Journeyman Electrician’s Exam Journeyman Electrician's Conductor Sizing and Ampacity 3 — Questions and Answers
Question 1: Nine current-carrying conductors are installed in a single conduit. What derating factor applies per NEC 310.15(C)(1)?
- 70% (Correct answer)
- 50%
- 80%
- 60%
Correct answer: 70%
For 7–9 current-carrying conductors in a raceway, a 70% adjustment factor applies per NEC 310.15(C)(1).
Question 2: What is the minimum conductor size for a 20A branch circuit supplying continuous loads using 60°C-rated terminations?
- 14 AWG
- 12 AWG (Correct answer)
- 10 AWG
- 8 AWG
Correct answer: 12 AWG
12 AWG copper at 60°C is rated 20A, which satisfies the 20A continuous circuit requirement per NEC 310.14.
Question 3: An ambient temperature of 50°C is encountered in a boiler room. If the base table temperature is 30°C, which correction factor must be applied to conductor ampacity?
- Multiply by 1.0
- Multiply by 0.82
- Multiply by 0.71 (Correct answer)
- Multiply by 0.58
Correct answer: Multiply by 0.71
For 75°C-rated conductors in a 50°C ambient, NEC Table 310.15(B)(2)(a) requires a 0.71 correction factor.
Question 4: Which of the following conductors is NOT counted as a current-carrying conductor for bundling derating purposes?
- Hot leg of a single-phase circuit
- Neutral carrying unbalanced current only (Correct answer)
- Phase conductor of a 3-phase circuit
- Grounded conductor carrying harmonic currents
Correct answer: Neutral carrying unbalanced current only
A neutral that carries only unbalanced current in a 3-wire circuit is not counted as a current-carrying conductor per NEC 310.15(E).
Question 5: What is the ampacity of 1/0 AWG copper THHN at 90°C per NEC Table 310.15(B)(16)?
- 125A
- 150A
- 170A (Correct answer)
- 195A
Correct answer: 170A
1/0 AWG copper at 90°C has an ampacity of 170A per NEC Table 310.15(B)(16).
Question 6: A 4/0 AWG aluminum conductor is used in underground conduit. Which insulation type is required for a wet location?
- THHN
- TW or THW
- RHH only
- XHHW or THWN (Correct answer)
Correct answer: XHHW or THWN
XHHW or THWN insulation is rated for wet locations; THHN is dry-location only and cannot be used in underground conduit.
Question 7: A motor branch circuit requires conductors sized at least 125% of the motor full-load current (FLC). If FLC = 28A, what is the minimum conductor ampacity?
- 28A
- 32A
- 35A (Correct answer)
- 40A
Correct answer: 35A
125% of 28A = 35A, so conductors must have at least 35A ampacity per NEC 430.22.
Journeyman Electrician’s Exam Journeyman Electrician's Conductor Sizing and Ampacity 3
Nine current-carrying conductors are installed in a single conduit.
What derating factor applies per NEC 310.15(C)(1)?