

Required overcurrent device rating: 19.0 A
20 A breaker adequate? True
Most continuous load on a 20 A breaker: 16.0 A

Overcurrent is any current beyond the rated current of equipment or the ampacity of a conductor, whatever the cause. It includes overload, which is operation in excess of normal rating that would cause damage if it lasted long enough, and short circuits and ground faults, which are faults. The difference matters for motors: Article 430 separates overload protection, sized from the motor nameplate, from short-circuit and ground-fault protection, sized from tables. Knowing the definitions makes that division obvious.

Occupancy and location words

Many rules apply only in certain kinds of places, so the definitions of those places decide whether a rule applies at all.

The bathroom definition is a good example of why reading matters. A powder room with a basin and toilet is a bathroom. A laundry room with a utility sink and no other fixture is not, although other rules may still require ground-fault protection near the sink. A hotel suite with a kitchenette may or may not contain a dwelling unit, depending on whether all four provisions, living, sleeping, cooking and sanitation, are permanent. Exam questions are built on exactly these borderline cases.

A method for definition questions

Dana develops a three-step routine. First, underline every noun in the question that might be a defined term. Second, check any term she is not completely sure of in Article 100, with the related entries next to it. Third, reread the question with the definition in mind before looking at the answer choices. The routine adds perhaps thirty seconds to a question and, in her practice tests, removes almost all of her definition errors.