Overcurrent Protection practice questions

Correct: B — Documentation, a method to reduce clearing time, and performance testing

Per 240.67, fuses rated 1200 amperes or higher trigger all three subsections: (A) documentation, (B) a method to reduce clearing time, and (C) performance testing. Being current-limiting doesn't exempt a fuse from these rules, and GFCI/AFCI protection is unrelated to arc energy reduction.

Correct: D — By the actual adjusted current (long-time pickup) setting

Per 240.6(C), a breaker with restricted access to the adjusting means — including behind bolted equipment enclosure doors — is permitted an ampere rating equal to the actual adjusted current setting. The maximum-setting rule only applies when access isn't restricted, per 240.6(B).

Correct: C — The cord is considered protected when used within the luminaire's listing requirements

Per 240.5(B)(1), flexible cord approved for and used with a specific listed appliance or luminaire is considered protected when applied within that listing's requirements — no matching to the branch-circuit breaker size, no separate length limit, and no supplementary device is required by this rule. The 50 ft length limit is a fixture-wire tap rule under 240.5(B)(2), not a cord rule.

Correct: B — A requirement to document breaker location and provide an arc energy reduction method, with performance testing

240.87 applies whenever the highest continuous trip setting the breaker is rated for or adjustable to is 1200 A or higher, requiring documentation (240.87(A)), a clearing-time reduction method (240.87(B)), and performance testing (240.87(C)). Ground-fault protection of equipment and selective coordination are separate, unrelated requirements, and there is no 80% derating rule tied to this section.

Correct: D — It must open all ungrounded conductors of the circuit, both manually and automatically

240.15(B) requires circuit breakers to open all ungrounded conductors of the circuit both manually and automatically, unless one of the exceptions in 240.15(B)(1) through (B)(4) applies. Opening only the faulted conductor, or only automatically, doesn't meet this default requirement.

Correct: D — Not less than one-tenth of the feeder overcurrent device rating

Per 240.21(B)(1)(4), when a 10-ft tap conductor leaves the enclosure or vault where the tap is made, its ampacity must be not less than one-tenth of the feeder overcurrent device rating. The one-third figure is the threshold for the 25-ft tap rule in 240.21(B)(2), not the 10-ft rule.

Correct: C — They must terminate in a single circuit breaker or set of fuses limiting load current to the conductor ampacity

240.21(B)(3)(5) requires the secondary conductors to terminate in a single circuit breaker or set of fuses that limits the load current to not more than the conductor ampacity permitted by 310.14. Terminating without protection, requiring per-load breakers, or matching the feeder OCPD rating are not what this section specifies.

Correct: A — Ground-fault protection of equipment in accordance with 230.95

240.13 requires ground-fault protection of equipment per 230.95 for each building or structure main disconnect rated 1000 amperes or more on a solidly grounded wye system over 150V to ground but not exceeding 1000V phase-to-phase — a 1200A, 480Y/277V disconnect fits squarely in that trigger. The 2000A threshold is wrong (1000A is the actual cutoff), the system already qualifies since 480V phase-to-phase is under the 1000V ceiling, and the rule explicitly names building/structure disconnecting means, not just feeders.

Correct: C — The conductor is part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads

240.4(B)(1) excludes conductors in a branch circuit feeding more than one receptacle for cord-and-plug portable loads — those require an exact ampacity match. A gap between standard ratings is actually what allows rounding up, a rating of 400 A is well under the 800 A cap, and the rule applies to both breakers and fuses.

Correct: D — The center of the grip of the operating handle in its highest position, from the floor or working platform

240.24(A) measures to the center of the grip of the operating handle when in its highest position, not the enclosure edges, and not the handle's lowest position. That point cannot exceed 2.0 m (6 ft 7 in.) above the floor or working platform.

Correct: A — No, because 240.24(D) prohibits overcurrent devices near easily ignitible material such as clothes closets

240.24(D) specifically names clothes closets as the kind of easily ignitible material location where overcurrent protective devices cannot be installed. There's no exception for a locking cover, no size threshold, and the restriction is not limited to luminaires.

Correct: B — Low-voltage power circuit breaker

240.2(B) permits reconditioning of low-voltage power circuit breakers (and electromechanical protective relays with current transformers), provided the equipment is listed as reconditioned per 110.21(A)(2). Molded-case circuit breakers, GFCIs, and low-voltage nonrenewable fuses are all barred from reconditioning under 240.2(A).

Correct: B — 15 amperes

240.4(D)(4) sets 14 AWG copper at a flat 15 A maximum. 20 A is the 12 AWG copper limit and 30 A is the 10 AWG copper limit — mixing up adjacent wire sizes is the common error. 25 A applies to 10 AWG aluminum/copper-clad aluminum, not copper of any size in this table.

