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Electrician Practice Questions

10 original journeyman-level questions · 2023 NEC section numbers; check the edition your exam uses

1. A 4-inch square metal box with a plaster ring contains: two 12 AWG hots entering and spliced, one 12 AWG neutral entering and spliced, one 12 AWG equipment grounding conductor, one cable connector clamp inside the box, and one single-pole switch on a yoke. Using NEC 314.16(B), what is the total box fill volume in cubic inches?

  1. A. 13.5 in³
  2. B. 15.75 in³
  3. C. 18.0 in³
  4. D. 20.25 in³

2. Six THHN current-carrying conductors are installed in a single conduit in an ambient temperature of 86°F (30°C). The conductors are 10 AWG copper. Per NEC Table 310.16 and the adjustment factor for more than three current-carrying conductors, what is the maximum allowable ampacity for each conductor (using 75°C terminal rating)?

  1. A. 24 amperes
  2. B. 20 amperes
  3. C. 30 amperes
  4. D. 32 amperes

3. A wireman needs to pull four 12 AWG THHN conductors (each with a cross-sectional area of 0.0133 in²) into a trade size ½ in. EMT (internal cross-sectional area 0.304 in²). Does this installation comply with NEC Chapter 9 conduit fill rules?

  1. A. Yes — four conductors in ½ in. EMT is within the 40% maximum fill
  2. B. No — four 12 AWG THHN conductors exceed the 40% fill limit in ½ in. EMT
  3. C. Yes — the 53% single-conductor limit applies because THHN is compact-stranded
  4. D. No — four conductors require a 31% fill limit regardless of conduit size

4. A 20-ampere branch circuit supplies a commercial sign that draws 16 amperes continuously. Per NEC 210.19(A)(1), what is the minimum conductor ampacity required for this branch circuit, and what is the minimum wire size (copper, THHN, 75°C terminations)?

  1. A. 16A required; 14 AWG copper satisfies the load
  2. B. 20A required; 12 AWG copper is the minimum
  3. C. 25A required; 10 AWG copper is the minimum
  4. D. 20A required; 10 AWG copper is the minimum

5. A 400 A feeder OCPD supplies a panelboard. A tap is made from the feeder conductors inside the panel enclosure. The tap conductors run 22 ft in conduit to a 60 A breaker in a downstream panel. What is the minimum ampacity required for the tap conductors under the 25-foot tap rule?

  1. A. 133⅓ A — one-third of the 400 A feeder OCPD rating, before selecting a conductor
  2. B. 60 A — the tap conductors need only match the load-side breaker rating
  3. C. 40 A — one-tenth of the 400 A feeder OCPD, per the 10-foot tap rule
  4. D. 400 A — tap conductors must always equal the feeder OCPD rating

6. A new 400-ampere, 240/120V single-phase service uses 500 kcmil copper ungrounded conductors. Size the copper grounding electrode conductor to a metal underground water-pipe electrode using the full NEC Table 250.66, without an electrode-specific size cap. What is the minimum table size?

  1. A. 2 AWG copper
  2. B. 1/0 AWG copper
  3. C. 3/0 AWG copper
  4. D. 2/0 AWG copper

7. Type NM cable is run exposed in the unfinished basement of a one-family dwelling, stapled parallel along the side of a floor joist. How frequently must the cable be secured, and within what distance of each box?

  1. A. Every 4½ ft; within 12 in. of each box
  2. B. Every 6 ft; within 12 in. of each box
  3. C. Every 4½ ft; within 8 in. of each box
  4. D. Every 8 ft; within 24 in. of each box

8. Three 14/2 NM cables enter a plastic handy box. Each cable carries one 14 AWG hot, one 14 AWG neutral, and one 14 AWG bare EGC. The box has no internal clamps. What is the minimum box volume required per NEC 314.16(B)? (14 AWG = 2.0 in³ each)

