MyAICalc Logo
MyAICalc Free Online Calculators
Electrical Tool

Voltage Drop Calculator

Determine the voltage drop in electrical wiring based on current load, cable size, distance, and phase options.

Wiring Parameters

Enter valid load parameters to calculate voltage drop.
Electrical Engineering

Understanding
Voltage Drop

Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. This is caused by the internal resistance of the conductor cables.

Conductor Loss

All wires have resistance. Over long runs, this resistance causes a noticeable drop in terminal voltage.

Phase & Multipliers

Three-phase setups distribute the load across multiple conductors, modifying the multiplier from 2 to 1.732.

NEC Recommendations

National Electrical Code advises keeping branch voltage drops below 3% for optimal appliance lifespan.

The Voltage Drop Formulas

Depending on the phase of the system, different multipliers are applied:

  • Single Phase systems:
    V_drop = (2 * K * I * L) / CM
  • Three Phase systems:
    V_drop = (1.732 * K * I * L) / CM

Where:

  • K (Resistivity): 12.9 for Copper, 21.2 for Aluminum.
  • I (Current): Load current in Amperes.
  • L (Distance): One-way length of the cable in feet.
  • CM (Circular Mils): The cross-sectional area of the conductor. E.g. 10 AWG is 10,380 CM.

Conductor Materials comparison:

  • • Copper: More conductive, smaller required gauge sizes, but more expensive.
  • • Aluminum: Less conductive (approx 61% of copper), requires larger gauge sizes, but cheaper and lighter.
  • • Fittings: Ensure aluminum connections use antioxidant compound to prevent corrosion.

How the calculation works

Voltage drop along a cable grows with current and length and shrinks as the wire gets thicker.

Vdrop = (phase factor × K × current × length in feet) ÷ circular mils K is 12.9 for copper. The single-phase factor is 2.

This matches the calculator: 12 AWG copper is 6,530 circular mils. NEC practice often keeps branch-circuit drop near 3%.

Worked example: 15 A, 100 ft, 12 AWG copper, 120 V, single phase

  1. Numerator = 2 × 12.9 × 15 × 100 = 38,700.
  2. Drop = 38,700 ÷ 6,530 = 5.93 volts.
  3. Percent = 4.94% of 120 V.
  4. Voltage at the load ≈ 114.07 V.

15 A on 100 ft of copper, 120 V single phase

WireCircular milsDrop
14 AWG4,1109.42 V
12 AWG6,5305.93 V
10 AWG10,3803.73 V

Frequently asked questions

What is the drop for 15 amps over 100 feet of 12 AWG copper at 120 volts?▼
About 5.93 volts, or 4.9%.
Why is the phase factor 2?▼
The current travels out and back on a single-phase circuit.
What does a thicker wire do?▼
It has more circular mils, so the drop falls. 10 AWG drops less than 12 AWG on the same load.