You're installing a 100-amp subpanel in a detached garage 80 feet from the main panel. Using 75°C copper conductors in underground conduit, what minimum wire size is required for the feeder conductors?
Correct Answer
C) #1/0 AWG
For 100-amp feeder, #3 AWG copper is normally adequate, but with 80-foot run, voltage drop calculations require upsizing to #1/0 AWG to maintain voltage within acceptable limits.
Why This Is the Correct Answer
#1/0 AWG copper at 75°C is rated for 150 amps, which provides the necessary capacity for a 100-amp feeder. More importantly, an 80-foot run causes significant voltage drop; the NEC recommends keeping voltage drop to 3% or less for feeders. At 100 amps over 80 feet, a smaller conductor like #2 or #1 AWG would exceed that limit, requiring an upsize to #1/0 AWG to maintain acceptable voltage levels.
Why the Other Options Are Wrong
Option A: #1 AWG
#1 AWG copper at 75°C is rated for 130 amps, which meets ampacity requirements, but the 80-foot run causes too much voltage drop with this gauge. It is close but insufficient once voltage drop is calculated for this distance.
Option B: #2/0 AWG
#2/0 AWG is oversized for this application. While it would work electrically, it is not the minimum required size — the question asks for the minimum. #1/0 AWG satisfies both ampacity and voltage drop requirements without over-engineering.
Option D: #2 AWG
#2 AWG copper at 75°C is rated for 95 amps, which is below the 100-amp breaker rating and fails to meet ampacity requirements. It also produces unacceptable voltage drop over 80 feet at 100 amps.
Memory Technique
For long runs, think 'distance demands more wire.' When distance doubles the normal run, bump up the wire size. For a 100A feeder: normal would be #4 AWG, but 80 feet pushes you up to #1/0 AWG.
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