AWG Diameter and Resistance Chart

Complete American Wire Gauge (AWG) reference: diameter (inch/mm), cross-sectional area (kcmil, mm²), and resistance per 1000ft & per km for copper & aluminum. Includes temperature correction, length estimator, and dynamic visualization. Conforms to ASTM B258‑18 and NEC Chapter 9, Table 8.

4/0 AWG#10 AWG#40 AWG
10 AWG
Diameter (inch)

0.1019

in
Diameter (mm)

2.588

mm
Area (mm²)

5.261

mm²
Area (Circular mils / kcmil)

10380 CM / 10.38 kcmil

Base resistivity @20°C, temp coeff α=0.00393 (Cu) / 0.00403 (Al)
Resistance per 1000 ft

0.9989

Ω/1000ft
Resistance per km

3.277

Ω/km
Length Resistance Estimator
Total resistance: 0.0999 Ω
Includes temperature correction for selected material. For voltage drop, use Vdrop = I × R (single‑phase, one‑way length doubled for round‑trip).
Diameter vs AWG (exponential decay)
Diameter (inch)
Standard AWG formula: d = 0.005 · 92⁽⁽³⁶⁻ⁿ⁾/³⁹⁾ inches (n = AWG number, 4/0 → n=-3)
Quick Reference: Typical Values (Copper @20°C)
AWGmm²Ω/1000ftΩ/km
Values match NEC Chapter 9 Table 8 for DC resistance at 20°C.
Precision engineering data: Calculations based on ASTM B258-18 and IEC 60228. Resistance values computed using 100% IACS conductivity (ρ_Cu = 0.017241 Ω·mm²/m @20°C). All computations run locally in your browser – no data leaves your device.

What is AWG? The American Wire Gauge Standard

The American Wire Gauge (AWG), also known as the Brown & Sharpe wire gauge, is a standardized wire sizing system used primarily in North America for electrically conductive wires. AWG is based on a logarithmic scale: as the gauge number increases, the diameter decreases exponentially. This inverse relationship makes it intuitive for electrical engineers: larger gauge = thinner wire = higher resistance.

Core AWG formula: dn = 0.005 inch × 92(36-n)/39 , where n = AWG size. For sizes 4/0 (0000) n = -3, 3/0 n = -2, 2/0 n = -1, 1/0 n = 0.

Developed in 1857, the AWG system replaced disparate regional gauges, bringing consistency to telegraph and later power transmission industries. The step ratio between successive gauges is a constant factor of 921/39 ≈ 1.12293, meaning each gauge step changes cross-sectional area by a factor of about 1.261. This ensures that resistance per unit length scales predictably, enabling easy voltage drop and ampacity estimations.

Standardization & Authority: AWG dimensions are defined by ASTM B258‑18 (Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes). The resistance values in this calculator align with the National Electrical Code (NEC) Chapter 9, Table 8, which provides DC resistance for copper and aluminum conductors at 20°C (75°C for ampacity adjustments). Our interactive tool extends NEC data with real‑time temperature correction.

Resistance Calculation & Temperature Effects

DC resistance of a wire is determined by R = ρ · L / A, where ρ is resistivity, L length, and A cross-sectional area. At 20°C, standard annealed copper exhibits ρ = 0.017241 Ω·mm²/m (or 10.371 Ω·CM/ft). Our calculator uses exact geometric area to compute resistance per km and per 1000ft, then adjusts using the linear temperature coefficient: R(T) = R20 × [1 + α (T - 20)], where α = 0.00393 /°C for copper and 0.00403 /°C for aluminum. This temperature correction is critical for real-world installations where operating temperatures exceed ambient.

Engineering Application – Voltage Drop in Solar PV Strings

A 30A DC circuit uses 10 AWG copper wire (R ≈ 1.018 Ω/1000ft @75°C). For a 150ft round-trip length, voltage drop at 30A is Vdrop = 30A × (150/1000 × 1.018) = 4.58V, representing a 1.9% drop on a 240V system. NEC recommends <3% drop. The AWG calculator allows instant material & temperature adjustment to guarantee code compliance.

Practical Sizing Guide (Based on NEC 310.16)

  • For general branch circuits: 14 AWG (15A), 12 AWG (20A), 10 AWG (30A) copper, 60/75°C insulation.
  • For long feeder runs (>200 ft), consider one size larger to reduce voltage drop.
  • Aluminum requires approximately two AWG sizes larger than copper for equivalent ampacity (e.g., 8 AWG Al ≈ 10 AWG Cu).
  • Always derate for ambient temperature >30°C and for more than 3 current‑carrying conductors in a raceway.

Copper vs. Aluminum: Conductivity and Practical Use

Aluminum has approximately 61% conductivity of copper (IACS). For the same AWG size, aluminum wire has higher resistance (about 1.6× copper). However, it is lighter and cheaper, often used in overhead transmission lines and large feeders. Our calculator toggles between materials instantly, showing how resistance changes – essential for replacement or upgrade projects.

How to Use This AWG Tool – Step by Step

  • Select AWG: Use slider or dropdown (4/0 to 40 AWG). Diameter, area and resistance update live.
  • Choose Material & Temperature: Copper or aluminum with temperature correction up to 150°C (max 200°C for special insulation).
  • Length Estimator: Enter any cable length to compute total resistance and voltage drop reference.
  • Visual Plot: The embedded chart shows exponential diameter decay across gauges, highlighting the selected gauge.
  • Quick Reference Table: Scroll precomputed values for typical sizes (copper @20°C) – verified against NEC Table 8.

Frequently Asked Questions (AWG)

AWG is a North American standard based on diameter steps. mm² refers to the metric cross-sectional area. They are related via circular area: mm² = (π/4) × d_mm². The conversion is direct; e.g., 10 AWG ≈ 5.26 mm², 12 AWG ≈ 3.31 mm². Many international standards (IEC) use mm².

Historical: higher gauge numbers originally represented the number of drawing steps to reduce a wire from a standard rod. More draws = smaller wire = higher gauge number.

Our calculations assume solid round conductor cross-section. For stranded wire, effective resistance is nearly identical (same total cross-sectional area) at DC and low frequencies; skin effect at high frequencies (above 60 Hz for large conductors) may increase AC resistance. For power frequencies (50/60 Hz) and most building wire, stranded and solid have the same DC resistance per NEC.

Higher temperature increases resistance and reduces current-carrying capacity (ampacity). Using the temperature correction, our tool provides resistance at any operating temperature, essential for derating calculations per NEC.

Larger conductors (250 kcmil, 500 kcmil) are designated in thousands of circular mils (kcmil). Our tool covers the typical AWG range up to 40 AWG; for larger busbars, refer to separate kcmil tables. AWG 4/0 = 211.6 kcmil.

Stranded wire has a slightly larger overall diameter than solid wire of the same AWG because of inter‑strand gaps. However, the circular mil area (total copper cross‑section) remains the same. Our calculator gives the nominal solid diameter; for stranded, add ~5‑10% for overall diameter – consult manufacturer data.
Data standards & references
• ASTM B258‑18: Standard specification for AWG diameters.
• IEC 60228: Conductors of insulated cables (metric mm²).
• NEC 2023, Chapter 9, Table 8: Conductor properties.
• IACS (International Annealed Copper Standard): Resistivity of 0.017241 Ω·mm²/m @20°C.
• Verified by independent calculation against NIST data. For further reading: AWG (Wikipedia) | ASTM B258 | NFPA 70 (NEC)