Wire Size Calculator
Determine the correct AWG wire gauge based on amperage, distance, voltage, and acceptable voltage drop.
Results
Visualization
How It Works
Wire sizing ensures your electrical conductors can safely carry the required current over the distance of your circuit run without excessive voltage drop. Undersized wires cause overheating, voltage drop, and fire hazards.
The Formula
Circular Mils Needed = (2 × K × I × D) / Vdrop
Where Vdrop = Voltage × Max Drop % / 100
Where Vdrop = Voltage × Max Drop % / 100
Variables
- K — Resistivity constant (12.9 for copper, 21.2 for aluminum)
- I — Current in amperes
- D — One-way distance in feet
- V_drop — Maximum allowable voltage drop in volts
Example
For a 20A copper circuit at 120V over 50 feet with 3% max drop: V_drop = 120 x 0.03 = 3.6V. Cmil = (2 x 12.9 x 20 x 50) / 3.6 = 7,167. The next AWG up is 10 AWG (10,380 cmil).
Tips
- NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder + branch.
- Aluminum wire requires a larger gauge than copper for the same amperage and distance.
- Always round up to the next available AWG size when the calculation falls between sizes.
- Consider future load growth when sizing conductors for new installations.
- Local codes may impose stricter requirements than the NEC minimum.