American Wire Gauge (AWG) is the system used in the United States and Canada to identify the size of round electrical conductors. It appears on every wire spool, on terminal markings and on the datasheets of contactors, breakers, terminal blocks and power supplies. This article explains how the gauge is defined, gives a size table computed from the defining formula, and shows how to use resistance values for voltage-drop checks.
How AWG is defined
AWG is a geometric series anchored on two sizes: 4/0 (0000) = 0.4600 in and 36 AWG = 0.0050 in. There are 39 steps between them, and each step changes the diameter by the same ratio. The diameter of gauge n is therefore:
d(n) = 0.127 mm × 92^((36 − n) / 39)
For the "aught" sizes, use n = 0 for 1/0, −1 for 2/0, −2 for 3/0 and −3 for 4/0. A few practical consequences follow directly from the formula:
- A larger gauge number means a smaller wire.
- Each gauge step changes the diameter by a factor of about 1.1229.
- Every 6 gauges, the diameter roughly doubles (factor 2.005).
- Every 3 gauges, the cross-sectional area roughly doubles, and the resistance per length roughly halves.
- Every 10 gauges, the area changes by a factor of about 10.2.
Sizes larger than 4/0 are not given gauge numbers. They are specified by area in thousands of circular mils (kcmil, formerly written MCM), for example 250 kcmil. One circular mil is the area of a circle 0.001 in (one mil) in diameter, so the area of a round conductor in circular mils is simply its diameter in mils, squared.
AWG diameters to scale, computed from the AWG definition.
AWG size table (4/0 to 30 AWG)
| AWG | Diameter (in) | Diameter (mm) | Area (mm²) | Area (circular mils) | Copper resistance (Ω/km) | Copper resistance (Ω/1000 ft) |
|---|---|---|---|---|---|---|
| 4/0 | 0.4600 | 11.684 | 107.219 | 211,600 | 0.161 | 0.0490 |
| 3/0 | 0.4096 | 10.405 | 85.029 | 167,806 | 0.203 | 0.0618 |
| 2/0 | 0.3648 | 9.266 | 67.431 | 133,077 | 0.256 | 0.0779 |
| 1/0 | 0.3249 | 8.251 | 53.475 | 105,535 | 0.322 | 0.0983 |
| 1 | 0.2893 | 7.348 | 42.408 | 83,693 | 0.407 | 0.1239 |
| 2 | 0.2576 | 6.544 | 33.631 | 66,371 | 0.513 | 0.1563 |
| 3 | 0.2294 | 5.827 | 26.670 | 52,635 | 0.646 | 0.1970 |
| 4 | 0.2043 | 5.189 | 21.151 | 41,741 | 0.815 | 0.2485 |
| 5 | 0.1819 | 4.621 | 16.773 | 33,102 | 1.028 | 0.3133 |
| 6 | 0.1620 | 4.115 | 13.302 | 26,251 | 1.296 | 0.3951 |
| 7 | 0.1443 | 3.665 | 10.549 | 20,818 | 1.634 | 0.4982 |
| 8 | 0.1285 | 3.264 | 8.366 | 16,510 | 2.061 | 0.6282 |
| 9 | 0.1144 | 2.906 | 6.634 | 13,093 | 2.599 | 0.7921 |
| 10 | 0.1019 | 2.588 | 5.261 | 10,383 | 3.277 | 0.9988 |
| 11 | 0.0907 | 2.305 | 4.172 | 8,234 | 4.132 | 1.2595 |
| 12 | 0.0808 | 2.053 | 3.309 | 6,530 | 5.211 | 1.5882 |
| 13 | 0.0720 | 1.828 | 2.624 | 5,178 | 6.571 | 2.0027 |
| 14 | 0.0641 | 1.628 | 2.081 | 4,107 | 8.285 | 2.5254 |
| 15 | 0.0571 | 1.450 | 1.650 | 3,257 | 10.448 | 3.1844 |
| 16 | 0.0508 | 1.291 | 1.309 | 2,583 | 13.174 | 4.0155 |
| 17 | 0.0453 | 1.150 | 1.038 | 2,048 | 16.612 | 5.0634 |
| 18 | 0.0403 | 1.024 | 0.823 | 1,624 | 20.948 | 6.3849 |
| 19 | 0.0359 | 0.912 | 0.653 | 1,288 | 26.415 | 8.0512 |
| 20 | 0.0320 | 0.812 | 0.518 | 1,022 | 33.308 | 10.1524 |
| 21 | 0.0285 | 0.723 | 0.410 | 810 | 42.001 | 12.8019 |
| 22 | 0.0253 | 0.644 | 0.326 | 642 | 52.962 | 16.1429 |
| 23 | 0.0226 | 0.573 | 0.258 | 509 | 66.784 | 20.3558 |
| 24 | 0.0201 | 0.511 | 0.205 | 404 | 84.213 | 25.6682 |
| 25 | 0.0179 | 0.455 | 0.162 | 320 | 106.191 | 32.3670 |
| 26 | 0.0159 | 0.405 | 0.129 | 254 | 133.904 | 40.8140 |
| 27 | 0.0142 | 0.361 | 0.102 | 202 | 168.850 | 51.4655 |
| 28 | 0.0126 | 0.321 | 0.081 | 160 | 212.916 | 64.8968 |
| 29 | 0.0113 | 0.286 | 0.064 | 127 | 268.482 | 81.8334 |
| 30 | 0.0100 | 0.255 | 0.051 | 101 | 338.550 | 103.1900 |
Values for guidance, computed from the AWG definition for a solid round conductor. Resistance is for annealed copper at 20 °C (resistivity 1.7241 × 10⁻⁸ Ω·m, 100 % IACS), direct current.
