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For total resistance calculation
💡 AWG rule: every 3 AWG steps doubles resistance. Every 6 AWG steps halves the cross-sectional area. Higher AWG = thinner wire.
Diameter
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⚠️ Disclaimer: Ampacity values are approximate NEC Table 310.16 figures for copper in conduit at 30°C ambient. Actual safe current depends on installation method, ambient temperature, conduit fill, and local electrical code. Always consult a licensed electrician and follow your local code for all wiring decisions.
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Sources & Methodology
AWG diameter formula from ASTM B258. Resistivity from NIST. Ampacity from NEC Table 310.16 (NFPA 70-2023).
NFPA 70 — National Electrical Code (NEC) 2023, Table 310.16
NEC Table 310.16 is the authoritative US source for conductor ampacity ratings used in this calculator. Values shown are for copper conductors in conduit, 30°C ambient, 60°C column.
NIST — Electrical Resistivity of Copper and Aluminium
Copper resistivity ρ = 1.724×10⁻⁸ Ω·m at 20°C. Aluminium ρ = 2.82×10⁻⁸ Ω·m. Used for DC resistance per metre calculations.
Methodology: AWG diameter: d(mm) = 0.127 × 92ˆ((36−AWG)/39). For 4/0 use AWG = −3; 3/0 = −2; 2/0 = −1; 1/0 = 0. Cross-section A = πd²/4 (mm²). DC resistance: R = ρ/A (mΩ/m) where A in m². Copper ρ = 1.724×10⁻⁸ Ω·m; Al ρ = 2.82×10⁻⁸ Ω·m. Ampacity values from NEC Table 310.16; Al values approx. 84% of Cu.
⏱ Last reviewed: April 2026
AWG Wire Gauge Reference — Diameter, mm², Resistance & Ampacity
The American Wire Gauge (AWG) system numbers wires inversely to their size: higher numbers mean thinner wire. The system is based on a geometric progression — each step of 6 AWG halves the cross-sectional area, each step of 3 AWG doubles the resistance. The formula for diameter is d(mm) = 0.127 × 92ˆ((36−AWG)/39).
AWG Diameter Formula
d(mm) = 0.127 × 92ˆ((36−AWG)/39)
AWG 12: d = 0.127 × 92ˆ((36−12)/39) = 0.127 × 92ˆ0.6154 = 0.127 × 16.164 = 2.053 mm
AWG 14: d = 0.127 × 92ˆ((36−14)/39) = 1.628 mm
AWG 10: d = 0.127 × 92ˆ((36−10)/39) = 2.588 mm
AWG 14: d = 0.127 × 92ˆ((36−14)/39) = 1.628 mm
AWG 10: d = 0.127 × 92ˆ((36−10)/39) = 2.588 mm
Full AWG Reference Table
| AWG | Diameter (mm) | Area (mm²) | Cu R (mΩ/m) | NEC Ampacity (Cu) |
|---|---|---|---|---|
| 4/0 | 11.684 | 107.2 | 0.161 | 195A |
| 3/0 | 10.405 | 85.0 | 0.203 | 165A |
| 2/0 | 9.266 | 67.4 | 0.256 | 145A |
| 1/0 | 8.252 | 53.5 | 0.322 | 125A |
| 2 | 6.544 | 33.6 | 0.512 | 95A |
| 4 | 5.189 | 21.2 | 0.814 | 70A |
| 6 | 4.115 | 13.30 | 1.30 | 55A |
| 8 | 3.264 | 8.37 | 2.06 | 40A |
| 10 | 2.588 | 5.26 | 3.28 | 30A |
| 12 | 2.053 | 3.31 | 5.21 | 20A |
| 14 | 1.628 | 2.08 | 8.28 | 15A |
| 16 | 1.291 | 1.31 | 13.2 | 13A |
| 18 | 1.024 | 0.823 | 20.9 | 10A |
| 20 | 0.812 | 0.518 | 33.2 | 5A |
| 22 | 0.644 | 0.326 | 52.8 | 3A |
💡 Key AWG rules: Every 6 AWG steps halves the cross-sectional area and doubles resistance. Every 3 AWG steps doubles resistance (and halves conductance). AWG 12 and 14 are the most commonly used house wiring gauges in the US. The EU uses mm² directly: AWG 12 ≈ 3.31 mm², AWG 14 ≈ 2.5 mm², AWG 10 ≈ 6 mm².
