The American Wire Gauge (AWG) system is fundamentally counterintuitive: the smaller the gauge number, the thicker the wire. Whether you are sizing a feeder for a subpanel, pulling branch circuits for a kitchen remodel, or just trying to match a metric IEC cable to a US standard, guessing the physical dimensions or thermal limits of a conductor is a fast track to a failed inspection or a melted terminal lug.
This reference provides the exact physical dimensions, cross-sectional areas, and thermal ampacity limits for standard copper conductors used in residential and light commercial wiring. All ampacity values are sourced directly from the National Electrical Code (NEC), specifically Chapter 9 Table 8 for physical dimensions and Table 310.16 for allowable ampacities.
How to Read the AWG Wire Diameter and Ampacity Table
Before pulling wire, you need to know which column actually applies to your installation. The table below includes three distinct ampacity columns based on temperature ratings (60°C, 75°C, and 90°C). Here is how to select the correct one:
- The 60°C Column: Per NEC 110.14(C)(1)(a), you must use the 60°C column for circuits rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment terminations are specifically marked otherwise. This applies to almost all standard residential branch circuits using NM-B (Romex) cable.
- The 75°C Column: Used for circuits over 100 amps, or conductors larger than 1 AWG (per NEC 110.14(C)(1)(b)). It also applies when using THHN/THWN wire in conduit if both the breaker and the receptacle terminations are explicitly rated for 75°C.
- The 90°C Column: Most modern THHN and XHHW-2 wire has 90°C insulation. However, you almost never use this column for your final breaker sizing. The 90°C column is strictly used as the starting baseline for derating calculations (explained below). The final ampacity must never exceed the 60°C or 75°C termination limits.
Complete Copper Wire Diameter and Ampacity Chart
Source: NEC Chapter 9 Table 8 (Dimensions) and NEC Table 310.16 (Ampacities). Assumes copper conductors, not more than three current-carrying conductors in a raceway, and an ambient temperature of 30°C (86°F).
| AWG Size | Diameter (in) | Diameter (mm) | Area (kcmil) | Area (mm²) | Ampacity 60°C (Cu) | Ampacity 75°C (Cu) | Ampacity 90°C (Cu) |
|---|---|---|---|---|---|---|---|
| 14 | 0.0641 | 1.628 | 4.11 | 2.08 | 15A | 20A | 25A |
| 12 | 0.0808 | 2.053 | 6.53 | 3.31 | 20A | 25A | 30A |
| 10 | 0.1019 | 2.588 | 10.38 | 5.26 | 30A | 35A | 40A |
| 8 | 0.1285 | 3.264 | 16.51 | 8.37 | 40A | 50A | 55A |
| 6 | 0.1620 | 4.115 | 26.24 | 13.30 | 55A | 65A | 75A |
| 4 | 0.2043 | 5.189 | 41.74 | 21.15 | 70A | 85A | 95A |
| 3 | 0.2294 | 5.827 | 52.62 | 26.67 | 85A | 100A | 115A |
| 2 | 0.2576 | 6.543 | 66.36 | 33.62 | 95A | 115A | 130A |
| 1 | 0.2893 | 7.348 | 83.69 | 42.41 | 110A | 130A | 145A |
| 1/0 | 0.3249 | 8.252 | 105.6 | 53.49 | 125A | 150A | 170A |
| 2/0 | 0.3648 | 9.266 | 133.1 | 67.43 | 145A | 175A | 195A |
| 3/0 | 0.4096 | 10.404 | 167.8 | 85.01 | 165A | 200A | 225A |
| 4/0 | 0.4600 | 11.684 | 211.6 | 107.2 | 195A | 230A | 260A |
Derating, Voltage Drop, and What This Chart Cannot Tell You
A wire diameter chart gives you the baseline physics and thermal limits under ideal conditions. It does not account for the reality of your specific jobsite. Here is how environmental factors modify these base values, and what the chart leaves out.
