Question 1

Which of the following is a standard rating for a fuse or inverse time circuit breaker?

Accepted Answer

NEC 240.6(A) lists the standard ampere ratings. 55, 65, and 75 are NOT standard ratings (standard are 50, 60, 70, 80, 90...). Wait, 70 is standard. 80 is standard. 90 is standard. 85 is NOT standard. 60 is standard. 50 is standard. Let's re-check 240.6(A). Standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125... So 55, 65, 75, 85 are ALL non-standard? No, 70 is standard. 80 is standard. 45 is standard. Is 55 standard? No. Is 65 standard? No. Is 75 standard? No. Is 85 standard? No. Wait, I need one correct option. 110 is standard. Let's adjust options.

Question 2

Which of the following is a standard ampere rating for a circuit breaker per NEC 240.6(A)?

Accepted Answer

NEC 240.6(A) lists 110 Amps as a standard rating. (65, 75, and 115 are not standard ratings).

Question 3

Conductors must generally be protected against overcurrent at the point where:

Accepted Answer

NEC 240.21 requires overcurrent protection to be provided in each ungrounded circuit conductor at the point where the conductors receive their supply.

Question 4

The 'Next Size Up' rule (240.4(B)) permits using a higher rated overcurrent device if the conductor ampacity does not match a standard rating, provided the rating does not exceed:

Accepted Answer

NEC 240.4(B) applies only to overcurrent devices rated 800 amperes or less.

Question 5

Under the '10-foot Tap Rule' (240.21(B)(1)), tap conductors are permitted without overcurrent protection at the tap point if their ampacity is at least:

Accepted Answer

NEC 240.21(B)(1) requires tap conductors not over 10 feet long to have an ampacity not less than the combined calculated load AND not less than 1/10 the rating of the overcurrent device protecting the feeder.

Question 6

Supplementary overcurrent protection (like a fuse in a light fixture) is:

Accepted Answer

NEC 240.10 specifies that supplementary overcurrent protection is not a substitute for branch-circuit protection. NEC 240.24(A) Exception 2 states that supplementary OCPDs are not required to be readily accessible.

Question 7

Circuit breakers used to switch 120V or 277V fluorescent lighting circuits on a daily basis must be marked:

Accepted Answer

NEC 240.83(D) requires breakers used as switches for 120V/277V fluorescent lighting to be marked SWD (Switching Duty) or HID (High Intensity Discharge).

Question 8

Overcurrent devices shall be readily accessible. Which of the following locations is permitted for installing an OCPD?

Accepted Answer

NEC 240.24 prohibits OCPDs in dwelling bathrooms (E), over steps of a stairway (F), and near easily ignitable material (clothes closets) (D). Guest rooms are permitted (provided they are accessible to the occupant, though management often holds keys to service areas, guest access limits apply. However, dwelling bathrooms and stairways are explicit NOs. Guest rooms are generally allowed if not in prohibited spots).

Question 9

Where a circuit breaker is rated 1200A or higher, what special method must be provided to reduce clearing time?

Accepted Answer

NEC 240.87 requires an Arc Energy Reduction method (such as zone-selective interlocking, differential relaying, or maintenance switch) for breakers rated (or adjustable to) 1200A or higher.

Question 10

Plug fuses of the Edison-base type shall be used only:

Accepted Answer

NEC 240.51(B) states that Edison-base plug fuses shall be used only as replacements in existing installations where there is no evidence of overfusing or tampering.

Question 11

For a circuit supplied by a transformer arc welder, the overcurrent protection is permitted to be rated at not more than ____ of the primary current.

Accepted Answer

NEC 630.12(A) allows the overcurrent protection for an arc welder to be rated or set at not more than 200% of the rated primary current.

Question 12

Circuit breakers used in 3-phase, 4-wire, delta-connected systems where the voltage to ground exceeds 120V (e.g., High-Leg Delta) must be rated:

Accepted Answer

NEC 240.85 states that a circuit breaker with a slash rating (e.g., 120/240V) shall be permitted only in a grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values. In a High-Leg Delta (208V to ground on the high leg), a 120/240V breaker is NOT permitted on the high leg because 208V > 120V. A straight rated 240V breaker is required.

Question 13

A fuse is stamped with 'Class RK1'. This indicates the fuse is:

Accepted Answer

Class R (RK1 and RK5) fuses are Current-Limiting fuses designed to reject non-current limiting fuses in the same holder (rejection clips).

Question 14

Standard fuse holders for cartridge fuses shall be so designed that it is difficult to put a fuse of any given class into a fuse holder that is designed for a ____ lower or higher ampere rating.

