NEC 240 — Overcurrent Protection (Washington Journeyman)

Learning Objectives

Explain the scope of Article 240 and name the devices recognized as OCPDs under 240.1, 240.2, and 240.7.

Apply 240.4, including the next-size-up rule of 240.4(B), the over-800-ampere rule of 240.4(C), and the small-conductor limits of 240.4(D).

Select ratings from Table 240.6(A) and apply the adjustable-trip rules of 240.6(B) through 240.6(D).

Determine when ground-fault protection of equipment is required by 240.13 and 230.95 and how the grounded conductor connects to ground under 250.50.

Apply the 240.21(B) tap rules, the 240.21(C) transformer secondary rules, and the 240.86 and 240.24 series-rating and location rules.

1.1 Scope and Recognized OCPDs (240.1, 240.7)

Article 240 Parts I through VII give the general requirements for overcurrent protection and OCPDs not over 1000 volts ac, 1500 volts dc, nominal; Part VIII covers supervised industrial installations at those limits (240.1). Section 240.2 requires listing of OCPDs, relays and circuit breakers providing GFPE, ground-fault circuit interrupters, and fuse reducers. Section 240.3 permits reconditioning of low-voltage power circuit breakers, electromechanical protective relays, and current transformers, but bars it for GFPE equipment, GFCIs, low-voltage fuseholders and nonrenewable fuses, molded-case circuit breakers, and power circuit breaker electronic trip units.

Section 240.7 lists the permitted OCPDs: molded-case, low-voltage power, and solid-state breakers; Class C through Class T fuses; Edison-base, Type C, or Type S fuses; and other fuses or breakers labeled as suitable. Part I also bars thermal relays from short-circuit or ground-fault protection (240.9), requires an OCPD in series with each ungrounded conductor (240.15), sets a minimum interrupting rating of 5000 amperes (240.16), and bars an OCPD in series with an intentionally grounded conductor (240.22).

1.2 Protection of Conductors and the Next-Size-Up Rule (240.4)

Conductors other than flexible cords, flexible cables, and fixture wires must be protected in accordance with their ampacities in 310.14 unless otherwise permitted or required in 240.4(A) through 240.4(H).

(B) OCPDs rated 800 amperes or less. The next higher standard rating above the conductor ampacity is permitted only where: the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads; the ampacity does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating; and the next higher standard rating does not exceed 800 amperes. An adjustable trip device so installed may be set to the next higher standard value in Table 240.6(A) where restricted access per 240.6(C) applies. An ampacity of 185 amperes takes 200 amperes; 225 amperes is two steps up and 175 amperes is below the ampacity.

(C) Where the OCPD is rated over 800 amperes, the conductor ampacity must be equal to or greater than the rating of the OCPD defined in 240.6.

(D) Small conductors. Unless permitted in 240.4(E) or 240.4(G), overcurrent protection may not exceed the following after correction factors for ambient temperature and number of conductors are applied:

Conductor	Maximum overcurrent protection
18 AWG copper	7 amperes (continuous loads not over 5.6 amperes; listed OCPD or a Class CC, CF, J, or T fuse)
16 AWG copper	10 amperes (continuous loads not over 8 amperes; same marking or fuse types)
14 AWG copper-clad aluminum	10 amperes (continuous loads not over 8 amperes)
14 AWG copper	15 amperes
12 AWG aluminum and copper-clad aluminum	15 amperes
12 AWG copper	20 amperes
10 AWG aluminum and copper-clad aluminum	25 amperes
10 AWG copper	30 amperes

(E) Taps may be protected per 210.19(C), 210.19(D), 240.5(B)(2), 240.21, 368.17(B), 368.17(C), and 430.53(D).

(F) Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not protected by the primary OCPD; a 2-wire single-phase or 3-wire delta-delta secondary may be protected on the primary side per 450.5, where that protection does not exceed the secondary ampacity multiplied by the secondary-to-primary voltage ratio.

(G) Table 240.4(G) lists conductors whose overcurrent protection is provided elsewhere — air-conditioning and refrigeration, capacitors, Type ITC control and instrumentation, welders, fire alarm, motor-operated appliances, motors and motor controls, phase converters, remote-control and power-limited circuits, and secondary ties — the applications where the limits of 240.4(D) do not apply.