  1. A. 12.0 in³
  2. B. 14.0 in³
  3. C. 16.0 in³
  4. D. 18.0 in³

9. A 12 AWG THHN copper conductor (90°C-rated insulation) is installed in a raceway in an engine room with a 50°C (122°F) ambient temperature. All terminations are rated 75°C. Using the Table 310.15(B)(1)(1) correction factor for 90°C conductors at 50°C ambient, what is the corrected ampacity? (Base: 30A at 90°C; correction factor for 90°C wire at 50°C = 0.82)

  1. A. 20 amperes — limited by terminal rating to the 75°C column before applying the correction factor
  2. B. 24.6 amperes — applying the 0.82 correction factor to the 90°C base ampacity of 30A
  3. C. 16.4 amperes — applying the 0.82 correction factor to the 75°C terminal-limited ampacity of 20A
  4. D. 25 amperes — limited by the 75°C column with no correction factor needed because 90°C conductor is used

10. A single 2 AWG THWN-2 conductor (area 0.1158 in² per Chapter 9 Table 5) is pulled as an equipment grounding conductor in 1 in. rigid metal conduit (total area 0.887 in² per Chapter 9 Table 4). What is the maximum fill percentage that applies, and is this installation within that limit?

  1. A. 40% applies; fill = 13.1% — compliant
  2. B. 31% applies; fill = 13.1% — compliant
  3. C. 53% applies; fill = 13.1% — compliant
  4. D. 53% applies; fill = 54.4% — non-compliant

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Answer key

1. B. 15.75 in³

Use the Table 314.16(B)(1) allowance for 12 AWG, 2.25 in³. Three conductors entering the box (two hots and one neutral) = 3 × 2.25 = 6.75 in³ [314.16(B)(1)]. All internal cable clamps together = one allowance, 2.25 in³ [314.16(B)(2)]. The switch yoke = two allowances, 4.50 in³ [314.16(B)(4)]. The equipment grounding conductor = one allowance, 2.25 in³ [314.16(B)(5)]. Total: 6.75 + 2.25 + 4.50 + 2.25 = 15.75 in³.

Reference: NEC 314.16(B)(1)-(5), Table 314.16(B)(1)

2. D. 32 amperes

THHN is a 90°C conductor, and 110.14(C) lets you apply adjustment factors to its 90°C ampacity: Table 310.16 lists 10 AWG copper at 40 A in the 90°C column. Table 310.15(C)(1) requires 80% for 4 to 6 current-carrying conductors: 40 A × 0.80 = 32 A. The result must not exceed the 75°C terminal ampacity of 35 A, and 32 A does not, so the adjusted ampacity is 32 A. This is not a breaker-size recommendation: for an ordinary circuit without a specific 240.4(E) or (G) permission, 240.4(D) still limits 10 AWG copper overcurrent protection to 30 A.

Reference: NEC Table 310.16, Table 310.15(C)(1), 110.14(C)

3. A. Yes — four conductors in ½ in. EMT is within the 40% maximum fill

Chapter 9 Table 1 allows 40% fill for more than two conductors. Table 5 gives 12 AWG THHN at 0.0133 in², and Table 4 gives ½ in. EMT a 40% area of 0.122 in². Four conductors use 4 × 0.0133 = 0.0532 in², which is under 0.122 in² (about 17.5% of the 0.304 in² total). Annex C shows up to nine 12 AWG THHN conductors fit in ½ in. EMT. This checks physical conduit fill only. Any required current-carrying-conductor ampacity adjustment is a separate check.

Reference: NEC Chapter 9 Table 1, Table 4, Table 5

4. B. 20A required; 12 AWG copper is the minimum

210.19(A)(1) requires branch-circuit conductors with an ampacity of at least the noncontinuous load plus 125% of the continuous load: 16 A × 1.25 = 20 A. Table 310.16 lists 12 AWG copper at 25 A in the 75°C column, which covers that, and 240.4(D) limits 12 AWG copper to a 20 A overcurrent device, which matches this circuit. 14 AWG copper is listed at 20 A at 75°C, but 240.4(D) limits it to 15 A protection, so it cannot go on a 20 A circuit. 12 AWG copper is the minimum.