Accepted Answer

NEC 240.60(B) requires fuse holders to be designed so that it is difficult to put a fuse of a given class into a holder designed for a current lower or higher than that of the class to which the fuse belongs. (Wait, checking specific text: 'designed so that it is difficult to put a fuse of any given class into a fuse holder that is designed for a current lower, or higher, or...'). Actually, 240.60(B) is about voltage. 240.61 is about screw shell. The correct citation for non-interchangeability of cartridge fuses is 240.60(B): 'Fuseholders shall be so designed that it is difficult to put a fuse of any given class into a fuse holder that is designed for a current lower, or higher, than that of the class to which the fuse belongs.' The key is Current ranges (0-30, 31-60, 61-100, etc.).

Question 15

What is the maximum overcurrent protection allowed for a 20A branch circuit supplying a single non-motor appliance?

Accepted Answer

NEC 210.20(B) states that the rating of the overcurrent device shall not exceed the rating of the branch circuit (20A). For a single appliance, 422.11(E) says it shall not exceed 20A if the appliance is rated 13.3A or less (standard 20A limit applies).

Question 16

Ground-fault protection of equipment (GFPE) shall be provided for solidly grounded wye electric services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnect rated _____ or more.

Accepted Answer

NEC 230.95 requires GFPE for solidly grounded wye services (like 480Y/277V) rated 1000 amperes or more.

Question 17

The maximum setting for Ground-Fault Protection of Equipment (GFPE) shall be _____ and the time delay shall not exceed 1 second for ground faults of 3000 amperes or more.

Accepted Answer

NEC 230.95(A) sets the maximum setting for GFPE at 1200 Amperes.

Question 18

Fuses and circuit breakers must be marked with their ampere rating. For hex-shaped fuses or fuses with a hex-shaped opening in the cap, the rating is:

Accepted Answer

NEC 240.50(C) requires plug fuses of 15 amperes and lower to be identified by a hexagonal configuration.

Question 19

Overcurrent devices for panelboards supplied by a transformer (secondary side) must be located:

Accepted Answer

NEC 408.36 requires that panelboards be protected by an overcurrent device having a rating not greater than that of the panelboard, located within or at any point on the supply side of the panelboard.

Question 20

A tap conductor is 20 feet long. To comply with the '25-foot Tap Rule' (240.21(B)(2)), the tap conductor ampacity must be at least ____ of the rating of the overcurrent device protecting the feeder.

Accepted Answer

NEC 240.21(B)(2) states that for taps not over 25 feet, the ampacity must be at least 1/3 of the rating of the feeder OCPD.

Question 21

Which Article provides the specific requirements for overcurrent protection of Air Conditioning and Refrigerating Equipment?

Accepted Answer

Article 440 Parts III and VI cover the specific overcurrent protection rules for HVAC equipment (often allowing higher ratings than standard motors to accommodate hermetic compressors).

Question 22

If a circuit breaker has an adjustable trip setting, the 'rating' of the breaker is considered to be:

Accepted Answer

NEC 240.6(B) states that the rating of an adjustable trip circuit breaker is the maximum setting possible, unless access is restricted (sealed cover, locked door, password protected).

Question 23

Overcurrent devices shall not be located in the vicinity of easily ignitable material, such as in:

Accepted Answer

NEC 240.24(D) explicitly prohibits installing overcurrent devices in clothes closets.

Question 24

A 'Series Rated' system allows a downstream circuit breaker to have an interrupting rating lower than the available fault current if:

Accepted Answer

NEC 240.86(B) permits series ratings where the combination of the line-side (protecting) device and load-side device is tested and listed for the available fault current.

Question 25

The handle position of a circuit breaker must indicate:

Accepted Answer

NEC 240.81 requires that circuit breakers clearly indicate whether they are in the open (off) or closed (on) position. If vertically mounted, 'up' must be 'on'.

Question 26

NEC 2026 240.24(G) adds a new prohibition for locating overcurrent devices in:

Accepted Answer

Safety updates in 2026 focus on keeping OCPDs out of critical egress paths to prevent obstruction during emergencies.

Question 27

Under NEC 2026, reconditioned circuit breakers must be listed as 'reconditioned' and the original listing mark must be:

Accepted Answer

NEC 2026 clarifies reconditioning rules (110.21/240.88). Typically, the original listing mark is removed or invalidated because the reconditioner takes responsibility, applying a new 'reconditioned' listing mark. Wait, check 240.88? Actually, the *original* listing mark is usually required to be removed to avoid confusion, or the new label supersedes it. The key rule is that it must be identified as reconditioned.

Question 28

What is the maximum overcurrent protection for an 18 AWG fixture wire used in a 120V circuit?

Accepted Answer

NEC 240.5(B)(2) or 240.4(D)(1) limits 18 AWG copper protection to 7 Amperes.

Question 29

A 200A fuse is installed in a circuit with a fault current of 50,000A. The fuse must have an Interrupting Rating (AIC) of at least:

Accepted Answer

NEC 110.9 requires that equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals (must be >= Available Fault Current).

Question 30

The '25-foot Tap Rule' applies to taps supplying:

Accepted Answer

Actually, the 25-ft tap rule (240.21(B)(2)) allows the tap to supply a single circuit breaker or set of fuses, which can then feed a panelboard, switchboard, or other load. The rule focuses on the *termination* (single device).

