Voltage Drop Calculator

Voltage Drop
1.6 V
–

What Is a Voltage Drop Calculator?

Voltage drop is the reduction in voltage that occurs as electrical current travels through wire resistance over a distance. This calculator estimates voltage drop based on current, wire length, gauge, and system voltage — important for ensuring circuits deliver adequate voltage at the far end.

How to Use the Voltage Drop Calculator

  1. Enter the circuit's current draw.
  2. Enter the one-way wire length.
  3. Select the wire gauge (AWG).
  4. Enter the system voltage.

Voltage Drop Formula

Voltage Drop
VD = 2 × K × I × L / 1000 (using resistance per 1000 ft, K = wire resistance factor)
I = current in amps
L = one-way length in feet
K = resistance per 1000 ft for the gauge (ohms)

Worked Example

Example: 20A, 100 ft, 12 AWG, 120V System

12 AWG resistance: approximately 1.98 ohms per 1000 ft

Voltage drop: 2 × 1.98 × 20 × 100 / 1000 ≈ 7.92V

Percentage drop: 7.92 ÷ 120 ≈ 6.6%

Understanding Your Results

Electrical codes generally recommend keeping voltage drop under 3% for branch circuits (5% for combined feeder and branch circuit) to ensure equipment operates properly. Excessive voltage drop can cause dim lighting, reduced motor performance, and inefficient power delivery.

Common Mistakes to Avoid

  • Using the total wire length instead of one-way length (the formula already accounts for the round trip).
  • Undersizing wire gauge for long circuit runs, which significantly increases voltage drop.

Frequently Asked Questions

The reduction in voltage as electrical current travels through a wire, caused by the wire's inherent resistance — greater for longer wires, higher currents, and thinner (higher gauge number) wire.

Many electrical codes recommend keeping voltage drop under 3% for branch circuits and under 5% for combined feeder and branch circuits, though specific requirements vary by code and application.

Thicker wire (lower AWG number) has lower resistance and therefore less voltage drop for the same current and length, compared to thinner wire.

Longer wire runs have more total resistance, since resistance accumulates along the wire's length, resulting in greater voltage drop over longer distances.

Dim lighting, reduced motor performance and lifespan, inefficient power delivery, and potentially equipment malfunction if voltage drops too low.