Reference: NEC 210.19(A)(1), Table 310.16, 240.4(D)

5. A. 133⅓ A — one-third of the 400 A feeder OCPD rating, before selecting a conductor

240.21(B)(2) covers feeder taps not over 25 ft long. The tap conductors need an ampacity of at least one-third of the feeder overcurrent device rating, must end in a single circuit breaker or set of fuses that limits the load to the tap ampacity, and must be protected from physical damage (for example, in a raceway). 400 A ÷ 3 = 133⅓ A minimum. Do not round this minimum down to 133 A; select a conductor whose permitted ampacity meets or exceeds the exact threshold and satisfies the other tap conditions. The 10-foot rule in 240.21(B)(1) does not apply because this tap is 22 ft long.

Reference: NEC 240.21(B)(2)

6. B. 1/0 AWG copper

Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service-entrance conductor (or the equivalent area of parallel sets). For copper conductors over 350 kcmil through 600 kcmil, it requires a 1/0 AWG copper GEC, and 500 kcmil falls in that row. The other choices are rows for other sizes: 2 AWG is for over 3/0 through 350 kcmil, 2/0 is for over 600 through 1100 kcmil, and 3/0 is for over 1100 kcmil copper. Where the GEC runs only to a rod, pipe or plate electrode, 250.66(A) does not require a conductor larger than 6 AWG copper for that sole electrode connection; the water-pipe connection in this question needs the full table size.

Reference: NEC Table 250.66

7. A. Every 4½ ft; within 12 in. of each box

334.30 requires NM cable to be secured at intervals not exceeding 4½ ft and within 12 in. of every cabinet, box or fitting. Running along the side of the joist gives the cable a surface to be stapled to. If the cable crossed the joists instead, 334.15(C) would allow it to be secured directly to the lower edges of the joists only if it is at least two 6 AWG or three 8 AWG conductors; smaller cables have to run through bored holes in the joists or on running boards.

Reference: NEC 334.30, 334.15(C)

8. B. 14.0 in³

Each conductor that originates outside the box counts once [314.16(B)(1)]: 3 hots + 3 neutrals = 6 × 2.0 = 12.0 in³. In the 2023 NEC, up to four equipment grounding conductors together take one allowance, with a quarter allowance for each one beyond four [314.16(B)(5)], so the three bare EGCs add 2.0 in³. External cable connectors take no allowance. Total: 12.0 + 2.0 = 14.0 in³.

Reference: NEC 314.16(B)(1), 314.16(B)(5), Table 314.16(B)(1)

9. B. 24.6 amperes — applying the 0.82 correction factor to the 90°C base ampacity of 30A

110.14(C) allows conductors with a higher temperature rating than the terminations to use that higher rating for ampacity adjustment, correction or both. So start from the 90°C column: Table 310.16 lists 12 AWG copper at 30 A. Table 310.15(B)(1)(1) gives 0.82 for 90°C conductors at 46-50°C: 30 A × 0.82 = 24.6 A. The 75°C value for 12 AWG copper is 25 A, and 24.6 A is below it, so the corrected ampacity is 24.6 A. This ampacity calculation does not select the breaker. For an ordinary circuit without a specific 240.4(E) or (G) permission, 240.4(D) still limits 12 AWG copper overcurrent protection to 20 A.

Reference: NEC 110.14(C), Table 310.16, Table 310.15(B)(1)(1)

10. C. 53% applies; fill = 13.1% — compliant

Chapter 9 Table 1 allows 53% fill for a single conductor in a raceway. Fill = 0.1158 in² ÷ 0.887 in² = 0.131, or 13.1%, well under 53%. Two table traps: 0.1333 in² is the 2 AWG area for TW, THW, THHW and THW-2, not THWN-2, and 0.864 in² is the area of 1 in. EMT, not RMC.

Reference: NEC Chapter 9 Table 1, Table 4, Table 5

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