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Overcurrent Protection Practice Test 2 (NEC 240) — NEC 2023 Edition

Free NEC 2023 practice test on overcurrent protection. Covers conductor protection limits, standard fuse/breaker sizing, next-size-up rounding, branch-circuit device location, multiwire circuits, and special equipment protection (welders, transformers). Simple English, full explanations.

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The National Electrical Code (NFPA 70) is the benchmark standard for safe electrical installation in the US. It is updated every 3 years by the NFPA.

Study the edition adopted by your state. As of 2025, most states use the 2020 or 2023 NEC. These tests are written to the 2023 edition.

The most tested articles are 100 (Definitions), 200–250 (Wiring & Grounding), 300–314 (Wiring Methods), 400–411 (Conductors), 430 (Motors), and 700–701 (Emergency Systems).

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This Overcurrent Protection Practice Test 2 (NEC 240) — NEC 2023 Edition has 30 questions with detailed answers and NEC references. Take the full test, review every wrong answer, and look up the NEC article it references before moving on.

Tip: Use the NEC index (back of the book) — find answers by keyword, not by article number.

Topics covered: NEC articles 100, 210, 220, 240, 250, 300, 310, 430, and special occupancies.
Passing score: Aim for 80%+ on NEC code tests before exam day.
Questions: 30 multiple-choice, one correct answer each

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Overcurrent Protection Practice Test 2 (NEC 240) — NEC 2023 Edition — Question List

Question 1: What is the maximum standard overcurrent device size generally allowed to protect a 12 AWG copper conductor?

20 amperes
15 amperes
25 amperes
30 amperes

NEC Reference: NEC 240.4(D)(5)

Question 2: On a residential electric service, which conductors generally require overcurrent protection?

Ungrounded conductors
Grounded conductors
Neutral conductors
Equipment grounding conductors

NEC Reference: NEC 240.21

Question 3: A feeder supplies three single-phase 240 V motors with full-load currents of 20 A, 25 A and 30 A, each protected by time-delay fuses at 175%. What is the maximum feeder fuse size?

90 A
100 A
110 A
125 A

NEC Reference: NEC 430.62(A)

Question 4: How must a fuse or circuit breaker be connected in the circuit it protects?

In series with each ungrounded conductor
In parallel with the load
Between the neutral and ground
In the equipment grounding conductor

NEC Reference: NEC 240.15(A)

Question 5: Where must the branch-circuit overcurrent protective device generally be located in relation to the branch circuit it protects?

At the equipment ground location
At the end of the branch circuit
At the panelboard where it receives power
At the power outlet where power is consumed

NEC Reference: NEC 240.21

Question 6: A multiwire branch circuit that is protected by fuses (not a breaker with common internal trip) may supply only what type of loads?

Line-to-line loads
Line-to-neutral loads
Receptacle loads only
Service loads

NEC Reference: NEC 210.4(C)

Question 7: For a resistance welder, the maximum overcurrent device rating is generally what percent of the welder's rated primary current?

Not over 100%
Not over 200%
Not over 300%
Not over 400%

NEC Reference: NEC 630.32(A)

Question 8: A 600-volt-class autotransformer has a rated full-load input current of 50 A. What is the maximum standard overcurrent device permitted?

50 A
60 A
70 A
80 A

NEC Reference: NEC 450.4(A)

Question 9: A busway or conductor has an allowable ampacity of 48 amperes, which is not a standard overcurrent device rating. What is the maximum standard overcurrent device rating that may protect it?

50 amperes
40 amperes
60 amperes
70 amperes

NEC Reference: NEC 240.4(B)

Question 10: Per the NEC standard ampere ratings, what is generally the smallest standard fuse or breaker rating commonly used for a 120-volt residential branch circuit?

15 amperes
20 amperes
12 amperes
18 amperes

NEC Reference: NEC 240.6(A)

Question 11: Transformer secondary conductors are not over 10 ft long. Their ampacity must be not less than what?

One-third of the primary device rating
The primary conductor ampacity
125% of the transformer rating
The calculated load and the rating of the device they terminate in

NEC Reference: NEC 240.21(C)(2)

Question 12: Where are overcurrent devices NOT permitted to be located?

In a dwelling unit garage
In a basement
On an exterior wall
Over the steps of a stairway

NEC Reference: NEC 240.24(F)

Question 13: Which of the following is a standard ampere rating listed in NEC 240.6(A)?

