Overcurrent Protection (Article 240)

Overloads, short circuits, and ground faults

Article 240 covers all three forms of overcurrent, and the exam expects you to name them correctly. An overload is excess current flowing in a normal, intact circuit — a motor running at 130 percent of its rating, or too many loads plugged into one circuit. Current is higher than the design limit, but the wiring is intact. A short circuit is a fault between two conductors that are not supposed to touch, producing an extremely high current spike in milliseconds. A ground fault is a fault between an ungrounded conductor and a grounded part of the system — the equipment grounding conductor, a metal enclosure, or the earth.

Overcurrent protective devices — fuses and circuit breakers — are designed to clear all three, but they do it differently. A time-delay fuse tolerates a brief overload before opening (useful for motor starting), while an instantaneous trip on a breaker responds to the high-current spike of a short circuit or ground fault in milliseconds. Knowing which type of OCPD applies where is part of the Article 240 picture, even if the journeyman exam keeps the calculation math straightforward.

Standard ampere ratings (240.6(A))

Section 240.6(A) lists the standard ampere ratings for fuses and inverse-time circuit breakers. These are not arbitrary — they are the ratings that manufacturers produce and that the NEC treats as the reference points for conductor protection. The series runs from 15 amps through 6,000 amps. The ratings you will encounter most often on the journeyman exam are in the lower end of the list: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, and 225 amperes. Memorizing where the gaps are — notably between 60 and 70, and between 100 and 110 — matters for the next-size-up problems.

Adjustable-trip circuit breakers introduce a nuance: when the trip setting is not locked and accessible only to qualified persons, the highest trip setting is what the NEC treats as the "rating" for protection purposes. This prevents someone from dialing up an adjustable breaker to defeat the conductor protection.

The next-size-up rule (240.4(B))

The conductors in a circuit have a calculated ampacity — and that ampacity almost never lands exactly on a standard OCPD rating. Section 240.4(B) handles this: when a conductor's ampacity falls between two standard ratings and the next larger rating does not exceed 800 amperes, the next larger standard OCPD is permitted. The conditions that must all be met:

• The circuit is not a multi-outlet branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
• The conductor ampacity does not correspond to a standard rating in 240.6(A) — if it does match, that matching device is required.
• The next higher standard rating selected does not exceed 800 amperes.
• The conductor is not subject to a specific overcurrent protection rule elsewhere in the Code (motors, air-conditioning, transformers, and similar loads have their own sizing rules that override 240.4(B)).

Worked example: a single load circuit is served by conductors whose ampacity — after applying correction and adjustment factors — comes out to 46 amps. The standard OCPD ratings nearest 46 amps are 45 amps and 50 amps. A 45-amp device would protect the conductors conservatively (it trips before the conductor reaches its 46-amp capacity), but the standard rating list includes no 46-amp device. Under 240.4(B), rounding up to the next standard size (50 A) is permitted because 50 A is within the 800-amp ceiling and the circuit supplies a single load, not a multi-outlet branch circuit. The conductors are protected — if current exceeds 50 amps, the device opens.

Small-conductor caps (240.4(D))

The next-size-up permission has a hard limit when small conductors are involved. Section 240.4(D) sets maximum OCPD ratings for the most common branch-circuit wire sizes, and these maximums cannot be bumped up by 240.4(B):

These caps apply regardless of the conductor's actual ampacity in the tables. A 12 AWG copper conductor in free air might be capable of carrying more than 20 amps under certain conditions, but 240.4(D) stops the OCPD at 20 amps for a 12 AWG circuit conductor. The rule reflects the insulation system and the real-world scenarios these wire sizes appear in.

On the exam, 240.4(D) most often appears as a trap: a problem gives you a load that calculates to, say, 22 amps, and asks what wire and breaker combination is correct. The correct answer is to upsize to 10 AWG (protected by a 30-amp breaker), not to put 12 AWG on a 25-amp breaker — which 240.4(D) prohibits outright.

Keep in mind that 240.4(D) lists copper conductor sizes. Aluminum conductors of the same AWG carry lower ampacity, and the equivalent caps for aluminum are listed separately in 240.4(D). Exam questions will usually specify the conductor material; if they do not, assume copper unless context says otherwise.