For a #8 copper conductor (16,510 cmil) carrying 39 A over 215 feet one-way at 120 V, what is the voltage drop?

• A. 2.3 Volts
• B. 5.1 Volts
• C. 16.0 Volts
• D. 10.5 Volts

Answer: (D)

Voltage drop is found from Ohm’s law: V_drop = I × R_total, with R_total the resistance of the complete path (out and back). For copper #8, the resistance is about 0.628 ohms per 1,000 ft. The run is 215 ft one-way, so the round-trip length is 430 ft. The circuit resistance is (430/1000) × 0.628 ≈ 0.270 Ω. Multiply by the current: 39 A × 0.270 Ω ≈ 10.5 V. So the voltage drop is about 10.5 volts.