40 amperes
37 amperes
42 amperes
33 amperes

NEC Reference: NEC 240.6(A)

Question 14: Supplementary overcurrent protection inside a luminaire or appliance may be used for what?

As the required branch-circuit protection
Additional protection of internal circuits, not in place of branch-circuit protection
To protect the feeder conductors
As the service overcurrent device

NEC Reference: NEC 240.10

Question 15: Per NEC 240.24(A), the center of the grip of a circuit breaker handle, in its highest position, must generally not be higher than what height above the floor?

6 feet 7 inches
5 feet
8 feet
10 feet

NEC Reference: NEC 240.24(A)

Question 16: Overcurrent devices must generally not be located in which of the following rooms?

Bathrooms
Garages
Basements
Kitchens

NEC Reference: NEC 240.24(E)

Question 17: Under the 10-foot tap rule, a feeder tap conductor not over 10 feet long generally must have an ampacity of not less than what?

The load served, and the terminating device rating
Any size at all
Half the feeder conductor size
A fixed 15 amperes always

NEC Reference: NEC 240.21(B)(1)

Question 18: Under the 25-foot tap rule, feeder tap conductors must have an ampacity of not less than what fraction of the ampacity of the feeder overcurrent device?

One-third
One-half
Two-thirds
The full ampacity

NEC Reference: NEC 240.21(B)(2)

Question 19: How must a series-rated combination of overcurrent devices be applied?

As a tested combination selected per its listing and field marked
Series ratings apply only to fuses
Any breakers of the same brand may be combined
The downstream breaker may be any interrupting rating

NEC Reference: NEC 240.86

Question 20: A Type S fuse and its adapter are designed to be what, once installed?

Non-removable and non-interchangeable
Removable and interchangeable with any fuse
Not evaluated for safety at all
Made only of glass

NEC Reference: NEC 240.54(B)

Question 21: Cartridge fuses and fuseholders of the 300-volt type may be used in which circuits?

Circuits not exceeding 300 V between conductors
Only dc circuits
Any circuit up to 600 V
Only 480 V three-phase circuits

NEC Reference: NEC 240.60(A)

Question 22: A circuit breaker will be used regularly as the switch for 277-volt fluorescent lighting. How must it be marked?

GFPE
SWD or HID
CL
HACR

NEC Reference: NEC 240.83(D)

Question 23: An overcurrent device's interrupting rating must be sufficient for what?

The available fault current at that point
The device's physical size
The room temperature
Nothing -- interrupting rating is not regulated

NEC Reference: NEC 110.9

Question 24: A lighting and appliance branch-circuit panelboard must generally be protected by an overcurrent device rated no more than what, relative to the panelboard?

Not more than the panelboard rating
150% of the panelboard rating
Any rating at all
Half the panelboard rating

NEC Reference: NEC 408.36

Question 25: Transformer secondary conductors installed under the 25-foot tap rule for transformers must be protected from physical damage and generally must terminate in what?

A single overcurrent device
No overcurrent device at all
Two separate services
A grounding rod only

NEC Reference: NEC 240.21(C)(6)

Question 26: Conductors with a 90 C insulation rating may use the 90 C ampacity column of NEC Table 310.16 for what specific purpose, even if terminations are only rated for 60 C or 75 C?

Applying ampacity adjustment/correction factors
Ignoring termination ratings completely
Nothing -- it is never permitted
Doubling the conductor's final rating

NEC Reference: NEC 110.14(C)

Question 27: Feeder conductors must generally be protected against overcurrent in accordance with their what?

Ampacity
Color
Length only
Manufacturer preference

NEC Reference: NEC 215.3

Question 28: When may an overcurrent device be placed in series with the grounded (neutral) conductor?

In any single-pole application
Whenever the circuit is 120 volts
When it opens all conductors of the circuit at the same time
Only on circuits over 50 amperes

NEC Reference: NEC 240.22

Question 29: May the "next size up" rule of 240.4(B) be used for transformer secondary conductors installed under 240.21(C)?

Yes, always
Only for three-phase transformers
Yes, but only below 100 A
No, 240.21(C) states that 240.4(B) does not apply

NEC Reference: NEC 240.21(C)

Question 30: Which of the following is a standard ampere rating listed in NEC 240.6(A), commonly used for larger residential feeders?

