Note: This article is based on the 2023 NEC.

Overcurrent protection of conductors
Conductors must have overcurrent protection per their ampacities as specified in 310.14. If the conductor ampacity doesn’t correspond to a standard OCPD you can use the next higher standard OCPD rating for OCPDs rated 800A or less. That is permitted only if the three conditions of 240.4(B) are met. For example, the next higher OCPD can’t exceed 800A. Figure 01

If the circuit OCPD exceeds 800A, the conductor ampacity must be equal to or greater than the standard ampere rating or setting of the OCPD as shown in Table 240.6(A) [240.4(C)].

Overcurrent protection for conductors cannot exceed the values listed in 240.4(D)(1) through (8). For example, 15A for 14AWG copper. There are two exceptions in 240.4(E) and (G). Flexible cord must be protected by an OCPD per its ampacity as specified in Table 400.5(A)(1) and Table 400.5(A)(2) [240.5].

Standard ampere ratings
The standard ampere ratings for fuses and inverse time circuit breakers are in Table 240.6(A). It covers 38 ratings, from 10A to 6000A.

The ampere rating of “adjustable trip circuit breakers” with restricted access to the adjusting means is equal to the long-time pickup current settings. Restricted access is achieved by one of the methods listed in 240.6(C)(1) through (4). For example, locating them behind bolted equipment enclosure doors.

A circuit breaker that can be adjusted remotely to modify the adjusting means can have an ampere rating equal to the adjusted current setting (long-time pickup setting) [204.6(D)]. Remote access must be achieved by connecting through either a networked interface that complies with 204.6(D)(2)(a) and (b), or through or a local nonnetworked interface.

All phases
Circuit breakers must automatically open all phase conductors of the circuit during an overcurrent condition [240.15(B)]. Four exceptions exist in 240.15(B)(1) through (4). For example, individual single-pole circuit breakers rated 120/240V with handle ties identified for the purpose are permitted for each phase conductor of a three-phase circuit supplying line-to-line loads.

OCPD location
The location of an OCPD must be:

• Readily accessible [240.24(A)].
• Not in bathrooms, over steps, near easily ignitable materials, or where exposed to physical damage [240.24(E), (F), (D), and (C), respectively].
• Not too high. The center grip of the handle of the circuit breaker or switch containing fuses cannot (in its highest position) be more than 6 ft 7 in. above the floor or working platform unless one of the four exceptions to 240.24(A) applies.
  

Overcurrent protection is required at the point where conductors receive their supply, except as permitted by 240.21(A) through (H). The bulk of (A) through (H) is in 240.21(B)(1) through (4), which cover feeder taps. A few highlights:

• The feeder tap is permitted at any point on the load side of the feeder OCPD, including the load terminals of the feeder OCPD.
• You can’t use the next size up rule [240.4(B)] for tap conductors.
• Tap conductors up to 10 ft long are permitted when they comply with the four requirements listed in 240.21(B)(1)(1) through (4). For example, they can’t extend beyond the equipment they supply.
• Tap conductors up to 25 ft long are permitted when they comply with the three requirements listed in 240.21(B)(2)(1) through (3). For example, the tap conductors are enclosed in a raceway or other means approved by the authority having jurisdiction.
  

For transformer secondary conductors, check out some key points from 240.21(C):

• The secondary terminals of a transformer can supply multiple sets of secondary conductors [240.21(C)]. But you can’t apply the next size up rule of 240.4(B).
• Outside secondary conductors can be of unlimited length if they comply with 240.21(C)(4)(1) through (4). For example, they terminate in a circuit breaker or set of fuses and have an ampacity of at least the rating of the OCPD.
• Secondary conductors up to 25 ft long are permitted when they comply with 240.21(C)(6)(1) through (3). For example, they’re enclosed in raceway or a similar approved means.
  

Enclosures containing OCPDs
Enclosures containing fuses must be mounted in a vertical position [240.33]. Enclosures containing circuit breakers must be mounted vertically if the circuit-breaker handle is operated vertically in accordance with 240.81.
When the handle of a circuit breaker is operated vertically, the “up” position of the handle must be the “on” position [240.81]. So, in effect, an enclosure that contains one row of circuit breakers can be mounted horizontally, but an enclosure that contains a panelboard with multiple circuit breakers on opposite sides of each other must be mounted vertically.

Markings
Circuit breakers used to switch 120V or 277V fluorescent lighting circuits must be listed and marked “SWD” or “HID.” Circuit breakers used to switch high-intensity discharge lighting circuits must be listed and marked “HID” [240.83(D)].

A circuit breaker with a straight voltage marking (e.g., 240V or 480V) is permitted on a circuit if the nominal system voltage between any two conductors (line-to-neutral or line-to-line) does not exceed the circuit breakers’ marked voltage rating [240.85].

A circuit breaker with a slash rating (such as 120/240V or 277/480V) marking is permitted if the nominal system voltage of any one conductor to ground does not exceed the lower voltage marking, and the nominal system voltage between any two conductors does not exceed the higher voltage marking.

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an approved OCPD having a higher rating, the circuit breaker must meet the requirements of 240.86(A), (B), or (C). For example, the combination of the line-side OCPD and the load-side circuit breakers is tested and marked on the end-use equipment, such as switchboards and panelboards.

Adjusting to 1200A
Where the highest continuous current trip setting of an adjustable trip circuit breaker can be adjusted to 1200A or higher, 240.87(A) through 240.87(C) apply:
(A) Documentation. It is required to:

• Provide of the location of the arc-energy reduction circuit breaker(s).
• Be available to those authorized to design, install, operate, or inspect the installation.
• Demonstrate that the method chosen to reduce clearing time is set to operate at a value below the available arcing current.
  

(B) Method to reduce clearing time. One of the seven methods listed in 240.87(B) is required. For example, differential relaying. The method must be set to operate at less than the available arcing current.

(C) Performance testing. This must be done with primary current injection testing or another approved method when first installed onsite. It must be performed by qualified persons per the manufacturer’s instructions.

Where to start?
Article 240 requirements are intended to protect conductors and equipment from overcurrent. But Article 430, Parts III, IV, and V amend those requirements because motors have inrush current. And Article 695 amends them because a fire pump must run no matter what even if that means severely damaging the pump or its supply conductors.

Those exceptions aside, start the overcurrent protection aspect of your projects by asking what is needed to protect your circuit conductors and connected equipment from overcurrent.

Sidebar: Definitions
These are in Article 100:
Identified. Recognized as suitable for a specific purpose, function, or environment by listing, labeling, or other means approved by the authority having jurisdiction.
Ground Fault. Unintentional electrical connection between a phase conductor and equipment grounding conductors, metal parts of enclosures, metal raceways, or metal equipment.
Overcurrent. Current that is more than the conductor’s ampacity (caused by an overload, short circuit, or ground fault).
OCPD. Device capable of protecting a circuit from an overload, short circuit, or ground fault.
Overload. Condition that occurs when equipment operates above its current rating or at more current than the ampacity of its supply conductors. A short circuit or ground fault is not an overload.
Short Circuit. Abnormal connection of relatively low impedance, whether made accidentally or intentionally, between points of different potential.
Tap Conductor. A conductor, other than a service conductor, with overcurrent protection rated more than the ampacity of the conductor.