Electrical Wire Sizing & Voltage Drop

Running long electrical lines for a shop compressor, sub-panel, or off-grid solar system? Prevent dangerous overheating and equipment-damaging voltage drops by calculating the exact conductor gauge required for your run.

Electrical Wire Sizing & Voltage Drop Calculator

Size copper and aluminum conductors (AWG / kcmil) to limit circuit voltage drop according to National Electrical Code (NEC) standards.
Electrical Analysis Summary
Calculated Voltage Drop
0.00 V
Percentage Drop (%)
0.00 %
Voltage at Load Terminal
0.0 V
Power Loss in Conductor
0.0 W
Recommended Min. Gauge for Selected Drop Limit
12 AWG
Compliance Status
OK

Electrical Engineering Formulas & NEC Guidelines

Excessive voltage drop across long conductor runs causes equipment inefficiency, motor overheating, flickering lights, and wasted energy dissipated as heat.

1. Voltage Drop Equations

For Single-Phase AC and DC circuits (2-wire systems):

ΔV = (2 * K * I * L) / CM

For Three-Phase AC circuits (balanced line-to-line):

ΔV = (1.732 * K * I * L) / CM

  • K = Conductor resistivity constant (approx. 12.9 Ω·cmil/ft for Copper; 21.2 Ω·cmil/ft for Aluminum at 75°C)
  • I = Load current in Amperes
  • L = One-way length of the run in feet
  • CM = Conductor cross-sectional area in Circular Mils

2. National Electrical Code (NEC) Standards (Article 210.19 & 215.2)

The NEC recommends that branch circuit conductors be sized so that the maximum voltage drop does not exceed 3% at the farthest outlet. The maximum combined drop for both feeder conductors and branch circuits combined should not exceed 5% for overall system operating efficiency.

3. Standard AWG Conductor Cross-Section Reference

AWG / kcmil Size Circular Mils (cmil) Typical 75°C Copper Ampacity (THHN)
14 AWG 4,110 20 A (Max 15A Breaker)
12 AWG 6,530 25 A (Max 20A Breaker)
10 AWG 10,380 35 A (Max 30A Breaker)
8 AWG 16,510 50 A
6 AWG 26,240 65 A
4 AWG 41,740 85 A
2 AWG 66,360 115 A
1/0 AWG 105,600 150 A
4/0 AWG 211,600 230 A

The Governing Physics: Ohm's Law & Conductor Resistance

As electrical current travels through a wire, the physical resistance of the copper or aluminum converts a portion of that energy into heat, causing the voltage at the end of the line to drop. The longer the run and the higher the amperage, the thicker the wire cross-sectional area (AWG or kcmil) must be to prevent voltage from dropping below safe operating limits. The tool utilizes the standard resistance formula $V_d = \frac{2 \cdot K \cdot I \cdot L}{A}$ for single-phase circuits.

Step-by-Step Worked Example

A user is running a 240V, 1-phase circuit 150 feet to power a 30 Amp welder. Using standard 10 AWG wire, the resistance over 150 feet causes a 5.5% voltage drop, starved below the 3% code recommendation. The tool automatically recalculates and recommends stepping up to 8 AWG copper, returning the voltage drop to a safe 3.4% to protect the welder's internal motors.

Industry Standards & Compliance

Calculations are strictly referenced against the NFPA 70 National Electrical Code (NEC) Chapter 9 conductor properties and allowable ampacities.