Wire Size Guide: AWG Chart & NEC Ampacity Tables
American Wire Gauge (AWG) System
AWG is a standardized wire gauge system used in North America. The gauge number is inversely related to wire diameter — lower AWG = thicker wire = more ampacity. Going down 3 AWG numbers approximately doubles the cross-sectional area.
NEC Ampacity Table (Table 310.16)
| AWG | Diameter (in) | Area (cmils) | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C | Typical Use |
|---|---|---|---|---|---|---|---|
| 14 | 0.0641 | 4,110 | 15A | 20A | 25A | — | 15A branch circuits (lighting) |
| 12 | 0.0808 | 6,530 | 20A | 25A | 30A | 20A | 20A branch circuits (receptacles) |
| 10 | 0.1019 | 10,380 | 30A | 35A | 40A | 30A | 30A circuits (dryer, AC) |
| 8 | 0.1285 | 16,510 | 40A | 50A | 55A | 40A | 40-50A circuits (range, oven) |
| 6 | 0.1620 | 26,250 | 55A | 65A | 75A | 50A | 60A subpanel feeders |
| 4 | 0.2043 | 41,740 | 70A | 85A | 95A | 65A | 70-100A feeders |
| 2 | 0.2576 | 66,360 | 95A | 115A | 130A | 90A | 100A service entrance |
| 1/0 | 0.3249 | 105,600 | 125A | 150A | 170A | 120A | 150A service entrance |
| 2/0 | 0.3648 | 133,100 | 145A | 175A | 195A | 135A | 200A service entrance |
| 4/0 | 0.4600 | 211,600 | 195A | 230A | 260A | 180A | 200A+ service entrance |
Temperature ratings: 60°C = TW, UF insulation. 75°C = THW, THWN insulation. 90°C = THHN, XHHW insulation. Most residential uses 60°C column for safety margin.
Copper vs Aluminum Wire
| Property | Copper | Aluminum |
|---|---|---|
| Conductivity | 100% (baseline) | 61% of copper |
| Ampacity (same gauge) | Higher | Lower (need 2 AWG sizes larger) |
| Cost | Higher | Lower (40–60% savings) |
| Weight | Heavier | Lighter (50% of copper) |
| Oxidation | Minimal | Requires anti-oxidant compound |
| NEC Requirement | Standard | Must be rated AL/CU for connections |
| Typical Use | Branch circuits (15–50A) | Service entrance, feeders (100A+) |
Voltage Drop by Wire Size (per 100 ft round trip)
| AWG | Resistance (Ω/1000ft) | 10A @ 120V | 15A @ 120V | 20A @ 120V | 30A @ 240V |
|---|---|---|---|---|---|
| 14 | 2.525 | 5.05V (4.2%) | 7.58V (6.3%) | — | — |
| 12 | 1.588 | 3.18V (2.6%) | 4.76V (4.0%) | 6.35V (5.3%) | — |
| 10 | 0.999 | 2.00V (1.7%) | 3.00V (2.5%) | 4.00V (3.3%) | — |
| 8 | 0.628 | 1.26V (1.0%) | 1.88V (1.6%) | 2.51V (2.1%) | — |
| 6 | 0.395 | — | — | — | 2.37V (1.0%) |
NEC recommendation: Keep voltage drop under 3% for branch circuits, 5% total (feeder + branch).
Common Wire Sizing Mistakes
- Using 14 AWG on 20A circuits: NEC 240.4(D) limits 14 AWG to 15A maximum overcurrent protection, regardless of ampacity.
- Ignoring voltage drop on long runs: A 15A load on 14 AWG at 150 ft has 11.4V drop (9.5%) — way over 3%. Upsize wire for long runs.
- Not derating for bundling: When 4+ current-carrying conductors are in one conduit, ampacity must be derated (80% for 4-6 conductors).
- Mixing copper and aluminum: Use only AL/CU-rated connectors. Dissimilar metals cause galvanic corrosion and overheating.
Standards Reference
- NEC Table 310.16 — Allowable Ampacities
- NEC Article 240 — Overcurrent Protection
- NEC Article 210 — Branch Circuits
- NEC 310.14 — Conductor Applications and Insulations
Frequently Asked Questions
What wire size do I need for 20 amps?
Use 12 AWG copper wire for 20A circuits. NEC Table 310.16 rates 12 AWG at 25A (75°C insulation), but NEC 240.4(D) limits it to 20A maximum overcurrent protection. For long runs (>50 ft), consider upsizing to 10 AWG to stay under 3% voltage drop.
Can I use 14 AWG on a 20A breaker?
No. NEC 240.4(D) limits 14 AWG copper to 15A maximum overcurrent protection. Using 14 AWG on a 20A breaker is a code violation and fire hazard. The wire could overheat before the breaker trips.
What's the difference between copper and aluminum wire?
Copper has 61% higher conductivity than aluminum, so aluminum must be 2 AWG sizes larger for the same ampacity. Aluminum is cheaper and lighter, making it preferred for service entrance and large feeders (100A+). Always use AL/CU-rated connectors when terminating aluminum wire.
How do I calculate voltage drop?
V_drop = I × R × 2 × L / 1000, where I = current (amps), R = resistance per 1000 ft (from wire table), L = one-way length (ft). The factor of 2 accounts for the round trip. NEC recommends keeping voltage drop under 3% for branch circuits.
What does AWG stand for?
AWG stands for American Wire Gauge, the standard system for wire diameter in North America. The gauge numbers are inversely proportional to wire size: 10 AWG is thicker than 14 AWG. Going down 3 AWG numbers approximately doubles the cross-sectional area and ampacity.