Wire Sizing Calculator
Find the correct wire gauge for ampacity and voltage drop per NEC requirements.
Quick Reference
| Circuit | Typical Wire | Breaker |
|---|---|---|
| 15A Branch | #14 Cu | 15A |
| 20A Branch | #12 Cu | 20A |
| 30A Circuit | #10 Cu | 30A |
| 40A Circuit | #8 Cu | 40A |
| 50A Circuit | #6 Cu | 50A |
| 60A Feeder | #6 Cu | 60A |
| 100A Service | #3 Cu | 100A |
| 200A Service | #2/0 Cu | 200A |
*Based on 75°C copper, short runs. Always verify with full calculation for long runs.
Understanding Wire Sizing
Two Critical Factors
Wire sizing must satisfy two requirements:
- Ampacity - The wire must safely carry the load current without overheating (NEC 310.16)
- Voltage Drop - The wire must deliver adequate voltage to the load (NEC recommends max 3% for branch circuits, 5% total)
Derating Factors
Ampacity must be derated for:
- Temperature - Higher ambient temps reduce ampacity
- Conduit Fill - More than 3 current-carrying conductors reduces ampacity
- Continuous Loads - Loads operating 3+ hours require 125% sizing
Voltage Drop Formula
Single Phase: VD = (2 × L × I × R) / 1000
Three Phase: VD = (1.732 × L × I × R) / 1000
Where: L = one-way length (ft), I = current (A), R = resistance (Ω/1000ft)