(H) Dwelling unit service and feeder conductors may be protected at the ampacity values of 310.12, which covers one-family dwellings and the individual units of two-family and multifamily dwellings on single-phase, 120/240-volt systems and two-ungrounded-conductor-plus-neutral feeders from a 208Y/120 volt system; a service rated 100 amperes through 400 amperes may be sized by those values.

1.3 Standard Ampere Ratings and Adjustable Trip Settings (240.6)

Table 240.6(A) gives the standard ampere ratings for fuses and inverse time circuit breakers: 10, 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 are 1, 3, 6, and 601 amperes, and nonstandard ratings are permitted for use. Values not listed, such as 230, 260, 275, 310, 325, 550, 650, 780, 1100, 1300, 1400, and 1500, are not standard, so a rating above 1000 amperes lands on 1200, 1600, 2000, 2500, 3000, 4000, 5000, or 6000 amperes.

Section 240.6(B) rates an adjustable-trip breaker with external means for adjusting the current setting (long-time pickup setting) at the maximum setting possible unless 240.6(C) or 240.6(D) applies. Section 240.6(C) permits the adjusted setting where the breaker cannot be adjusted remotely and restricted access is achieved by removable and sealable covers, bolted doors, locked doors accessible only to qualified personnel, or password protection. Section 240.6(D) permits the adjusted setting for a remotely adjustable breaker only where that local restricted access exists and remote access is through a local nonnetworked interface or a networked interface meeting the cybersecurity conditions of 240.6(D). Otherwise the rating is the highest possible setting.

1.4 Ground-Fault Protection of Equipment (240.13 and 230.95)

GFPE protects equipment from damaging line-to-ground fault currents; it is not the 5-milliampere personnel protection of a Class A ground-fault circuit interrupter. Section 240.13(A) requires GFPE, provided in accordance with 230.95, for solidly grounded wye systems of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase at each individual device used as a building or structure main disconnecting means rated 1000 amperes or more; 240.13(B) requires it for solidly grounded dc systems of more than 150 volts to ground but not exceeding 1500 volts dc line-to-line at a main disconnecting means rated 1000 amperes or more. Both exclude the disconnecting means for continuous industrial processes where a nonorderly shutdown would introduce additional hazards, installations where ground-fault protection is provided by other requirements, and fire pumps.

Section 230.95 applies the same requirement to services: GFPE for solidly grounded wye services of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase at each service disconnect rated 1000 amperes or more (230.95(A)), and for solidly grounded dc services of more than 150 volts to ground but not exceeding 1500 volts dc line-to-line (230.95(B)). "1000 amperes or more" includes exactly 1000 amperes, so a 120/240-volt single-phase service and a 600-ampere disconnect are outside the rule.

The grounded conductor must be connected directly to ground through a grounding electrode system, as specified in 250.50, without inserting any resistor or impedance device; a grounding resistor that limits fault current defeats the low-impedance path the section requires, and connection through the service enclosure alone omits the electrode system. Section 250.50 builds that system from all grounding electrodes described in 250.52(A)(1) through 250.52(A)(7) present at each building or structure served. Where GFPE is installed it must be performance tested as a system considering all sources (230.95(C)).

1.5 Location in Circuit: Feeder Taps (240.21(B))

Overcurrent protection is provided in each ungrounded circuit conductor at the point where the conductors receive their supply, except as specified in 240.21(A) through 240.21(H), and conductors supplied under 240.21(B) through 240.21(H) may not supply another conductor except through an OCPD meeting 240.4. For taps the next-size-up rule is unavailable: Section 240.4(B) shall not be permitted for tap conductors.

240.21(B)(1), taps not over 3 m (10 ft) long. Ampacity not less than the combined calculated loads and not less than the rating of the equipment containing the OCPDs supplied or the OCPD at the termination; no extension beyond the enclosure of the switchboard, switchgear, panelboard, or control devices supplied; except at the point of connection, enclosed in a raceway running to that enclosure; and, where the installation leaves the enclosure or vault, ampacity at least one-tenth of the feeder OCPD rating.

240.21(B)(2), taps not over 7.5 m (25 ft) long. Ampacity at least one-third of the rating of the feeder OCPD; termination in a single breaker or set of fuses that limit the load to the tap ampacity, with additional OCPDs permitted on the load side; and protection from physical damage. For a 400-ampere feeder OCPD the threshold is _133.3 A_.