Question 31

Arc-Fault Circuit Interrupter (AFCI) protection is required for 120V, single-phase, 15A and 20A branch circuits in which dwelling unit areas?

Accepted Answer

NEC 210.12(A) requires AFCI protection for almost all dwelling unit living areas: Kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas.

Question 32

For a transformer with a primary current of less than 2 Amps, the primary overcurrent protection is permitted to be set at not more than:

Accepted Answer

NEC Table 450.3(B) allows primary overcurrent protection for transformers with primary current less than 2A to be set at a maximum of 300%.

Question 33

When locating overcurrent devices for a feeder supplying a mobile home, the disconnect must be located within _____ of the mobile home.

Accepted Answer

NEC 550.32(A) requires the mobile home service equipment (disconnect/OCP) to be located in sight from and not more than 30 feet from the exterior wall of the mobile home.

Question 34

A circuit breaker with a slash rating of 120/240V is installed in a panelboard. This breaker is permitted to be used on:

Accepted Answer

Slash-rated breakers (120/240V) can only be used where the voltage to ground does not exceed the lower value (120V). A 120/240V single-phase system has 120V to ground.

Question 35

Where a feeder taps into a busway, the overcurrent device is not required at the tap if the tap length does not exceed:

Accepted Answer

Busway taps invoke the 25-foot tap rule (240.21(B)(2)) or specific busway rules in 368.17(C) which allow 50 feet for industrial, but generally 25 feet applies for standard feeder taps.

Question 36

Supplementary overcurrent protective devices must have an interrupting rating:

Accepted Answer

NEC 240.101(B) requires supplementary OCPDs to have an interrupting rating at least equal to the available fault current at the line terminals of the device.

Question 37

According to 240.4(C), for overcurrent devices rated over 800 Amperes, the conductor ampacity must be:

Accepted Answer

The 'Next Size Up' rule does NOT apply to devices over 800A. The conductor ampacity must be equal to or greater than the OCPD rating.

Question 38

In a supervised industrial installation, tap conductors up to 100 feet long are permitted if the ampacity is not less than _____ of the feeder rating.

Accepted Answer

NEC 240.21(B)(5) allows 100-foot taps in supervised industrial installations if the ampacity is at least 1/3 of the rating of the overcurrent device protecting the feeder.

Question 39

What is the maximum standard rating for a fuse?

Accepted Answer

NEC 240.6(A) lists the standard ampere ratings. The list goes up to 6000 Amperes.

Question 40

Overcurrent protection for an electric boiler (heating load) using resistance elements shall be sized at:

Accepted Answer

NEC 424.3(B) generally considers fixed electric space heating equipment a continuous load, requiring OCPDs and conductors to be sized at 125% of the total load.

Question 41

Which fuse class is not permitted to be used in new installations of 250V switches?

Accepted Answer

Class H fuses (renewable) are generally restricted or non-current limiting and typically allowed only for replacement. NEC requires rejection hardware (Class R) or current-limiting fuses for high fault currents. However, specifically, Class H are often 'renewable' cartridge fuses - the Code restricts them in 240.60(D) to replacement only? Wait, 240.60(D) says renewable fuses permitted only for replacement. Class H *can* be non-renewable, but 'Renewable' is the key word. Let's rephrase: 'Renewable fuses shall be used only for...'.

Question 42

Renewable link cartridge fuses (Class H) shall be permitted to be used only:

Accepted Answer

NEC 240.60(D) states that Class H renewable fuses shall be used only for replacement in existing installations where there is no evidence of overfusing or tampering.

Question 43

If a multiwire branch circuit supplies line-to-line loads, the overcurrent device must:

Accepted Answer

NEC 240.15(B)(1) requires that for multiwire branch circuits supplying line-to-line loads, the OCPD must disconnect all ungrounded conductors simultaneously (multipole breaker or handle tie).

Question 44

When protection is required for a transformer secondary (600V or less), and the secondary current is 100A, the maximum breaker rating is:

Accepted Answer

Table 450.3(B) allows secondary protection at 125% of the rated secondary current. 100A * 1.25 = 125A.

Question 45

Arc-Fault Circuit Interrupters (AFCIs) function by detecting:

Accepted Answer

AFCIs use electronic logic to recognize the specific waveform characteristics (signatures) of hazardous arcing faults.

Question 46

A breaker protecting a fire pump circuit must be set to:

Accepted Answer

NEC 695.4(B)(2) requires the fire pump OCPD to hold locked-rotor current indefinitely to ensure the pump runs until destruction if necessary during a fire.

Question 47

The maximum rating of a time-delay fuse for a motor branch circuit (3-phase) is:

Accepted Answer

Table 430.52 sets the max rating for Dual Element (Time-Delay) fuses at 175% of Full Load Current.

Overcurrent Protection Practice Questions

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