Sizing conductors is a core skill for the journeyman exam. You need to read the ampacity tables, apply derating for more than three current-carrying conductors, and correct for temperature.
Every question here is based on the 2023 NEC. Some states are on a different edition, so check which one your exam uses.
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Question 1 of 15
Using NEC Table 310.16 (75°C column), what is the ampacity of 1/0 AWG copper?
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Answer: A. 150 A
In Table 310.16, 1/0 AWG copper is 150 A in the 75°C column.
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Question 2 of 15
In many exam problems, after applying adjustment/correction factors, you must still verify the final ampacity against:
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Answer: A. Termination temperature limitations (often 75°C or 60°C) per 110.14(C)
Even if corrected/adjusted ampacity is based on 90°C insulation, the final usable ampacity can be limited by termination temperature ratings per 110.14(C).
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Question 3 of 15
The insulation marking “-2” (as in XHHW-2 or THWN-2) MOST commonly indicates:
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Answer: A. 90°C rating (commonly wet and dry where applicable)
The “-2” designation commonly indicates a 90°C temperature rating (often for wet and dry locations depending on the specific insulation type).
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Question 4 of 15
A feeder requires at least 125 A ampacity. Using Table 310.16 (75°C column), what is the MINIMUM copper conductor size that meets this?
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Answer: A. 1 AWG Cu
At 75°C: 2 AWG Cu = 115 A (too small), 1 AWG Cu = 130 A (meets). So 1 AWG is the minimum.
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Question 5 of 15
Using Table 310.15(C)(1), what adjustment factor is typically applied when there are 4-6 current-carrying conductors in a raceway or cable?
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Answer: A. 80%
Table 310.15(C)(1) commonly applies an 80% adjustment factor for 4-6 current-carrying conductors.
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Question 6 of 15
Which thermoplastic nylon conductor insulation marking typically indicates a conductor is rated 90°C in both wet and dry locations?
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Answer: A. THWN-2
THWN-2 conductors are rated 90°C in both wet and dry locations. THHN is 90°C for dry locations, but THHN by itself is not the wet-location 90°C marking. TW is a lower-temperature wet-location insulation, and XHHW is not the same as XHHW-2.
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Question 7 of 15
You have four (4) current-carrying THHN copper conductors in a raceway longer than 24 in., in a dry location with a 40°C (104°F) ambient. Using NEC tables, what is the adjusted ampacity of 8 AWG copper, to the nearest whole ampere? (Assume 75°C terminations for final use.)
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Answer: A. 40 A
Start with 8 AWG copper THHN from the 90°C column: 55 A. Apply the 4-6 current-carrying conductor adjustment factor: 55 A × 0.80 = 44 A. Then apply the 40°C ambient correction factor for 90°C conductors: 44 A × 0.91 = 40.04 A, which rounds to about 40 A. This does not exceed the 75°C terminal limit for 8 AWG copper, which is 50 A.
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Question 8 of 15
You have THHN copper conductors in a raceway with 6 current-carrying conductors. The base ampacity (90°C column) is 40 A. Ignoring temperature correction and termination limits, what is the adjusted ampacity using an 80% adjustment factor?
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Answer: A. 32 A
80% = 0.80, so 40 × 0.80 = 32 A.
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Question 9 of 15
Which wiring method is Mineral-Insulated, Metal-Sheathed cable?
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Answer: A. Type MI
Type MI is Mineral-Insulated, Metal-Sheathed Cable, covered under Article 332.
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Question 10 of 15
A feeder requires at least 150 A ampacity. Using Table 310.16 (75°C column), what is the MINIMUM copper conductor size that meets this?
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Answer: A. 1/0 AWG Cu
At 75°C: 1/0 Cu = 150 A. 1 AWG is 130 A (too small). Therefore 1/0 is minimum.
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Question 11 of 15
Which of the following is MOST likely to increase conductor heating and require ampacity adjustment?
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Answer: A. More than three current-carrying conductors in the same raceway
As the number of current-carrying conductors increases, heat buildup can require ampacity adjustment using Table 310.15(C)(1).
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Question 12 of 15
Which term BEST describes the outer protective covering on a cable assembly (not the insulation on individual conductors)?
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Answer: A. Jacket
A cable jacket is the outer protective covering of a cable assembly, distinct from the insulation on each conductor inside.
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Question 13 of 15
A feeder requires at least 200 A ampacity. Using Table 310.16 (75°C column), what is the MINIMUM copper conductor size that meets this?
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Answer: A. 3/0 AWG Cu
At 75°C: 2/0 Cu = 175 A (too small), 3/0 Cu = 200 A (meets). So 3/0 is minimum.
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Question 14 of 15
A raceway contains 8 current-carrying conductors. What adjustment factor typically applies from Table 310.15(C)(1)?
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Answer: A. 70%
Eight current-carrying conductors falls in the 7-9 range, which commonly uses a 70% adjustment factor.
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Question 15 of 15
Which wiring method is commonly known as “Romex”?
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Answer: A. Nonmetallic-Sheathed Cable (Type NM)
Type NM is Nonmetallic-Sheathed Cable, commonly called “Romex” in the field.
Want more? Electrician Practice has 1,100+ practice questions with explanations.
Most ampacity mistakes come from using the wrong temperature column. Check the terminal rating before you pick a column.
Derating and temperature correction stack, so apply both when both apply.
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