Practice Test Geeks(E2) E2 Commercial Electrical Inspector Practice Test
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Conductor Ampacity and Derating Flashcards

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  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?

    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.

  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?

    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.

  3. Which NEC article provides the general rules for conductor ampacity calculations including adjustment and correction factors?

    Answer: Article 310

    NEC Article 310 covers conductors for general wiring, including ampacity tables and the application of adjustment and correction factors.

  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?

    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.

  5. Under NEC 310.15(E), what is the minimum allowable ampacity for a conductor used as an equipment grounding conductor?

    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.

  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?

    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.

  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?

    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.