Wire Size & Voltage Drop Calculator
Find the right AWG wire gauge and check voltage drop for a copper or aluminum run — using NEC 310.16 ampacity and the standard circular-mils method.
How it works
Voltage drop is calculated with the circular-mils method used throughout the trade:
Vdrop = (K × I × L × P) ÷ CM
- K — resistivity constant: 12.9 for copper, 21.2 for aluminum (ohm·cmil/ft, ~75 °C).
- I — load current in amps.
- L — one-way run length in feet.
- P — path factor: 2 for single-phase/DC, 1.732 (√3) for three-phase.
- CM — conductor cross-section in circular mils (from AWG).
Drop % = Vdrop ÷ system voltage × 100. The recommended wire is the smallest gauge that is both rated for the current (NEC 310.16, 75 °C column) and stays under your voltage-drop target.
AWG reference (copper, 75 °C)
| AWG | Circular mils | Ampacity (A) |
|---|
Ampacity values are the 75 °C column of NEC Table 310.16 for copper. Aluminum carries less current for the same size; this tool applies the aluminum resistivity for voltage drop but you should confirm aluminum ampacity separately.
Common questions
Why does voltage drop matter?
Excess drop means motors run hot, lights dim, and heaters underperform — and it wastes energy as heat in the wire. The NEC recommends (does not mandate) ≤3% on a branch circuit and ≤5% total from service to outlet.
Do I use one-way or round-trip distance?
Enter the one-way distance. The path factor (2 for single-phase) already accounts for current flowing out and back.
Copper or aluminum?
Aluminum is cheaper and lighter but needs a larger gauge for the same load and requires listed connectors and anti-oxidant. Copper is the default for branch circuits.