How to Use This Calculator
1
Enter Circuit Parameters
Input the current (amps), one-way wire distance (feet), and voltage of your circuit.
2
Set Voltage Drop Limit
Choose your acceptable voltage drop percentage — typically 3% for branch circuits and 5% for total feeder plus branch.
3
Select the Right Wire
The calculator recommends the minimum AWG wire gauge and shows the actual voltage drop, helping you choose copper or aluminum conductors.
Key Terms
- AWG
- American Wire Gauge — a standardized wire size system where smaller numbers mean larger wire. AWG 14 is common for 15A circuits; AWG 12 for 20A.
- Voltage Drop
- The loss of voltage along a wire due to its inherent resistance. Excessive voltage drop causes dim lights, slow motors, and wasted energy.
- Ampacity
- The maximum current a wire can safely carry without overheating, determined by insulation type, ambient temperature, and conduit fill.
- Circular Mil
- A unit of wire cross-sectional area. One circular mil equals the area of a circle with a 0.001-inch diameter.
- NEC
- National Electrical Code — the U.S. standard governing wire sizing, overcurrent protection, and electrical installation practices.
Real-World Examples
Example 1
Garage Subpanel Feed
Current: 60A, Distance: 80 feet, Voltage: 240V, Max drop: 3%
Result: Minimum wire size is AWG 4 copper (or AWG 2 aluminum). Actual voltage drop with AWG 4 copper is approximately 5.9V (2.5%).
Example 2
Outdoor Lighting Circuit
Current: 10A, Distance: 150 feet, Voltage: 120V, Max drop: 3%
Result: AWG 10 copper is required. Using AWG 14 (minimum for 15A circuits) would cause a 7.5% voltage drop, resulting in noticeably dim lights.
AWG Wire Sizes for Copper Conductors
| AWG | Diameter (in) | Ampacity (60°C) | Resistance (ohm/1000ft) |
|---|
| 14 | 0.064 | 15A | 2.52 |
| 12 | 0.081 | 20A | 1.59 |
| 10 | 0.102 | 30A | 0.999 |
| 8 | 0.128 | 40A | 0.628 |
| 6 | 0.162 | 55A | 0.395 |
Choosing the Right Wire Size for Safety and Efficiency
Voltage Drop vs. Ampacity
Wire sizing involves two independent checks: the wire must be thick enough to handle the current without overheating (ampacity), and thick enough to keep voltage drop within acceptable limits. For short runs, ampacity usually governs. For long runs to outbuildings, sheds, or well pumps, voltage drop often demands a larger wire than ampacity alone would require.
Copper vs. Aluminum
Copper has lower resistance and higher ampacity per gauge but costs significantly more. Aluminum is commonly used for service entrance cables and long feeder runs where the cost savings outweigh the need for a slightly larger conduit. Aluminum requires anti-oxidant compound and compatible connectors to prevent failures at terminations.