AWG Wire Size Calculator
Determine the correct electrical wire size (AWG or kcmil) based on allowable voltage drop and load current, compliant with standard engineering practices.
Recommended Wire Size
How to Calculate Wire Size based on Voltage Drop?
In electrical engineering, selecting the correct conductor size is not just about ensuring the wire doesn't melt under high currents (Ampacity). According to the National Electrical Code (NEC), specifically in its Informational Notes, a conductor must also be sized to prevent excessive voltage loss across long distances.
If a wire is too thin or the run is too long, the inherent resistance of the metal will cause the voltage at the load end (e.g., a motor or a panel) to drop significantly. This can lead to equipment malfunction, overheating, and decreased efficiency.
The Voltage Drop Formula (Circular Mils)
In North America, the standard mathematical approach for calculating wire cross-sectional area to limit voltage drop uses Circular Mils (cmil) instead of square millimeters.
Let's break down the components of this vital formula:
- K (Specific Resistance): The constant for the conductor material. Commonly accepted values are 12.9 ohms-cmil/ft for Copper and 21.2 ohms-cmil/ft for Aluminum at standard operating temperatures (75°C).
- I (Current): The maximum continuous load current in Amperes (Amps).
- L (Length): The one-way distance from the power source to the load in feet.
- System Multiplier: This accounts for the return path of the current. It is 2 for Single-Phase or DC circuits, and 1.732 (√3) for Three-Phase systems.
- Allowable Voltage Drop: The maximum voltage loss permitted (e.g., 3% of 480V = 14.4V).
Understanding AWG and kcmil
Once the mathematical calculation gives us the required area in Circular Mils (cmil), we must translate this theoretical number into a commercially available wire size. In the United States, this is done using the American Wire Gauge (AWG) standard.
AWG works backwards: the larger the AWG number (e.g., 14 AWG, 12 AWG), the thinner the wire. For cables larger than 4/0 AWG (0000), the industry switches to kcmil (thousands of circular mils), formerly known as MCM. Our calculator automatically handles this complex conversion, providing you with the exact commercial size you need to purchase.
NEC Voltage Drop Recommendations
While often cited as a strict rule, the NEC handles voltage drop as a recommendation for efficiency (FPN in Section 210.19(A) and 215.2(A)):
- Branch Circuits: Maximum 3% voltage drop at the farthest outlet.
- Feeders + Branch Circuits combined: Maximum total of 5% voltage drop.