E2 Conductor Ampacity and Derating 3 — Questions and Answers
Question 1: What is the maximum number of current-carrying conductors allowed in a conduit before a derating adjustment must be applied per NEC 310.15(C)(1)?
- 2
- 3 (Correct answer)
- 4
- 6
Correct answer: 3
NEC 310.15(C)(1) requires adjustment factors when more than 3 current-carrying conductors are in a raceway or cable.
Question 2: A 3/0 AWG copper THW conductor is installed in an ambient temperature of 40°C. Using NEC Table 310.15(B)(1), what correction factor applies to the 75°C column ampacity?
- 0.82 (Correct answer)
- 0.88
- 0.91
- 1.00
Correct answer: 0.82
For 75°C rated insulation at 40°C ambient, NEC Table 310.15(B)(1) specifies a correction factor of 0.82.
Question 3: Under NEC 310.15(A)(3), when can an engineer use an ampacity that differs from the NEC tables?
- Only when approved by the local AHJ in writing
- When using the engineering supervision method with a qualified engineer (Correct answer)
- Whenever the conductor is larger than 750 kcmil
- When conduit fill does not exceed 40%
Correct answer: When using the engineering supervision method with a qualified engineer
NEC 310.15(A)(3) permits ampacity to be calculated under engineering supervision using recognized calculation methods as an alternative to table values.
Question 4: A 10 AWG THHN conductor in a 90°C rated conduit system has a terminal rated at 60°C. What ampacity column governs the calculation?
- 90°C column — 40A
- 75°C column — 35A
- 60°C column — 30A (Correct answer)
- 90°C derated to 75°C — 33A
Correct answer: 60°C column — 30A
Per NEC 110.14(C)(1), the conductor ampacity cannot exceed the terminal's temperature rating; with a 60°C terminal, the 60°C column (30A for 10 AWG) governs.
Question 5: In a commercial building, a 4-wire, 3-phase circuit with a linear load uses a neutral conductor. How is the neutral counted for bundling adjustment purposes?
- It is always counted as a current-carrying conductor
- It is never counted regardless of load type
- It is counted only if it carries more than 50% of the phase current
- It is not counted because it only carries unbalanced current from linear loads (Correct answer)
Correct answer: It is not counted because it only carries unbalanced current from linear loads
NEC 310.15(E)(1) states that a neutral conductor on a 4-wire, 3-phase wye circuit carrying only unbalanced current from linear loads is not counted as a current-carrying conductor.
Question 6: What effect do nonlinear loads (such as computer equipment) have on the neutral conductor's current-carrying status for bundling purposes?
- Nonlinear loads reduce neutral current, so it is excluded from the count
- The neutral may carry harmonic currents exceeding phase current and must be counted (Correct answer)
- Nonlinear loads have no effect on how the neutral is counted
- The neutral is only counted when harmonic content exceeds 75%
Correct answer: The neutral may carry harmonic currents exceeding phase current and must be counted
NEC 310.15(E)(2) requires the neutral to be counted as current-carrying when it serves nonlinear loads that produce harmonic currents in the neutral.
Question 7: A commercial inspector finds 12 THHN conductors in a single conduit run in an ambient of 30°C. What combined adjustment and correction factor applies to the 90°C column ampacity?
- 0.50 × 1.00 = 0.50 (Correct answer)
- 0.50 × 0.91 = 0.455
- 0.70 × 1.00 = 0.70
- 0.60 × 1.00 = 0.60
Correct answer: 0.50 × 1.00 = 0.50
For 10-20 conductors, Table 310.15(C)(1) gives 0.50; at 30°C ambient, the 90°C correction factor is 1.00, so the combined factor is 0.50 × 1.00 = 0.50.
E2 Conductor Ampacity and Derating 3
What is the maximum number of current-carrying conductors allowed in a conduit before a derating adjustment must be applied per NEC 310.15(C)(1)?