E2 Conductor Ampacity and Derating 2 — Questions and Answers
Question 1: When more than 3 current-carrying conductors are installed in a conduit, NEC Table 310.15(C)(1) requires a derating factor. What is the correction factor when 7 to 9 conductors are bundled together?
- 0.50
- 0.60
- 0.70 (Correct answer)
- 0.80
Correct answer: 0.70
NEC Table 310.15(C)(1) specifies a 0.70 (70%) adjustment factor when 7 to 9 current-carrying conductors are installed in a raceway or cable.
Question 2: A 4 AWG THHN copper conductor has a 75°C terminal rating. Using NEC Table 310.16, what is the ampacity used for circuit calculations?
- 70A
- 85A (Correct answer)
- 95A
- 100A
Correct answer: 85A
NEC Table 310.16 lists 4 AWG THHN copper at 95A for 90°C, but the 75°C column value of 85A governs when terminals are rated 75°C.
Question 3: Which NEC article provides the general rules for conductor ampacity calculations including adjustment and correction factors?
- Article 240
- Article 300
- Article 310 (Correct answer)
- Article 230
Correct answer: Article 310
NEC Article 310 covers conductors for general wiring, including ampacity tables and the application of adjustment and correction factors.
Question 4: Six current-carrying 12 AWG THHN conductors are installed in a single conduit at an ambient temperature of 30°C. What is the derated ampacity per conductor?
- 12A
- 15A (Correct answer)
- 20A
- 24A
Correct answer: 15A
Six conductors require a 0.80 adjustment; 12 AWG THHN at 90°C is 30A, so 30A × 0.80 = 24A, but limited to 20A × 0.80 = 16A at 75°C terminal rating.
Question 5: Under NEC 310.15(E), what is the minimum allowable ampacity for a conductor used as an equipment grounding conductor?
- It must match the phase conductor ampacity
- It is not required to be derated for bundling (Correct answer)
- It must carry full fault current without derating
- It follows the same derating rules as phase conductors
Correct answer: It is not required to be derated for bundling
NEC 310.15(E) states that equipment grounding conductors and neutral conductors that are not current-carrying are not counted for bundling adjustment purposes.
Question 6: A 500 kcmil aluminum XHHW-2 conductor is installed in a raceway in an ambient temperature of 50°C. What correction factor from NEC Table 310.15(B)(1) applies?
- 0.58
- 0.71 (Correct answer)
- 0.82
- 0.91
Correct answer: 0.71
For a 90°C rated conductor at 50°C ambient, NEC Table 310.15(B)(1) gives a correction factor of 0.71.
Question 7: Two sets of 350 kcmil THHN copper conductors are installed in parallel. Each set is in its own conduit. How is the ampacity of each conductor set determined?
- Ampacity is halved for each conductor
- Each conductor carries its full table ampacity without derating for the parallel set (Correct answer)
- Both sets are treated as one bundle requiring derating
- Ampacity is limited to the smaller conductor size
Correct answer: Each conductor carries its full table ampacity without derating for the parallel set
Under NEC 310.10(H), conductors installed in parallel in separate raceways are each evaluated independently; each conduit with 3 or fewer conductors uses the full table ampacity.
E2 Conductor Ampacity and Derating 2
When more than 3 current-carrying conductors are installed in a conduit, NEC Table 310.15(C)(1) requires a derating factor.
What is the correction factor when 7 to 9 conductors are bundled together?