⚡ Voltage Drop Calculator

Voltage Drop Calculator

Calculate voltage drop and wire sizing

Circuit Details

One-way length of wire run
NEC recommends 3% max

Results

Voltage Drop
0 volts
Percentage Drop
0%
Voltage at Load
0V
Power Loss
0W
Wire Resistance
0Ω
Min Wire Size
–
Current Wire OK?
–

💡 Voltage Drop Guidelines

  • NEC Recommendation: 3% max for branch circuits, 5% total
  • Critical Loads: 2% or less recommended
  • Motors/HVAC: 5% acceptable during startup
  • Long Runs: Upsize wire to compensate for distance
  • Aluminum: 1.6× the resistance of copper
  • Temperature: Higher temps increase resistance
  • Conduit Fill: Affects ampacity, not voltage drop
    How to Calculate Voltage Drop | Formula, Example & Wiring Tips

    How to Calculate Voltage Drop: Formula, Definition, and Examples

    Voltage drop is the decrease in electrical potential as current flows through a wire or circuit. It occurs because conductors have resistance, which causes energy loss in the form of heat. Understanding voltage drop is essential for electrical design, wiring efficiency, and code compliance.

    🔹 Basic Voltage Drop Formula

    The most common formula derived from Ohm’s Law is:

    Vdrop = I × R
    • Vdrop = Voltage drop (volts)
    • I = Current (amps)
    • R = Resistance of the conductor (ohms)

    This formula applies to simple DC circuits or short AC runs where inductance and reactance are minimal.

    🔹 Extended Voltage Drop Formula for Conductors

    For longer wire runs or AC systems, use the full resistivity formula:

    Vdrop = (2 × K × I × L) / CM
    • K = Resistivity constant (12.9 for copper, 21.2 for aluminum)
    • I = Current (amps)
    • L = One-way length of the conductor (feet)
    • CM = Circular mil area of the wire

    🔹 Voltage Drop Example

    If a 10-amp current flows through 100 ft of 12 AWG copper wire:

    Vdrop = (2 × 12.9 × 10 × 100) / 6530 ≈ 3.95 V

    The total voltage drop is approximately 3.95 volts.

    🔹 How to Minimize Voltage Drop

    • Use larger-diameter wires (lower AWG numbers).
    • Keep wire lengths short whenever possible.
    • Use copper conductors for better conductivity.
    • Follow NEC recommendations — 3% voltage drop for branch circuits, 5% max overall.

    ⚡ Key Takeaways

    • Formula: Vdrop = I × R or (2 × K × I × L) / CM
    • Excessive voltage drop can cause equipment malfunction or energy loss.
    • Always check voltage drop for long-run electrical circuits to maintain efficiency and safety.