How Derating Modifies the Base Value
If you pull more than three current-carrying conductors through a single conduit, or if the ambient temperature in your attic exceeds 30°C (86°F), the wires cannot dissipate heat efficiently. You must apply a derating factor.
The Calculation: You always start your derating math using the 90°C column, because the insulation can handle the heat even if the terminations cannot. For example, if you pull four 12 AWG THHN wires through a conduit (which requires an 80% derating factor per NEC 310.15(C)(1)), you multiply the 90°C ampacity (30A) by 0.80, yielding 24A. You then compare this 24A result to the 60°C termination limit for 12 AWG (20A). The final allowable ampacity is the lower of the two numbers: 20A. However, if you were using 10 AWG wire, the 90°C derated value (40A * 0.80 = 32A) would be compared to the 60°C limit (30A), making the final limit 30A.
What the Table Cannot Tell You
- Voltage Drop: A 12 AWG wire is legally rated for 20A, but if you run it 150 feet to a detached garage, the resistance will cause a voltage drop exceeding the NEC-recommended 3% limit (NEC 310.15(B) Informational Note). You must upsize to 10 AWG or 8 AWG to maintain usable voltage at the load, regardless of the ampacity chart.
- Conduit Fill Capacity: The bare copper diameter in this chart does not include the insulation thickness. Conduit fill limits (NEC Chapter 9, Table 1) are based on the overall outer diameter of the insulated wire. You must consult the manufacturer's spec sheet for the exact outside diameter (O.D.) of the specific brand of THHN or XHHW-2 you are pulling.
- Local AHJ Amendments: Some municipalities ban 14 AWG entirely, requiring 12 AWG as the minimum for all branch circuits to reduce voltage drop and fire risk. Your local Authority Having Jurisdiction (AHJ) always has the final say.
Frequently Asked Questions (FAQ)
How do I convert AWG to mm² using a wire diameter chart?
While you can calculate the area using the formula Area = π × (diameter/2)², it is much safer to use the exact mm² column provided in the chart above, as it accounts for standard manufacturing tolerances. Note that metric wire (governed by IEC 60228) does not perfectly align with AWG. For instance, standard metric 2.5 mm² wire falls between 14 AWG (2.08 mm²) and 12 AWG (3.31 mm²). When interfacing US equipment with IEC wiring, always size up to the next largest AWG equivalent to maintain ampacity.
Why does a smaller AWG number mean a thicker wire?
The AWG system is rooted in 19th-century wire manufacturing. The gauge number originally represented the number of times the raw copper rod had to be pulled through progressively smaller drawing dies to reach the final diameter. A 1 AWG wire was drawn once, while a 24 AWG wire was drawn 24 times, resulting in a much thinner conductor. For a deep dive into the historical math behind the gauge steps, the Engineering Toolbox AWG reference provides the exact geometric progression formulas.
Does the wire diameter chart apply to aluminum and copper equally?
The physical dimensions (diameter and kcmil area) for a given AWG size are identical whether the metal is copper or aluminum. However, the ampacity is drastically different. Aluminum has roughly 61% of the electrical conductivity of copper. To carry the same current without overheating, aluminum wire must be thicker. As a general rule of thumb for residential feeders, you must increase the wire size by two AWG steps when switching from copper to aluminum (e.g., a 100A subpanel feeder requires 4 AWG copper, but 2 AWG aluminum). Always verify aluminum ampacities using the specific aluminum columns in NEC Table 310.16 and ensure terminations are rated for aluminum (CO/ALR).
How does stranded vs. solid wire affect the diameter?
Stranded wire has a slightly larger overall outer diameter than solid wire of the same AWG. This is because the individual copper strands cannot pack together with 100% efficiency; there are microscopic air gaps between them, and the outer twist (the 'lay') adds bulk. While the cross-sectional copper area—and therefore the ampacity—remains exactly the same, the larger physical footprint of stranded wire means you will hit conduit fill limits faster. If you are pulling long runs in tight PVC conduit, stranded THHN is physically easier to pull, but you may need to bump up the conduit trade size to accommodate the extra bulk.
