Free E2 Conductor Ampacity and Derating Questions and Answers 1 — Questions and Answers
Question 1: What is the allowable ampacity of a 3 AWG THWN copper conductor operating at 75°C?
- 85 A
- 115 A
- 100 A (Correct answer)
- 110 A
Correct answer: 100 A
Using NEC Table 310.16, find the row for '3' under the AWG size column. Then, move across to the 'Copper' section and down the column for 75°C (which covers THWN insulation). The value listed is 100 amperes.
Question 2: If ten 12 AWG THHN copper current-carrying conductors are installed in a single conduit, what adjustment factor must be applied to their ampacity?
- 80%
- 70%
- 60%
- 50% (Correct answer)
Correct answer: 50%
According to NEC Table 310.15(C)(1), when the number of current-carrying conductors is between 10 and 20, an adjustment factor of 50% must be applied. This is to account for the cumulative heating effect of multiple conductors in a confined space.
Question 3: A 2/0 AWG THWN copper conductor is to be installed in an area with an ambient temperature of 110°F (43°C). What is its approximate allowable ampacity?
- 175 A
- 144 A (Correct answer)
- 158 A
- 123 A
Correct answer: 144 A
From Table 310.16, a 2/0 AWG THWN copper conductor has a base ampacity of 175A at 75°C. Using Table 310.15(B)(1), the correction factor for 41-45°C (which includes 43°C) in the 75°C column is 0.82. The adjusted ampacity is 175A * 0.82 = 143.5A.
Question 4: Four 4 AWG THHN copper current-carrying conductors are run in a conduit on a rooftop where the ambient temperature is 125°F (52°C). What is the allowable ampacity of each conductor?
- 76.0 A
- 66.5 A
- 62.3 A (Correct answer)
- 54.4 A
Correct answer: 62.3 A
First, find the base ampacity of 4 AWG THHN (90°C column) from Table 310.16, which is 95A. Next, find the temperature correction factor from Table 310.15(B)(1) for 51-55°C (90°C column), which is 0.82. Then, find the adjustment factor for 4 conductors from Table 310.15(C)(1), which is 80% (0.80). The final ampacity is 95A * 0.82 * 0.80 = 62.3A.
Question 5: Which conductor is NOT considered a current-carrying conductor for the purposes of applying adjustment factors from Table 310.15(C)(1)?
- An ungrounded 'hot' conductor in a single-phase circuit.
- A neutral conductor of a balanced 3-phase, 4-wire, wye-connected system. (Correct answer)
- An equipment grounding conductor.
- The neutral conductor of a 120/240V single-phase system.
Correct answer: A neutral conductor of a balanced 3-phase, 4-wire, wye-connected system.
According to NEC 310.15(E)(1), a neutral conductor that carries only the unbalanced current from other conductors of the same circuit is not required to be counted as a current-carrying conductor. This is common in balanced 3-phase, 4-wire, wye-connected systems.
Question 6: A commercial air conditioning unit has a nameplate rating of 38A. The circuit will be run with THWN copper conductors. What is the minimum size conductor required?
- 10 AWG
- 8 AWG (Correct answer)
- 6 AWG
- 4 AWG
Correct answer: 8 AWG
For continuous loads like an AC unit, the conductors must be sized to carry 125% of the load (38A * 1.25 = 47.5A). Using Table 310.16, we look in the 75°C column for a conductor with an ampacity of at least 47.5A. An 8 AWG conductor is rated for 50A, making it the correct minimum size.
Free E2 Conductor Ampacity and Derating Questions and Answers 1
What is the allowable ampacity of a 3 AWG THWN copper conductor operating at 75°C?