Correct: B — No, because the 25-ft tap rule applies only to industrial installations, and 20 ft exceeds the 10-ft general rule

A commercial office building is not an industrial installation, so the 7.5 m (25 ft) tap in 240.21(C)(3) is unavailable. The general tap in 240.21(C)(2) caps out at 3 m (10 ft), and 20 ft exceeds that limit regardless of raceway enclosure. Neither exception applies here.

Correct: C — No, because 10,000 amperes is the rating exempted from marking

Under 240.60(C), the interrupting rating must be marked only where it is other than 10,000 amperes — so a 10,000 A rating is exempt. It's incorrect that marking is always required regardless of rating, and incorrect that interrupting rating is never required, since it is required whenever the rating differs from 10,000 A. Current-limiting status is a separate, independent marking requirement.

Correct: A — 240.83(C)

240.83(C) requires the interrupting rating to be marked on any breaker rated above 5000A, with an exception for supplementary protection devices. 240.83(A) only addresses that markings be durable and visible, not what must be marked, and 240.86(B) deals with tested series-rated combinations, not individual breaker marking.

Correct: C — The combination must be a tested combination marked on the equipment, or selected under engineering supervision per 240.86(A) or (B), and meet 240.86(C)

240.86 requires the combination to satisfy 240.86(A) (engineered and PE-stamped) or 240.86(B) (tested and marked), and 240.86(C) (motor contribution limit) in all cases. Simply having a higher-rated upstream device isn't sufficient by itself, individual full-rating isn't the only option since series ratings exist as an alternative, and AHJ sign-off alone doesn't substitute for the documented engineering or testing required.

Correct: A — A solidly grounded wye system

Per 240.85, slash-rated breakers are only permitted on solidly grounded circuits, checking both conductor-to-ground and conductor-to-conductor voltages. Ungrounded and corner-grounded delta systems don't meet the solidly grounded requirement, and any-system application ignores the grounding condition entirely.

Correct: B — At the conductor's ampacity as specified in 310.14, unless 240.4(A) through (H) applies

240.4 states conductors are protected in accordance with their ampacities per 310.14 unless otherwise permitted or required by 240.4(A) through (H). The 'always round up' and 'always use smallest conductor' options describe specific exception scenarios (like tap rules or small conductor rules), not the general rule itself, and sizing to load alone ignores the ampacity basis entirely.

Correct: A — The overcurrent device is rated over 800 amperes

Per 240.91, conductors follow 240.4 (the general rule) unless the overcurrent device is rated over 800 amperes, in which case 240.91(B) applies instead. Conductor material, location, and feeder length don't determine which subsection applies.

Correct: D — Replacing it with another Edison-base fuse of the correct rating

Per 240.51(B), Edison-base fuses are permitted for replacements in existing installations where there's no evidence of overfusing or tampering. There's no requirement to convert to Type S at that point, and 240.51(A) caps Edison-base classification at 30 amperes and below.

Correct: A — The building must be a high-bay manufacturing building over 35 ft high at the walls

240.21(B)(4) applies only to high-bay manufacturing buildings over 11 m (35 ft) high at the walls. The 25 ft figure describes the tap's horizontal length limit, not the building height, and there's no footprint or 3-phase service requirement in this rule.

Correct: A — There is no length limit as long as all listed conditions are met

240.21(B)(5) removes the length restriction entirely for taps located outside a building, provided the four conditions (physical protection, single sized overcurrent device, device placement, and accessible disconnect location) are satisfied. The 10-ft, 25-ft, and 100-ft figures belong to other tap rules, not this unlimited-length exception.

Correct: C — 240.22

240.22 specifically bars an overcurrent device from being connected in series with a grounded conductor unless it opens all conductors together, with no independent pole operation. 240.21(D) instead governs where service conductor overcurrent protection is located, not the grounded-conductor series restriction, so it doesn't answer this question.

Correct: A — At the point where the conductor receives its supply

240.21 states the general rule: overcurrent protection is located at the point where the conductor receives its supply, except as permitted by 240.21(A) through (H). Locating it at the load end, at a midpoint, or wherever is convenient are not the code default — those patterns only exist because a specific tap-rule exception allows them.

Correct: D — Electric service and electrical maintenance are provided by building management under continuous supervision

240.24(B)(1) requires that electric service and electrical maintenance be provided by building management and be under continuous building management supervision before service or feeder overcurrent devices supplying multiple occupancies can be restricted to authorized personnel. A locked room or separate metering alone doesn't satisfy this — the continuous-supervision condition is what matters.

Correct: B — 240.92(D)

240.92(D) is the outside-feeder-tap rule that lists the five conditions (physical protection, six-device limit, outdoor routing, integral disconnect location, readily accessible disconnect) permitting unprotected taps or connections made outdoors, including at a transformer secondary. 240.92(B) covers taps made from a feeder run indoors under the 10-ft/25-ft/outside-building tap rules, not a transformer secondary connection outdoors, and 240.92(C) governs separately derived system secondary conductors rather than outside taps.

