Sub-panel feeder sizing must satisfy two independent requirements: minimum ampacity per NEC 215.2 (which references the conductor ampacity tables in Article 310) and the NEC 2% feeder voltage drop recommendation. In practice, voltage drop often controls conductor selection for feeders to detached garages, basement sub-panels, or panels in outbuildings. The feeder conductor must also be sized for the calculated load, not necessarily the sub-panel breaker rating — though sizing for the sub-panel rating is common to allow for future load growth. For feeders over 100 feet, always check both ampacity and voltage drop before selecting wire size.
100A sub-panel feeder, 75 ft, aluminum, 240V: Ampacity — 2/0 AWG aluminum (90A at 75°C) or 3/0 AWG aluminum (120A at 75°C) for 100A rating. Voltage drop at 100A, 75 ft, 3/0 AWG aluminum: VD = 2 × 21.2 × 100 × 75 / 167,800 ≈ 1.89V (0.79%). Use 3/0 AWG aluminum with 100A disconnect.
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