240.21(B)(3), taps supplying a transformer, primary plus secondary not over 7.5 m (25 ft) long. Primary ampacity at least one-third the feeder OCPD rating; secondary ampacity not less than the primary-to-secondary voltage ratio multiplied by one-third of that rating; total length of one primary plus one secondary conductor, excluding any primary portion protected at its ampacity, not over 7.5 m (25 ft); and the secondary terminating in a single breaker or set of fuses limiting the load current to not more than the ampacity permitted by 310.14.

240.21(B)(4), taps over 7.5 m (25 ft) long in high bay manufacturing buildings. With the feeder in a high bay manufacturing building over 11 m (35 ft) high at the walls and qualified persons servicing the systems: tap not over 7.5 m (25 ft) horizontally and not over 30 m (100 ft) total; ampacity at least one-third the feeder OCPD rating; termination at a single breaker or set of fuses limiting the load to its ampacity; protected from physical damage; continuous end-to-end with no splices; 6 AWG copper or 4 AWG aluminum or larger; no penetration of walls, floors, or ceilings; and made no less than 9 m (30 ft) from the floor.

240.21(B)(5), outside taps of unlimited length. Conductors outside a building or structure, except at the point of load termination, must be protected from physical damage; terminate at a single breaker or set of fuses limiting the load to their ampacity; be protected by an OCPD integral to a disconnecting means or located immediately adjacent; and have that disconnecting means readily accessible — outside, inside nearest the point of entrance, or, per 230.6, nearest the point of entrance.

1.6 Transformer Secondary Conductors (240.21(C))

A set of conductors feeding a single load, or each set feeding separate loads, may connect to a transformer secondary without overcurrent protection at the secondary as specified in 240.21(C)(1) through 240.21(C)(6). Section 240.4(B) shall not be permitted for transformer secondary conductors.

240.21(C)(1) — a 2-wire (single-voltage) single-phase secondary or a 3-wire delta-delta secondary may be protected on the primary side per 450.5, where that protection does not exceed the secondary ampacity multiplied by the secondary-to-primary voltage ratio; other secondaries are not protected by the primary device.

240.21(C)(2), not over 3 m (10 ft) long — ampacity not less than the combined calculated loads and not less than the rating of the equipment containing the OCPDs supplied; no extension beyond the equipment enclosure; enclosed in a raceway from the transformer to that enclosure; and, for field installations leaving the enclosure or vault, ampacity at least the primary-to-secondary voltage ratio multiplied by one-tenth of the primary OCPD rating.

240.21(C)(3), industrial installations not over 7.5 m (25 ft) long — switchgear or switchboard supply only, with qualified persons: ampacity not less than the secondary current rating of the transformer, with the sum of the OCPD ratings not exceeding that ampacity; all OCPDs grouped; physical damage protection.

240.21(C)(4), outside secondary conductors — protected from physical damage; terminated at a single breaker or set of fuses limiting the load to the conductor ampacity; protected by an OCPD integral to a disconnecting means or immediately adjacent; disconnecting means readily accessible.

240.21(C)(5) — secondary conductors installed per 240.21(B)(3) are protected as that section specifies.

240.21(C)(6), not over 7.5 m (25 ft) long — ampacity not less than the primary-to-secondary voltage ratio multiplied by one-third of the primary OCPD rating; termination in a single breaker or set of fuses limiting the load current to not more than the ampacity permitted by 310.14; physical damage protection.

The arithmetic is the exam favorite. With a 400-ampere primary OCPD on a transformer rated 480 volts to 208/120 volts, the one-third term is _133.3 A_ and the voltage-ratio term is 400 multiplied by 208/480, or _173.3 A_. The larger governs, so the secondary conductors must carry at least _173.3 A_, and the termination must be a standard rating at or above that value. Those decimals are products of the rules, not code text and not NEC sections.

1.7 Series Ratings and OCPD Location (240.86 and 240.24)

Where a breaker is used on a circuit whose available fault current exceeds its marked interrupting rating by connecting it on the load side of an approved OCPD with a higher rating, the breaker must meet 240.86(A), 240.86(B), and 240.86(C). A combination selected under engineering supervision in existing installations must be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations, documented and stamped, and field marked on the end-use equipment with the series combination rating including identification of the upstream device. Engineered combinations are marked per 110.22(B). Field-marking a breaker with a higher interrupting rating, or paralleling it with the upstream device, is not a series-rating method.