Notes on the table
- Stranded wire of the same gauge has about the same copper area, but a larger overall diameter and slightly higher resistance because the strands are twisted. For exact values, use the manufacturer's data or the conductor properties table of the electrical code (NEC Chapter 9, Table 8).
- Temperature raises copper resistance by roughly 0.39 % per °C. At a conductor temperature of 75 °C, resistance is about 1.22 times the 20 °C value.
- Aluminum conductors have a higher resistivity than copper, so an aluminum conductor needs a larger size to carry the same current with the same losses.
- Alternating current in large conductors also has a small extra resistance due to skin effect; for the control and branch circuit sizes common in panels, the DC values above are a good approximation at 60 Hz.
Using resistance to estimate voltage drop
For a two-wire circuit, the voltage drop is the current multiplied by the resistance of both conductors (the outgoing and the return path):
Voltage drop = I × R per length × 2 × one-way length
- Example 1: 12 AWG copper, 100 ft one way, 10 A. Drop ≈ 3.18 V at 20 °C, which is about 2.6 % of 120 V.
- Example 2: 18 AWG copper, 50 m one way to a sensor or valve on a 24 V DC circuit, 0.5 A. Drop ≈ 1.05 V at 20 °C, about 4.4 % of 24 V.
The second example is typical of long control runs: a 24 V DC device far from its power supply can see noticeably less voltage than the supply delivers, especially when the conductors are warm or several loads share them. Three-phase circuits use a different multiplier; follow the method in your design guide or electrical code.
Why this table has no ampacity column
Ampacity, the current a conductor may carry continuously, is not a property of the copper alone. It depends on:
- the temperature rating of the insulation (for example 60 °C, 75 °C or 90 °C);
- the temperature rating of the terminals the wire connects to;
- the installation method: free air, cable, conduit, raceway or cable tray;
- the number of current-carrying conductors grouped together and the ambient temperature;
- specific rules for small conductors, motor circuits, control circuits and machinery.
The values you may use are set by the ampacity tables and adjustment rules of the electrical code (NEC Article 310 in the United States, CEC Section 4 in Canada) and, for machines and panels, by NFPA 79 or UL 508A. Take ampacity from those sources, never from a generic size chart.
Comparing AWG with metric (mm²) sizes
IEC equipment is often specified in square millimeters. Metric and AWG sizes do not line up exactly, so a metric size always falls between two AWG sizes:
| Metric size | Next smaller even AWG | Next larger even AWG |
|---|---|---|
| 0.5 mm² | 22 AWG (0.33 mm²) | 20 AWG (0.52 mm²) |
| 0.75 mm² | 20 AWG (0.52 mm²) | 18 AWG (0.82 mm²) |
| 1 mm² | 18 AWG (0.82 mm²) | 16 AWG (1.31 mm²) |
| 1.5 mm² | 16 AWG (1.31 mm²) | 14 AWG (2.08 mm²) |
| 2.5 mm² | 14 AWG (2.08 mm²) | 12 AWG (3.31 mm²) |
| 4 mm² | 12 AWG (3.31 mm²) | 10 AWG (5.26 mm²) |
| 6 mm² | 10 AWG (5.26 mm²) | 8 AWG (8.37 mm²) |
| 10 mm² | 8 AWG (8.37 mm²) | 6 AWG (13.30 mm²) |
| 16 mm² | 6 AWG (13.30 mm²) | 4 AWG (21.15 mm²) |
| 25 mm² | 4 AWG (21.15 mm²) | 2 AWG (33.63 mm²) |
Values for guidance, computed from the AWG definition. Odd gauges exist but are rarely used for building and panel wire.
When a terminal is marked in both systems, follow the range printed on the device or in its datasheet, and use the tightening torque given by the manufacturer. Fine-stranded conductors may need ferrules or terminals approved for that conductor class.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Key takeaways
- AWG is a geometric series between 4/0 (0.4600 in) and 36 AWG (0.0050 in) with 39 equal-ratio steps.
- Three gauges smaller doubles the area and halves the resistance; six gauges smaller doubles the diameter.
- Sizes above 4/0 are given in kcmil.
- Use the resistance column for voltage-drop estimates, corrected for temperature.
- Take ampacity only from the applicable code tables, which depend on insulation, terminals and installation.
Related parts at 3JIndustry
Check the wire range of the terminals on our used contactors and circuit breakers before you order, and read our article on North American voltage supply networks. Looking for a discontinued part? Send the full part number to sales@3jindustry.com or use our contact page.
