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Practice Questions: Voltage Drop

Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.

NEC NEC 210.19(A)(1) - Voltage Drop FormulaQuestion 1

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

NEC NEC 210.19(A)(1) Informational NoteQuestion 2

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).

NEC NEC 210.19(A) Informational Note, 215.2(A) Informational NoteQuestion 3

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.

  • A2.89 volts (2.41%)
  • B3.45 volts (2.88%)
  • C4.12 volts (3.43%)
  • D5.67 volts (4.73%)
Show explanation

Per NEC 210.19(A), use single-phase formula: VD = (2 × K × I × D) / CM. With copper K=12.9, 12 AWG has CM=6530. VD = (2 × 12.9 × 20 × 150) / 6530 = (77400) / 6530 = 3.45 volts. Percent VD = (3.45 / 120) × 100 = 2.88%. This is within the 3% branch circuit recommendation.

NEC NEC 210.19(A) Informational NoteQuestion 4

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?

  • A4.87 volts (2.03%)
  • B6.53 volts (2.72%)
  • C8.19 volts (3.41%)
  • D10.24 volts (4.27%)
Show explanation

For aluminum, K=21.2. Single-phase VD = (2 × K × I × D) / CM. 10 AWG = 10380 CM. VD = (2 × 21.2 × 30 × 200) / 10380 = (254400) / 10380 = 8.19 volts. Percent VD = (8.19 / 240) × 100 = 3.41%. This slightly exceeds the 3% recommendation, suggesting larger wire might be preferred.

NEC NEC 215.2(A) Informational NoteQuestion 5

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%)
  • B2.37 volts (1.98%)
  • C3.56 volts (2.97%)
  • D4.73 volts (3.94%)
Show explanation

Single-phase formula: VD = (2 × K × I × D) / CM. K=12.9 (copper), 14 AWG = 4110 CM. VD = (2 × 12.9 × 15 × 100) / 4110 = (38700) / 4110 = 1.49 volts. Percent VD = (1.49 / 120) × 100 = 1.24%. Well within acceptable limits.

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