Section 240.24 requires breakers and switches containing fuses to be readily accessible with the center of the grip of the operating handle in its highest position not more than 2.0 m (6 ft 7 in.) above the grade, floor, or working platform, unless the location is a busway per 368.17(C), supplementary protection per 240.10, an OCPD per 225.40 or 230.92, or (240.24(A)(4)) an OCPD adjacent to the utilization equipment it supplies, where access by portable means is permitted. Service and feeder OCPDs supplying more than one occupancy may be management-only where building management provides electric service and maintenance under continuous supervision in multiple-occupancy buildings and guest rooms or guest suites (240.24(B)(1)); branch-circuit OCPDs for guest rooms, guest suites, or sleeping rooms in dormitory units without permanent cooking provisions may be similarly restricted (240.24(B)(2)). OCPDs must not be exposed to physical damage (240.24(C)), in the vicinity of easily ignitible material such as clothes closets (240.24(D)), in bathrooms, showering facilities, or locker rooms with showering facilities other than supplementary protection (240.24(E)), or over the steps of a stairway (240.24(F)).

Elsewhere in the article, 240.50 through 240.54 limit plug fuses to circuits not exceeding 125 volts between conductors and 150 volts line-to-neutral, and 240.87 requires arc energy reduction where the highest continuous current trip setting of a breaker is 1200 amperes or higher.

1.8 Flexible Cords, Fixture Wire, and Supervised Industrial Installations (240.5, 240.91)

Flexible cord and flexible cable, must be protected in accordance with their ampacity as specified in Table 400.5(A)(1) and Table 400.5(A)(2), and fixture wire in accordance with Table 402.5; supplementary protection per 240.10 is permitted as an acceptable means (240.5(A)). Under 240.5(B), cord supplied by a branch circuit is protected by 240.5(B)(1) (supply cord of a listed appliance or luminaire), 240.5(B)(3) (listed extension cord sets), or 240.5(B)(4) (field assembled extension cord sets on a 20-ampere circuit with 16 AWG and larger); fixture wire is protected by 240.5(B)(2), permitting 18 AWG up to 15 m (50 ft), 16 AWG up to 30 m (100 ft), and 14 AWG and larger on 20-ampere circuits. For supervised industrial installations used exclusively for manufacturing or process control activities (240.90), 240.91(B) permits conductor ampacity at 95 percent of the rating of an OCPD rated over 800 amperes.

1.9 Code Navigation

Topic	NEC Section	What it covers
Recognized devices	240.7	Permitted breakers and fuses; molded-case, low-voltage power, solid-state, Class C through Class T, Edison-base, Type S
Next-size-up and over 800 amperes	240.4(B), 240.4(C)	Next higher standard rating capped at 800 amperes; ampacity at least the rating above 800 amperes
Small conductors and specific applications	240.4(D), 240.4(G), Table 240.4(G)	Limits from 18 AWG copper through 10 AWG copper, and the applications where they do not apply
Standard ratings and trip settings	240.6, Table 240.6(A)	Standard ratings; additional fuse ratings of 1, 3, 6, and 601 amperes; adjustable-trip rules of 240.6(B) through 240.6(D)
Ground-fault protection of equipment	240.13, 230.95, 250.50	GFPE for grounded wye and dc systems; direct connection of the grounded conductor through the electrode system
Feeder taps and transformer secondary conductors	240.21(B), 240.21(C)	10 ft, 25 ft, transformer, high bay, outside, industrial, and primary-protection rules
Accessibility and location	240.24	Readily accessible, the 2.0 m (6 ft 7 in.) handle height, portable access, closets, bathrooms, stairways

Common Exam Traps

Treating 240.4(B) as an unconditional shortcut. The next higher standard rating requires that the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, that the ampacity does not correspond with a standard rating, and that the next higher rating does not exceed 800 amperes. Distractors offer 600 or 1200 amperes as the limit, or claim there is none.

Confusing 240.4(D) with 240.4(G). 240.4(D) sets the small-conductor limits (14 AWG copper at 15 amperes, 12 AWG copper at 20 amperes, 10 AWG copper at 30 amperes); 240.4(G) lists the applications where those limits do not apply, and is not limited to conductors larger than 10 AWG.

Using the next-size-up rule inside a tap. Section 240.4(B) is prohibited for tap conductors under 240.21(B) and for transformer secondary conductors under 240.21(C); each is sized by its own rule.

