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

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  1. What is the allowable ampacity of a 3 AWG THWN copper conductor operating at 75°C?

    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.

  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?

    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.

  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?

    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.

  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?

    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.

  5. Which conductor is NOT considered a current-carrying conductor for the purposes of applying adjustment factors from Table 310.15(C)(1)?

    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.

  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?

    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.