Voltage drop calculations are crucial for ensuring electrical systems operate efficiently and safely. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% for the total combined voltage drop of feeders and branch circuits. These questions frequently appear on electrical licensing exams, testing your ability to determine appropriate conductor sizes to maintain proper voltage levels.

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Example Voltage Drop Questions on NEC Electrical Exams

If a heat pump rated at 40 A, 240 V, 1-phase was installed 230 ft from its supply, the minimum size of XHHW conductor with k-factor of 12.9 that would not allow more than a 3 percent voltage drop is __________.
A 480V, 3-phase circuit supplies a 50A load located 300 feet from the panel. What conductor size with a k-factor of 12.9 is required to maintain a voltage drop of no more than 3%?
Determine the voltage drop percentage for a 100A, 208V single-phase load supplied by #2 AWG THHN copper conductors with a k-factor of 12.9 at a distance of 150 feet from the source.

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How to Identify a Voltage Drop Question on NEC Electrical Exams

Key phrases to look out for:

  • "Voltage drop"
  • "Percent drop"
  • "Distance from supply/source"
  • "Circuit length"
  • References to "3%" or "5%"
  • "Maximum allowable voltage drop"
  • “k-factor”

When you spot these elements:

  • Confirm it's a voltage drop calculation (not ampacity)
  • Refer to NEC Chapter 9, Table 8 for circular mils on conductors
  • Check if it's single-phase (2 wire) or three-phase (3 wire)

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Voltage Drop Articles: NEC Chapter 9, Table 8

To correctly apply voltage drop calculations, focus on these main elements:

  1. Voltage Drop Formula:
    • Single-phase: CM = (2 × K × I × D) / VD
    • Three-phase: CM = (√3 × K × I × D) / VD
  2. Where:
    • CM = Circular mil area required
    • K = Conductor material constant (12.9 for copper, 21.2 for aluminum)
    • I = Current in amperes
    • D = Distance in feet (one-way)
    • VD = Allowable voltage drop (voltage × percentage)
  3. Key Constants:
    • Copper (Cu) K = 12.9
    • Aluminum (Al) K = 21.2
  4. Common Voltage Drop Limits:
    • Branch circuits: 3% maximum
    • Feeders: 2% maximum
    • Total (feeder + branch): 5% maximum

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Walkthrough for a NEC Electrical Exam Voltage Drop Question

Question:

If a heat pump rated at 40 A, 240 V, 1-phase was installed 230 ft from its supply, the minimum size of XHHW conductor with k-factor of 12.9 that would not allow more than a 3 percent voltage drop is __________.

Step 1: Identify Known Values

  • Current (I) = 40 A
  • Voltage (V) = 240 V
  • Distance (D) = 70 m = 229.66 ft
  • Maximum voltage drop = 3%
  • Single-phase circuit
  • Material constant (K) = 12.9 (copper)

Step 2: Calculate Allowable Voltage Drop

  • VD = 240 V × 0.03 = 7.2 V

Step 3: Apply Voltage Drop Formula

  • CM = (2 × K × I × D) / VD
  • CM = (2 × 12.9 × 40 × 230) / 7.2
  • CM = 32,967

Step 4: Select Conductor Size

  • Using Chapter 9, Table 8
  • Find next size up from 32,967 CM
  • #4 AWG = 41,740 CM
  • Answer: #4 AWG

Additional Tips:

  1. Watch for single-phase vs. three-phase circuits
  2. Don't forget to multiply voltage by percentage to get VD
  3. Round UP to next standard conductor size
  4. Be careful with units and conversions
  5. Remember that most questions assume copper unless specified

Common Pitfalls:

  1. Using wrong K factor (aluminum vs. copper)
  2. Forgetting to convert meters to feet
  3. Using wrong formula (single-phase vs. three-phase)
  4. Not rounding up to next available conductor size
  5. Calculating total voltage drop instead of one-way distance

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