Frequently Asked Questions
AWG (American Wire Gauge) is a standardised system for specifying wire diameter used in North America. Higher AWG numbers mean thinner wire: AWG 40 (0.080 mm) is the thinnest; 4/0 (11.68 mm) is among the thickest. Formula: d(mm) = 0.127 × 92ˆ((36−AWG)/39). Each 6-step increase halves the cross-sectional area; each 3-step increase doubles resistance.
NEC requires minimum AWG 12 copper for 20A branch circuits. AWG 12 has a 20A ampacity rating at 60°C in conduit. For 15A circuits: AWG 14. For 30A: AWG 10. For 40A: AWG 8. For 50A: AWG 6. These are NEC minimums; always follow your local electrical code and consult a licensed electrician.
Calculate diameter using d(mm) = 0.127 × 92ˆ((36−AWG)/39), then area A = πd²/4. AWG 14 = 2.08 mm². AWG 12 = 3.31 mm². AWG 10 = 5.26 mm². AWG 6 = 13.3 mm². AWG 2 = 33.6 mm². EU metric equivalents: AWG 14 ≈ 2.5 mm², AWG 12 ≈ 4 mm², AWG 10 ≈ 6 mm².
AWG 12 copper at 20°C: diameter 2.053 mm, area 3.31 mm², resistance = ρ/A = 1.724×10⁻⁸ / (3.31×10⁻⁶) = 5.21 mΩ/m. For a 30 m run (15 m each way): R = 5.21 × 30 = 156 mΩ = 0.156 Ω. At 20A: voltage drop = 20 × 0.156 = 3.12 V. Resistance increases 0.393% per °C above 20°C.
AWG 12 is thicker (2.053 mm diameter, 3.31 mm²) than AWG 14 (1.628 mm diameter, 2.08 mm²). In AWG, lower numbers are thicker. AWG 12 can carry 20A vs AWG 14’s 15A. AWG 12 has lower resistance: 5.21 vs 8.28 mΩ/m. For safety: never use AWG 14 on a 20A protected circuit.
NEC 310.16 ampacity for copper in conduit at 60°C: AWG 14 = 15A. AWG 12 = 20A. AWG 10 = 30A. AWG 8 = 40A. AWG 6 = 55A. AWG 4 = 70A. AWG 2 = 95A. AWG 1/0 = 125A. AWG 2/0 = 145A. AWG 3/0 = 165A. AWG 4/0 = 195A. Derate for ambient >30°C, high conduit fill, or continuous loads (80% rule).
Voltage drop V = 2 × I × R/1000 × L (V), where I = current (A), R = resistance (mΩ/m), L = one-way length (m). Factor of 2 for round trip. For AWG 12 at 20A over 15 m: VD = 2 × 20 × 5.21/1000 × 15 = 3.1 V (1.3% on 240V, 2.6% on 120V). NEC recommends keeping total voltage drop under 3% on branch circuits.
USB power conductors: AWG 28 (0.321 mm, 0.0804 mm²) for data cables carrying up to 500 mA. AWG 24 (0.511 mm, 0.205 mm²) for charging cables. USB-C cables rated for 5A (USB PD 100W) often use AWG 20 (0.812 mm, 0.518 mm²). AWG 28 resistance = 213 mΩ/m; 3 m cable has 640 mΩ total, causing a 0.32 V drop at 500 mA.
AWG (American Wire Gauge) and SWG (British Standard Wire Gauge) are different standards with different diameter progressions. SWG 14 = 2.032 mm ≈ AWG 12 (2.053 mm). SWG 16 = 1.626 mm ≈ AWG 14 (1.628 mm). Always compare using the actual diameter in mm or cross-sectional area in mm², as the gauge numbers don’t correspond directly between standards.
Standard US residential wiring: AWG 14 for 15A circuits (general lighting/outlets). AWG 12 for 20A circuits (kitchen, bathroom, garage). AWG 10 for 30A circuits (dryer). AWG 8 for 40A (ranges). AWG 6 for 50A. Service entrance: AWG 2 for 100A panel, AWG 2/0 for 200A service. These are NEC minimums; local codes may require larger gauges for specific applications.
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