Calculate the required wire size (AWG) for your project based on voltage, current, and distance. Prevent overheating and voltage drop with this NEC-aligned tool.
Wire Size (AWG) Calculator
The Wire Sizing Formula
Professional wire sizing is calculated using the Circular Mils (CM) method. Circular Mils is a unit of area used to describe the cross-section of a wire. The goal is to ensure the wire has enough surface area to carry current over a specific distance without losing too much voltage to resistance.
CM = (2 × K × I × L) / Vdrop
K = Specific resistivity (Copper: 12.9, Aluminum: 21.2)
I = Current in Amps
L = One-way distance in feet
Vdrop = Maximum allowed voltage drop in volts
AWG Quick Reference Table
This table shows common AWG sizes and their respective Circular Mil areas. The calculator selects the smallest wire that exceeds the required CM for your load.
| AWG Gauge | Circular Mils (CM) | Typical Usage |
|---|---|---|
| 14 AWG | 4,110 | Lighting circuits (15A) |
| 12 AWG | 6,530 | Outlets/Appliances (20A) |
| 10 AWG | 10,380 | Dryers/Water Heaters (30A) |
| 6 AWG | 26,240 | EV Chargers/Ranges (50A) |
Why Does Distance Affect Wire Size?
Every foot of wire has a small amount of electrical resistance. Over a short distance (like 10 feet), the resistance is negligible. However, over a long distance (like 200 feet), that resistance adds up. If the wire is too thin, the resistance will cause the voltage to “drop” by the time it reaches the end, and the energy lost will be converted into heat, which can melt insulation and cause fires.
Critical vs. Non-Critical Loads
- 3% Drop (Critical): Used for sensitive electronics, branch circuits, and motor-driven appliances. This ensures your devices receive the voltage they were designed for.
- 5% – 10% Drop (Non-Critical): Sometimes acceptable for outdoor lighting or simple heating elements where slight performance loss is not a safety issue.
Material Choice: Copper vs. Aluminum
Copper is a superior conductor but is more expensive. Aluminum is lighter and cheaper but has higher resistance (represented by a higher K factor). When using aluminum, you typically need to move up two wire sizes compared to copper to achieve the same performance.