60 amperes
55 amperes
65 amperes
58 amperes

NEC Reference: NEC 240.6(A)

Key NEC References for This Test

Article	What It Covers
Ch. 1–4	General rules — apply to all installations
Ch. 5–7	Special rules — override Ch. 1–4 where they conflict
Ch. 8	Communications — completely independent of Ch. 1–7
Ch. 9	Tables — referenced by articles, not standalone enforceable rules
Art. 210.12	AFCI — major expansion in NEC 2026
Art. 230.67	Surge protection at service equipment — new NEC 2026

Study Tip: Learn the NEC index — use keywords, not article numbers. Practice finding 5 different topics in under 2 minutes each.

Overcurrent Protection Practice Test 2 (NEC 240) — NEC 2023 Edition

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Overcurrent Protection Practice Test 2 (NEC 240) — NEC 2023 Edition — Question List

Question 1: What is the maximum standard overcurrent device size generally allowed to protect a 12 AWG copper conductor?

Question 2: On a residential electric service, which conductors generally require overcurrent protection?

Question 3: A feeder supplies three single-phase 240 V motors with full-load currents of 20 A, 25 A and 30 A, each protected by time-delay fuses at 175%. What is the maximum feeder fuse size?

Question 4: How must a fuse or circuit breaker be connected in the circuit it protects?

Question 5: Where must the branch-circuit overcurrent protective device generally be located in relation to the branch circuit it protects?

Question 6: A multiwire branch circuit that is protected by fuses (not a breaker with common internal trip) may supply only what type of loads?

Question 7: For a resistance welder, the maximum overcurrent device rating is generally what percent of the welder's rated primary current?

Question 8: A 600-volt-class autotransformer has a rated full-load input current of 50 A. What is the maximum standard overcurrent device permitted?

Question 9: A busway or conductor has an allowable ampacity of 48 amperes, which is not a standard overcurrent device rating. What is the maximum standard overcurrent device rating that may protect it?

Question 10: Per the NEC standard ampere ratings, what is generally the smallest standard fuse or breaker rating commonly used for a 120-volt residential branch circuit?

Question 11: Transformer secondary conductors are not over 10 ft long. Their ampacity must be not less than what?

Question 12: Where are overcurrent devices NOT permitted to be located?

Question 13: Which of the following is a standard ampere rating listed in NEC 240.6(A)?

Question 14: Supplementary overcurrent protection inside a luminaire or appliance may be used for what?

Question 15: Per NEC 240.24(A), the center of the grip of a circuit breaker handle, in its highest position, must generally not be higher than what height above the floor?

Question 16: Overcurrent devices must generally not be located in which of the following rooms?

Question 17: Under the 10-foot tap rule, a feeder tap conductor not over 10 feet long generally must have an ampacity of not less than what?

Question 18: Under the 25-foot tap rule, feeder tap conductors must have an ampacity of not less than what fraction of the ampacity of the feeder overcurrent device?

Question 19: How must a series-rated combination of overcurrent devices be applied?

Question 20: A Type S fuse and its adapter are designed to be what, once installed?

Question 21: Cartridge fuses and fuseholders of the 300-volt type may be used in which circuits?

Question 22: A circuit breaker will be used regularly as the switch for 277-volt fluorescent lighting. How must it be marked?

Question 23: An overcurrent device's interrupting rating must be sufficient for what?

Question 24: A lighting and appliance branch-circuit panelboard must generally be protected by an overcurrent device rated no more than what, relative to the panelboard?

Question 25: Transformer secondary conductors installed under the 25-foot tap rule for transformers must be protected from physical damage and generally must terminate in what?

Question 26: Conductors with a 90 C insulation rating may use the 90 C ampacity column of NEC Table 310.16 for what specific purpose, even if terminations are only rated for 60 C or 75 C?

Question 27: Feeder conductors must generally be protected against overcurrent in accordance with their what?

Question 28: When may an overcurrent device be placed in series with the grounded (neutral) conductor?

Question 29: May the "next size up" rule of 240.4(B) be used for transformer secondary conductors installed under 240.21(C)?

Question 30: Which of the following is a standard ampere rating listed in NEC 240.6(A), commonly used for larger residential feeders?

Key NEC References for This Test

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