1.3 NEC Article 100 Essential Definitions

1.3 NEC Article 100 Essential Definitions

Exam Focus: Accessibility classifications, circuit hierarchy (service, feeder, branch circuit), grounding/bonding terminology, continuous load rules, ampacity, and disconnecting means.

Article 100 contains all defined terms essential for applying the National Electrical Code. On the Michigan Journeyman examination, definition questions appear in two forms: direct definition identification and situational application questions where code compliance hinges on the precise legal meaning of a single word.

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Accessibility Terms: Accessible vs. Readily Accessible

One of the most frequently tested distinctions in Article 100 is the difference between equipment that is Accessible, Accessible (as applied to wiring methods), and Readily Accessible.

                      ACCESSIBILITY COMPARISON MATRIX
+-------------------------------------------------------------------------+
| READILY ACCESSIBLE (e.g., Service Disconnects, Panel OCPDs)             |
| - Reachable QUICKLY for operation, renewal, or inspection               |
| - NO portable ladders permitted                                         |
| - NO climbing over obstacles or moving heavy equipment                 |
| - NO resorting to portable tools to gain access                         |
+-------------------------------------------------------------------------+
                                     |
                                     v
+-------------------------------------------------------------------------+
| ACCESSIBLE (As Applied to Equipment)                                    |
| - Admitting close approach; not guarded by locked doors or elevation    |
| - May require opening an access door or stepping onto a fixed platform   |
+-------------------------------------------------------------------------+
                                     |
                                     v
+-------------------------------------------------------------------------+
| ACCESSIBLE (As Applied to Wiring Methods) (e.g., Junction Boxes)       |
| - Capable of being exposed without damaging building structure          |
| - Installed behind removable lay-in ceiling tiles or access panels      |
+-------------------------------------------------------------------------+

Key Application Examples:

• Overcurrent Devices (240.24): Circuit breakers and fuses must be readily accessible. Installing a panelboard above a stairwell or behind heavy stock shelving violates the readily accessible rule.
• Junction Boxes above Drop Ceilings (314.29): A junction box located above a lay-in suspended ceiling tile is accessible (as applied to wiring methods) because removing a ceiling tile does not damage the building structure. However, it is NOT readily accessible.

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Circuit Hierarchy: Service, Feeder, & Branch Circuit

Electrical power distribution systems are divided into three distinct wiring tiers. Misidentifying where a circuit transitions from a feeder to a branch circuit leads to sizing errors on the exam.

                        POWER DISTRIBUTION SYSTEM HIERARCHY

  Utility Power Source
           |
           v
  [ Service Point ]
           |
           | <--- SERVICE CONDUCTORS (From Service Point to Service Disconnect)
           v
 [ Service Disconnect / Main Panel ]
           |
           | <--- FEEDER CONDUCTORS (Between Service Equipment & Branch OCPD)
           v
  [ Distribution Subpanel ]
           |
           | <--- BRANCH CIRCUIT CONDUCTORS (From Final OCPD to Outlet)
           v
     [ Receptacle / Luminaire / Motor Outlet ]

1. Service Conductors

The conductors from the service point (utility connection point) to the service disconnecting means.

2. Feeder

All circuit conductors between the service equipment, the source of a separately derived system, or another power supply source and the final branch-circuit overcurrent device.

3. Branch Circuit

The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Branch Circuit Classifications:

• General Purpose Branch Circuit: A branch circuit that supplies two or more receptacles or outlets for lighting and appliances.
• Individual Branch Circuit: A branch circuit that supplies only one utilization equipment (e.g., a dedicated 50A circuit supplying a single electric range).
• Appliance Branch Circuit: A branch circuit that supplies one or more outlets to which appliances are to be connected; no permanently connected luminaires.
• Multiwire Branch Circuit: A branch circuit consisting of two or more ungrounded (hot) conductors that have a voltage potential between them, and a grounded (neutral) conductor having equal voltage potential between it and each ungrounded conductor of the circuit.

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Grounding and Bonding Terminology

Understanding the distinct role of each grounding conductor is critical for Article 250 questions.

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Load & Operational Definitions

Continuous Load (CRITICAL EXAM CALCULATIONS)

A load where the maximum current is expected to continue for 3 hours or more.

• Examples: Office building general lighting, store display lighting, commercial water heaters under 120 gallons (422.13).
• Sizing Rule: Under 210.19(A), 215.2(A), and 240.4, branch circuit conductors and overcurrent protective devices supplying a continuous load must be sized at 125% of the continuous load + 100% of the non-continuous load.

$\text{Minimum Rating} = (\text{Continuous Load} \times 1.25) + \text{Non-Continuous Load}$

Ampacity

The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Disconnecting Means

A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.

In Sight From (Within Sight)

Where specified that one equipment shall be "in sight from", "within sight from", or "within sight of" another equipment, the specified equipment is to be visible and not more than 50 ft (15 m) distant from the other.

[!IMPORTANT] Two-Part Test for 'In Sight From': Equipment is considered within sight ONLY IF it is physically visible AND located within 50 feet. If equipment is 40 feet away but behind a wall, it is NOT within sight. If it is visible down a straight hallway but 60 feet away, it is NOT within sight.