7.1 Article 240 Fundamentals: Ratings, Locations & Conductor Protection

The Purpose of Overcurrent Protection

240.1 Informational Note puts it well: overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation.

Note what the device is protecting. A branch-circuit breaker protects the conductor, not the appliance and not the person. Equipment protection is a separate design decision, and personnel protection comes from GFCIs and grounding.

Standard Ratings — 240.6

240.6(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings for fuses and inverse time circuit breakers shall be considered:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150,
175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000,
1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.

Additional standard ratings for fuses shall be considered to be 1, 3, 6, 10, and 601 amperes.

240.6(B) Adjustable-Trip Circuit Breakers. The rating of adjustable-trip circuit breakers having external means for adjusting the current setting shall be the maximum possible setting.

240.6(C) Restricted Access Adjustable-Trip Circuit Breakers. Where a circuit breaker's adjusting means are behind removable and sealable covers, behind bolted equipment enclosure doors, or behind locked doors accessible only to qualified personnel, the ampere rating shall be equal to the adjusted current setting.

Memorize where the sequence changes step size — it goes up by 5 from 15 to 50, then by 10 to 100, then irregularly. Missing 110 A or 225 A is a common error on round-up questions.

Conductor Protection — 240.4

240.4 General. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.14, unless otherwise permitted or required in 240.4(A) through (G).

240.4(A) Power Loss Hazard. Conductor overload protection shall not be required where the interruption of the circuit would create a hazard, such as in a material-handling electromagnet circuit or a fire pump circuit. Short-circuit protection shall still be provided.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating above the ampacity of the conductors being protected shall be permitted provided all of the following conditions are met:

1. The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads;
2. The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload adjustments;
3. The next higher standard rating selected does not exceed 800 amperes.

240.4(C) Overcurrent Devices Rated Over 800 Amperes. Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device as defined in 240.6.

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed the values in 240.4(D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied:

240.4(E) Tap Conductors. Tap conductors shall be permitted to be protected against overcurrent in accordance with 210.19(A)(3) and (A)(4), 240.5(B)(2), 240.21, 368.17(B), 368.17(C), and 430.53(D).

240.4(F) Transformer Secondary Conductors. Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire secondary, or a three-phase delta-delta connected transformer having a 3-wire secondary, shall be permitted to be protected by overcurrent protection provided on the primary side, provided the protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

240.4(G) Specific Conductor Applications. Overcurrent protection for the conductors in the following applications shall be permitted to be provided as referenced: air-conditioning and refrigeration equipment circuit conductors (440 Parts III, VI), capacitor circuit conductors (460.8), control and instrumentation circuit conductors (Article 727), electric welder circuit conductors (630.12, 630.32), fire alarm circuits (760.43, 760.45, 760.121), motor-operated appliance circuit conductors (422.10, 422.11), motor and motor-control circuit conductors (Article 430 Parts II, III, IV, V, VI, VII), phase converter supply conductors (455.7), remote-control, signaling, and power-limited circuit conductors (Article 725), and secondary tie conductors (450.6).

Location of Overcurrent Devices — 240.21 and 240.24

240.21 Location in Circuit. Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply, except as specified in 240.21(A) through (H). No conductor supplied under the provisions of 240.21(A) through (H) shall supply another conductor under those provisions, except through an overcurrent protective device meeting the requirements of 240.4 — the "no tap off a tap" rule.

240.24(A) Accessibility. Overcurrent devices shall be readily accessible and shall be installed so that the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 2.0 m (6 ft 7 in.) above the floor or working platform. The exceptions cover busway overcurrent devices per 368.17(C), supplementary overcurrent devices per 240.10, overcurrent devices in a listed handle-operated switch or breaker where the handle is within 6 ft 7 in., and overcurrent devices adjacent to utilization equipment they supply, accessible by portable means.

240.24(B) Occupancy. Each occupant shall have ready access to all overcurrent devices protecting the conductors supplying that occupancy. Exceptions permit devices to be accessible only to authorized management personnel in multiple-occupancy buildings and guest rooms where electrical maintenance is provided by continuous building management.

240.24(C) Not Exposed to Physical Damage. Overcurrent devices shall be located where they will not be exposed to physical damage.

240.24(D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets.

240.24(E) Not Located in Bathrooms. In dwelling units, dormitory units, and guest rooms or guest suites, overcurrent devices, other than supplementary overcurrent protection, shall not be located in bathrooms.

240.24(F) Not Located over Steps. Overcurrent devices shall not be located over steps of a stairway.

That set of four prohibited locations — closets, bathrooms, over steps, and physical-damage exposure — is a standard exam item.

Enclosures and Protection from the Environment — 240.32 and 240.33

240.32 Damp or Wet Locations. Enclosures for overcurrent devices in damp or wet locations shall comply with 312.2 — weatherproof, with a 1/4-inch airspace behind the enclosure where mounted on a concrete, masonry, tile, or similar surface, and drainage where necessary.

240.33 Vertical Position. Enclosures for overcurrent devices shall be mounted in a vertical position unless that is impracticable. Circuit breaker enclosures shall be permitted to be installed horizontally where the circuit breaker is installed in accordance with 240.81 — the up-is-on rule.

240.81 Indicating. Circuit breakers shall clearly indicate whether they are in the open (off) or closed (on) position. Where circuit breaker handles are operated vertically rather than rotationally or horizontally, the up position of the handle shall be the on position.

Circuit Breaker Marking and Application

240.83(A) Durable and Legible. Circuit breakers shall be marked with their ampere rating in a manner that will be durable and visible after installation.

240.83(C) Interrupting Rating. Every circuit breaker having an interrupting rating other than 5000 amperes shall have its interrupting rating shown on the circuit breaker. This is why a breaker with no AIC marking is understood to be 5000 AIC.

240.83(D) Used as Switches. Circuit breakers used as switches in 120-volt and 277-volt fluorescent lighting circuits shall be listed and marked SWD or HID. Circuit breakers used as switches in HID lighting circuits shall be listed and marked HID.

240.83(E) Voltage Marking. Circuit breakers shall be marked with a voltage rating not less than the nominal system voltage that is indicative of their capability to interrupt fault currents between phases or phase to ground.

240.85 Applications. A circuit breaker with a straight voltage rating, such as 240 V or 480 V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker's voltage rating. A circuit breaker with a slash rating, such as 120/240 V or 480Y/277 V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values and the nominal voltage between any two conductors does not exceed the higher value.

The slash-rating rule is the one that catches people: a 480Y/277 V breaker may not be used on a 480 V corner-grounded delta or ungrounded delta system, because the voltage to ground on that system is 480 V, not 277 V. That application requires a straight 480 V rated breaker.

Supplementary Overcurrent Protection — 240.10

Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment, or for internal circuits and components of equipment, it shall not be used as a substitute for branch-circuit overcurrent devices or in place of the branch-circuit protection specified in Article 210. Supplementary overcurrent devices shall not be required to be readily accessible.

The distinction between a branch-circuit overcurrent protective device and a supplementary overcurrent protective device was moved into Article 100 in the 2020 edition. A supplementary device — typically a small in-line fuse or a UL 1077 supplementary protector — is evaluated only for the specific application inside the equipment, not as a branch-circuit device.