Basics
How AWG works — and how it maps to mm²
American Wire Gauge is a logarithmic numbering system: the bigger the number, the thinner the wire. It is anchored at 36 AWG = 0.005 in (0.127 mm) and 4/0 (0000) AWG = 0.4600 in, with 39 geometric steps in between. That gives three handy rules of thumb: every 3 gauges doubles (or halves) the cross-section, every 6 gauges doubles the diameter, and every 10 gauges changes resistance by roughly 10×. 10 AWG copper is about 1 Ω per 1,000 ft — memorise that one and you can estimate the rest. Outside North America, wire is specified by metric cross-section in mm² from the IEC 60228 series (0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16 mm² …), so when you source parts or read European and Asian datasheets you constantly convert between the two.
AWG diameter: d = 0.127 mm × 92^((36 − n) / 39) (1/0 = 0, 2/0 = −1, 3/0 = −2, 4/0 = −3) Area: A = π/4 × d² circular mils = (d in mils)² Resistance: R' = ρ(T) / A, ρ(T) = 1.72×10⁻⁸ × (1 + 0.00393 × (T − 20 °C)) Ω·m Voltage drop: Vd = k × I × R' × L k = 2 (DC / single-phase), √3 (three-phase) Minimum area: A_min = k × I × L × ρ / (V × drop%)
AWG quick reference
| AWG | Diameter (in / mm) | Area mm² / kcmil | Ω per 1,000 ft @ 20 °C | NEC 310.16 75 °C | Typical use |
|---|---|---|---|---|---|
| 24 | 0.0201 / 0.511 | 0.205 / 0.404 | 25.6 | — | Signal, Cat5e/6 conductors |
| 22 | 0.0253 / 0.644 | 0.326 / 0.642 | 16.1 | — | Hookup wire, sensors |
| 18 | 0.0403 / 1.02 | 0.823 / 1.62 | 6.37 | 14 A (90 °C col.) | Lamp cord, LED strips |
| 16 | 0.0508 / 1.29 | 1.31 / 2.58 | 4.01 | 18 A (90 °C col.) | Extension cords, auto accessories |
| 14 | 0.0641 / 1.63 | 2.08 / 4.11 | 2.52 | 20 A (15 A OCPD) | 15 A branch circuits |
| 12 | 0.0808 / 2.05 | 3.31 / 6.53 | 1.58 | 25 A (20 A OCPD) | 20 A circuits, inverters |
| 10 | 0.1019 / 2.59 | 5.26 / 10.4 | 0.996 | 35 A (30 A OCPD) | 30 A, dryers, solar strings |
| 8 | 0.1285 / 3.26 | 8.37 / 16.5 | 0.627 | 50 A | Ranges, high-current DC |
| 4 | 0.2043 / 5.19 | 21.2 / 41.7 | 0.248 | 85 A | Battery cables, sub-panels |
Ampacity
Ampacity is really an insulation temperature limit
Copper won't melt at any current you'd sensibly run, but its insulation will: PVC/THHN-class insulation is rated 60, 75 or 90 °C. Ampacity is the current at which I²R heating balances heat loss so the conductor stays at that rating. That's why NEC Table 310.16 has three columns, and why it assumes no more than three current-carrying conductors in a raceway or cable at 30 °C (86 °F) ambient. More conductors bundled together get the 310.15(C)(1) adjustment (4–6 → 80 %, 7–9 → 70 %, 10–20 → 50 %), and hotter ambients get the 310.15(B) correction, √((Tc − Ta)/(Tc − 30)) — 0.88 for a 75 °C conductor at 40 °C. Two traps catch people every day: 110.14(C) says that if terminals are only rated 60/75 °C (most breakers and lugs), you must use that column even with 90 °C wire; and 240.4(D) caps the breaker at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper regardless of the table. For metric wire the calculator also offers IEC 60364-5-52 Table B.52.4 (PVC, 70 °C conductor), the basis for BS 7671 and most national codes outside the US.
Worked examples
When voltage drop, not ampacity, picks the wire
- 12 V DC, 20 A load, 10 ft from the battery, 3 % target (0.36 V): A_min = 2 × 20 × 3.048 m × 1.72×10⁻⁸ ÷ 0.36 = 5.83 mm². 10 AWG (5.26 mm²) falls short, so use 8 AWG (8.37 mm²). Ampacity alone would have allowed 12 AWG, which drops 0.63 V (5.3 %); 14 AWG would drop 1.0 V (8.4 %).
- 120 V single-phase, 15 A, 100 ft one-way, 3 % (3.6 V): A_min = 2 × 15 × 30.48 × 1.72×10⁻⁸ ÷ 3.6 = 4.37 mm², i.e. 10 AWG instead of the 14 AWG the 15 A breaker would otherwise allow. Long garage and shed runs are where this bites.
- Same power at 240 V instead of 120 V: half the current and twice the allowable drop, so a quarter of the copper area. That's why EV chargers, well pumps and long feeders run at 240 V.
Pitfalls
Common mistakes
- Entering the round-trip length: the calculator wants the one-way distance and applies the return conductor itself (k = 2). Entering 2 × the run doubles the result.
- Trusting 'chassis wiring' charts: the widely copied table claiming 10 AWG = 55 A assumes a single wire in free air at high conductor temperature, with no published basis. Inside a bundle, conduit or a hot enclosure it is dangerous.
- Ignoring termination ratings: 90 °C wire on a 75 °C breaker is a 75 °C circuit. Use the 90 °C column only as the starting point for derating.
- Rounding mm² down when substituting: replacing 12 AWG (3.31 mm²) with 2.5 mm² wire adds 32 % resistance. Always go to the same or larger area — 4 mm².
- Forgetting crimp ranges: terminals are rated for a specific gauge span (for example 28–22 AWG) printed in the contact datasheet. Oversized wire crimps badly and the terminal overheats first.
- Treating aluminium like copper: aluminium's resistivity is about 2.82×10⁻⁸ Ω·m (1.64× copper), roughly two AWG sizes larger for the same drop. This calculator is copper-only.
In practice
Tips from KEMI's bench
On electronics harnesses the current rating is set by the weakest of three links: the wire, the connector contact and the PCB trace. A 0.1 in (2.54 mm) header pin is good for about 3 A, a JST XH contact 3 A and a PH contact 2 A, so upsizing the wire often just moves the bottleneck to the connector. Parallel several pins or switch to a terminal block for high-current paths, and size the copper on the board with our PCB trace width calculator. When you order a harness, specify it fully — 'UL1007 22 AWG red, 150 mm, XH crimp both ends' — to avoid a second round. For 12–24 V systems carrying tens of amps (vehicles, solar, robotics), voltage drop almost always governs, so check the voltage-drop minimum before you look at any ampacity table.