Wire sizing is fundamental to electrical work. Undersized wire creates heat, wasted energy, and fire hazards. This guide covers American Wire Gauge (AWG) sizes, metric equivalents, ampacity ratings, and how to calculate proper wire size for your application.
Understanding AWG
American Wire Gauge uses a counterintuitive numbering system: smaller numbers mean larger wire. The scale runs from 40 AWG (finest) to 0000 (4/0, largest standard size). Each step of 3 gauges doubles the cross-sectional area.
This matters because current-carrying capacity (ampacity) is directly related to cross-sectional area, while resistance is inversely related.
AWG to Metric Conversion
Most of the world uses mm² for wire sizing. Here are the common equivalents:
| AWG | mm² | Diameter (mm) |
|---|---|---|
| 18 | 0.82 | 1.02 |
| 16 | 1.31 | 1.29 |
| 14 | 2.08 | 1.63 |
| 12 | 3.31 | 2.05 |
| 10 | 5.26 | 2.59 |
| 8 | 8.37 | 3.26 |
| 6 | 13.3 | 4.11 |
| 4 | 21.2 | 5.19 |
| 2 | 33.6 | 6.54 |
| 1/0 | 53.5 | 8.25 |
Ampacity Ratings
Ampacity depends on wire size, insulation temperature rating, and installation conditions. These are NEC-based values for copper wire at 75°C in free air:
| AWG | Ampacity (75°C) | Common Use |
|---|---|---|
| 14 | 20A | 15A circuits |
| 12 | 25A | 20A circuits |
| 10 | 35A | 30A circuits |
| 8 | 50A | 40A circuits |
| 6 | 65A | 50-60A circuits |
| 4 | 85A | Large appliances |
| 2 | 115A | Sub-panels |
| 1/0 | 150A | Service entrance |
Calculate Wire Size for Your Application
Enter your load current and distance to find the right wire size with voltage drop calculations.
Voltage Drop Calculations
For long runs, voltage drop becomes critical. The NEC recommends no more than 3% drop for branch circuits and 5% total from service entrance to final outlet.
The voltage drop formula:
VD = (2 × K × I × L) / CM
Where K is the resistivity constant (12.9 for copper), I is current in amps, L is one-way length in feet, and CM is the circular mil area of the wire.
For a 20A load over 100 feet using 12 AWG (6,530 CM):
VD = (2 × 12.9 × 20 × 100) / 6,530 = 7.9 volts (6.6% on 120V)
This exceeds 3%, so you'd need to upsize to 10 AWG for this run.
Copper vs. Aluminum
Aluminum wire is lighter and cheaper but has higher resistance. Generally, you need to go two sizes larger with aluminum to match copper ampacity:
- Copper 4 AWG = Aluminum 2 AWG
- Copper 2 AWG = Aluminum 1/0 AWG
Aluminum also requires anti-oxidant compound and special terminations rated for aluminum. It's common for service entrances and feeders but less suitable for branch circuits.
Derating Factors
You must reduce ampacity for:
More Than 3 Conductors
- 4-6 conductors: 80% of ampacity
- 7-9 conductors: 70%
- 10-20 conductors: 50%
High Ambient Temperature
Standard ratings assume 30°C (86°F) ambient. Higher temperatures require derating per NEC tables—typically 10-20% reduction for industrial environments.
Continuous Loads
Loads running 3+ hours continuously must be calculated at 125% of the actual load when sizing conductors and breakers.
Stranded vs. Solid Wire
- Solid wire: Easier to terminate, holds shape in boxes. Standard for 14-10 AWG building wire.
- Stranded wire: More flexible, better for conduit pulls and moving applications. Required for 8 AWG and larger.
Stranded wire of the same gauge has slightly lower ampacity due to skin effect at high frequencies, but this is negligible for 60Hz power.
Quick Sizing Rules
- 14 AWG minimum for 15A circuits
- 12 AWG minimum for 20A circuits
- 10 AWG for 30A (dryers, large window AC)
- 8 AWG for 40A (ranges, sub-panels)
- 6 AWG for 50-60A
- Always check voltage drop for runs over 50 feet
Wire sizing isn't just about meeting minimum code—it's about efficiency, safety, and longevity. When in doubt, go one size larger. The cost difference is minimal compared to the long-term benefits of reduced heat and voltage drop.