Putting the one-third factor in the 10 ft rule. The 10 ft rule of 240.21(B)(1) requires ampacity not less than the combined calculated loads and not less than the rating of the OCPD supplied; the one-third factor belongs to the 25 ft rules of 240.21(B)(2), 240.21(B)(3), 240.21(B)(4), and 240.21(C)(6). For a 400-ampere feeder, one-third is _133.3 A_.

Mistaking computed decimals for code text. Values such as _133.3 A_ and _173.3 A_ are products of the tap and transformer rules, and _601.5 A_ offered as an additional standard fuse rating comes from an older list. The 2026 text of 240.6(A) adds 1, 3, 6, and 601 amperes.

Applying GFPE by habit instead of by trigger. Both 240.13(A) and 230.95(A) require a solidly grounded wye system of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase with a main or service disconnecting means rated 1000 amperes or more. A 120/240-volt single-phase service, a 600-ampere disconnect, and an ungrounded or corner-grounded delta system do not qualify.

Redesigning the grounded conductor connection. Section 230.95(A) requires the grounded conductor to connect directly to ground through a grounding electrode system as specified in 250.50, with no resistor or impedance device inserted; an option that adds a grounding resistor inverts the requirement.

Reading 240.24 too loosely. Handles must be readily accessible with the grip no more than 2.0 m (6 ft 7 in.) above the floor or platform; OCPDs adjacent to the utilization equipment they supply may be reached by portable means (240.24(A)(4)); and management-only access under 240.24(B) covers service and feeder OCPDs, plus guest-room branch circuits, under continuous building management supervision.

Practical Field Points

Check the tap before adding a load. A conductor tapped from a feeder is governed by 240.21(B)(1) for 10 ft taps, 240.21(B)(2) for 25 ft taps, or 240.21(B)(3) for transformer primary-plus-secondary taps; the ampacity test, length limit, and physical damage protection all change with the rule.

Never field-mark a breaker with a higher interrupting rating. A series-rated installation needs either an engineered combination documented and stamped by a licensed professional engineer under 240.86(A) or a tested combination listed for the specific upstream device, marked as 110.22 requires.

Verify the grounded conductor on any wye service with GFPE. It must land on the grounding electrode system required by 250.50 with no resistor or impedance device in that path, and testing under 230.95(C) must consider all sources.

Treat adjustable-trip breakers as set at their maximum. Outside the restricted-access cases of 240.6(C), the rating is the highest setting the breaker can be adjusted to.

Watch the small conductors. On 14 AWG, 12 AWG, and 10 AWG copper branch circuits, 240.4(D) governs unless the circuit is one of the specific applications in Table 240.4(G).

Expect arc energy reduction at 1200 amperes and above, under 240.87 for breakers, with a performance test record available to the authority having jurisdiction.

Summary

Article 240 answers two questions: what protects the conductor, and where the protective device may be installed. Section 240.4 states the general rule that conductors are protected in accordance with their ampacities in 310.14, with its exceptions in 240.4(A) through 240.4(H) — among them the next-size-up rule capped at 800 amperes (240.4(B)), the requirement that ampacity be at least the OCPD rating above 800 amperes (240.4(C)), the small-conductor limits from 18 AWG copper through 10 AWG copper (240.4(D)), the tap and transformer permissions of 240.4(E) and 240.4(F), the applications of Table 240.4(G), and the dwelling ampacities of 310.12 reached through 240.4(H). Ratings come from Table 240.6(A), with 1, 3, 6, and 601 amperes added for fuses, and an adjustable-trip breaker is rated at the maximum setting possible unless the restricted access of 240.6(C) applies. Ground-fault protection of equipment is required for solidly grounded wye systems over 150 volts to ground and not over 1000 volts phase-to-phase at a main or service disconnect rated 1000 amperes or more, with the grounded conductor connected directly to ground through a grounding electrode system as specified in 250.50 and no resistor or impedance device inserted. The location rules of 240.21 drive the field work — the 10 ft, 25 ft, transformer, high bay, and outside tap rules of 240.21(B) and the transformer secondary rules of 240.21(C), with 240.4(B) prohibited in both — while 240.86 limits series-rated systems to documented engineered or tested combinations marked as 110.22 requires, and 240.24 keeps OCPDs readily accessible and out of bathrooms, closets, and stairway steps.

NEC 240 — Overcurrent Protection (Washington)