Correct: C — 150 volts

Per 240.50(A)(2), plug fuses are permitted on systems with a grounded neutral point where the line-to-neutral voltage does not exceed 150 volts. The 125-volt figure is the separate limit for line-to-line voltage under 240.50(A)(1), not the line-to-neutral allowance on a grounded system. 208 and 277 volts both exceed the permitted line-to-neutral maximum.

Correct: D — None of these alone — short-circuit/ground-fault protection, overload protection, and physical protection must all be satisfied together

240.92(C) requires the conditions of 240.92(C)(1), (C)(2), and (C)(3) to all be met — short-circuit/ground-fault protection, overload protection, and physical protection. Raceway enclosure alone only satisfies (C)(3); the 30 m/150% method alone only satisfies part of (C)(1); a single terminating device alone only satisfies (C)(2). None of them individually is sufficient.

Correct: A — 601 amps

240.6(A) states that additional standard ampere ratings for fuses shall be 1, 3, 6, and 601 amps. 500, 50, and 1000 amps are not part of that supplemental list (500 and 50 fall within the main Table 240.6(A) progression, and 1000 is not called out at all here).

Correct: C — 240.4(E)

240.4(E) is the list of permitted alternative overcurrent-protection schemes for tap conductors, including the single-motor tap rule (referring out to 430.53(D)); 240.4(F) instead covers transformer secondary conductors, a different tap scenario entirely.

Correct: C — Once inserted, it cannot be removed from the fuseholder

Per 240.54(C), Type S adapters are designed so that once inserted in a fuseholder, they cannot be removed. This prevents reverting to an Edison base that would accept oversized fuses. It does not become removable, and per 240.54(B) it restricts the holder to Type S fuses only, not standard Edison-base fuses.

Correct: D — No, 240.21(C) specifically prohibits using 240.4(B) for transformer secondary conductors

240.21(C) states plainly that 240.4(B) shall not be permitted for transformer secondary conductors, so the next-size-up rounding allowance can't substitute for meeting one of the (C)(1) through (C)(6) exceptions. The 25 ft length figure is a condition found in one of those specific exceptions, not a blanket allowance, and secondary conductors are not always required to have protection at the secondary — that's the entire point of the 240.21(C) exception family.

Correct: A — The conductors must also satisfy the primary tap conditions in 240.21(B)(3)

240.21(C)(6) is a standalone rule with its own three conditions: the ampacity/voltage-ratio requirement, single-OCPD termination, and physical protection. It does not require also meeting 240.21(B)(3) — that cross-reference to the primary tap rule belongs to the separate allowance in 240.21(C)(5), not (C)(6).

Correct: D — 240.60(D)

240.60(D) is the specific rule permitting renewable-type Class H cartridge fuses only as replacements in existing installations with no evidence of overfusing or tampering. 240.51(B) is the parallel replacement-only rule but it governs Edison-base plug fuses, not cartridge fuses.

Correct: C — 240.60(A)

240.60(A) is the maximum-voltage rule that specifically permits 300-volt type cartridge fuses on line-to-neutral circuits from a solidly grounded 3-phase, 4-wire source not exceeding 300 volts. 240.61 only classifies fuses by voltage/amperage range in general, not this specific usage; 240.50(A) is the analogous maximum-voltage rule for plug fuses, not cartridge fuses.

Correct: B — 240.60(B)

240.60(B) is the noninterchangeability rule for 0-6000-ampere cartridge fuseholders, covering both class/voltage rejection and current-limiting-only fuseholders. 240.60(A) only sets the 300-volt maximum voltage marking rule, and 240.61 merely classifies fuses by ampere/voltage/interrupting rating rather than requiring physical fuseholder rejection.

Correct: D — No, because breakers rated 100 A or less and 1000 V or less must have the rating molded, stamped, or etched into the handle or escutcheon

Under 240.83(B), breakers rated 100 A or less and 1000 V or less must have the ampere rating molded, stamped, etched, or similarly marked into the handle or escutcheon area. This 90 A, 480 V breaker falls within that threshold, so an adhesive label alone does not comply. The rule applies at or below the 100 A / 1000 V threshold, not above it, and there is no voltage-based exemption at 480 V.

Correct: C — 240.81

240.81 specifically requires breakers to indicate open/closed and sets the 'up equals on' rule for vertically operated handles. 240.80 only covers the method of operation (manual plus automatic trip), and 240.83 governs ampere-rating marking, not on/off position indication.

Correct: A — The ampacity of the cord itself, per Table 400.5(A)(1) or Table 400.5(A)(2)

240.5(A) requires flexible cord to be protected by an overcurrent device sized to its own ampacity from Table 400.5(A)(1) or Table 400.5(A)(2), not by the ampacity of the upstream branch-circuit conductors or plug rating.

Correct: D — 240.41(B)

240.41(B) specifically requires that handles or levers of circuit breakers likely to strike nearby persons be guarded or isolated. 240.41(A) instead addresses burn/injury protection from fuse and breaker operation generally (arcing/location), and 240.30(B) covers operating handle accessibility for supply-side equipment, not injury guarding.