How to Size Electrical Wire
Proper wire sizing ensures safety and code compliance. Undersized wire causes voltage drop, overheating, and fire risk. The two main factors are ampacity (current capacity) and voltage drop over distance.
Voltage Drop Formula
Voltage drop increases with distance and current, and decreases with larger wire gauge. For single-phase circuits:
Vdrop=2×L×I×R1000
Where L is the one-way distance in feet, I is current in amps, and R is wire resistance in ohms per 1000 ft.
Voltage Drop Percentage
The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total (feeder + branch combined):
Drop %=VdropVsource×100
Safety First
All electrical work must comply with the National Electrical Code (NEC) and local building codes. Many jurisdictions require a licensed electrician for new circuits. Always turn off power at the breaker before working on wiring. When in doubt, hire a professional.AWG Wire Size Reference
Common residential wire sizes and their ratings for copper conductors at 75 C (NM-B / Romex):
| AWG | Ampacity | Max Breaker | Typical Use |
|---|---|---|---|
| 14 AWG | 15A | 15A | Lighting, general outlets |
| 12 AWG | 20A | 20A | Kitchen, bath, garage |
| 10 AWG | 30A | 30A | Dryers, water heaters |
| 8 AWG | 40A | 40A | Ranges, large appliances |
| 6 AWG | 55A | 60A | Sub-panels, EV chargers |
Breaker Sizing Rule
The NEC 80% rule states that continuous loads should not exceed 80% of the breaker rating. A 20A breaker on 12 AWG wire should carry no more than 16A of continuous load (loads lasting 3+ hours).