Wire SizeCalculator
Determine the optimal wire size based on both ampacity (NEC Table 310.16) and voltage drop requirements. This calculator identifies the controlling factor and recommends the minimum wire size for your application.
Circuit Parameters
Enter your requirements
Recommended Wire Size
Size Comparison
| Requirement | Min. Size | Status |
|---|---|---|
| Ampacity (20A load) | #12 AWG | OK |
| Voltage Drop (3% max) | #8 AWG | Controlling |
NEC Compliance Note
This calculation uses NEC Table 310.16 ampacities for 75°C rated conductors. Apply temperature and conduit fill derating factors as required. Always verify with local codes and AHJ requirements.
Quick Reference
- Branch circuits:3% max
- Feeders:2% max
- Combined total:5% max
Per NEC 210.19(A) Informational Note No. 4
Sizing Process
- 1.Calculate minimum size for ampacity (NEC 310.16)
- 2.Calculate minimum size for voltage drop
- 3.Select the larger of the two sizes
- 4.Apply derating factors if needed
Related Resources
How to Size Electrical Wire Correctly
Two Critical Factors
Proper wire sizing requires balancing two requirements: ampacity (the wire's ability to safely carry current without overheating) and voltage drop (maintaining adequate voltage at the load). Both must be satisfied for a safe, efficient installation.
The NEC provides ampacity tables (Table 310.16) that define maximum current for each wire size based on insulation temperature rating and installation conditions. Voltage drop, while advisory in the NEC, is essential for equipment performance.
When Voltage Drop Controls
For long runs, voltage drop often requires larger wire than ampacity alone would dictate. This is common in outdoor lighting, well pumps, agricultural buildings, and any circuit exceeding approximately 100 feet at 120V or 200 feet at 240V.
When voltage drop is the controlling factor, you're essentially oversizing the wire beyond minimum ampacity requirements. This provides a safety margin and improved efficiency, though at additional material cost.