Practice Questions: Voltage Drop
Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.
What is the approximate voltage drop for a single-phase circuit with 40 amperes of load, 150 feet of copper conductors (K = 12.9 ohms·cmil/ft), using 6 AWG wire (26,240 cmil)?
- A3.7 volts
- B5.9 volts
- C7.4 volts
- D11.8 volts
Show explanation
VD = 2 × K × I × D / CM = 2 × 12.9 × 40 × 150 / 26,240 = 154,800 / 26,240 = 5.9V
For a 240-volt single-phase circuit, what is the maximum recommended voltage drop per NEC Informational Note for branch circuits and feeders combined?
- A2%
- B3%
- C5%
- D10%
Show explanation
NEC 210.19(A)(1) Informational Note recommends max 5% VD for branch circuits and feeders combined (3% branch, 2% feeder ideal).
A single-phase 120V circuit supplies a 20-amp load located 150 feet away. The circuit uses 12 AWG copper wire. Calculate the voltage drop.
- A3.45 volts (2.88%) — within 3% limit
- B7.42 volts (6.18%) — exceeds 3% limit
- C11.85 volts (9.88%) — exceeds 3% limit
- D14.82 volts (12.35%) — exceeds 3% limit
Show explanation
Single-phase VD = 2 × K × I × L / CM. K = 12.9 (copper), 12 AWG Cu CM = 6,530, I = 20A, L = 150 ft. VD = 2 × 12.9 × 20 × 150 / 6,530 = 77,400 / 6,530 = 11.85 volts. %VD = 11.85 / 120 × 100 = 9.88%. This far exceeds the NEC 210.19(A) Informational Note 3% recommendation. A larger conductor or shorter run would be required to meet the guideline.
A 240V single-phase branch circuit serves a 30-amp water heater 200 feet away using 10 AWG aluminum conductor. What is the voltage drop?
- A8.19 volts (3.41%) — exceeds 3% limit
- B12.28 volts (5.12%) — exceeds 3% limit
- C24.51 volts (10.21%) — exceeds 3% limit
- D4.87 volts (2.03%) — within 3% limit
Show explanation
Single-phase VD = 2 × K × I × L / CM. K = 21.2 (aluminum), 10 AWG Al CM = 10,380, I = 30A, L = 200 ft. VD = 2 × 21.2 × 30 × 200 / 10,380 = 254,400 / 10,380 = 24.51 volts. %VD = 24.51 / 240 × 100 = 10.21%. This far exceeds the NEC 210.19(A) Informational Note 3% recommendation. A much larger aluminum conductor or shorter run is needed.
A 120V single-phase feeder serves a 15-amp load at 100 feet using 14 AWG copper. Calculate the voltage drop.
- A1.49 volts (1.24%) — within 3% limit
- B4.71 volts (3.93%) — exceeds 3% limit
- C9.42 volts (7.85%) — exceeds 3% limit
- D12.55 volts (10.46%) — exceeds 3% limit
Show explanation
Single-phase VD = 2 × K × I × L / CM. K = 12.9 (copper), 14 AWG Cu CM = 4,110, I = 15A, L = 100 ft. VD = 2 × 12.9 × 15 × 100 / 4,110 = 38,700 / 4,110 = 9.42 volts. %VD = 9.42 / 120 × 100 = 7.85%. This exceeds the NEC 215.2(A) Informational Note 3% recommendation. A larger conductor (e.g., 10 AWG) would be needed to stay within 3%.
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