The exact bare diameter of a solid round copper wire is strictly governed by the ASTM B258 standard. For a direct answer: a 12 AWG solid copper conductor measures exactly 0.0808 inches (2.053 mm) across, while a 10 AWG measures 0.1019 inches (2.588 mm). However, on the bench or jobsite, you are rarely measuring bare wire. You are working with insulated conductors like THHN or NM-B, where the overall outside diameter (OD) dictates your conduit fill limits, and the insulation temperature rating dictates your maximum breaker size.
This reference guide provides the bare AWG diameter alongside practical insulated dimensions and National Electrical Code (NEC) ampacities, giving you the exact numbers needed for wire sizing, conduit planning, and termination.
How to Read the AWG Diameter and Ampacity Table
The table below merges physical dimensions from ASTM B258 wire gauge standards with ampacity limits from NFPA 70 (NEC) Table 310.16.
Bookmark Quick-Jumps for Most Queried Sizes: Residential branch circuits heavily rely on 14, 12, and 10 AWG. Feeders and subpanels typically step up to 8, 6, 4, and 2 AWG. Service entrances often require 1/0 through 4/0 AWG.
| AWG Size | Bare Dia. (in) | Bare Dia. (mm) | Approx. THHN OD (in) | 60°C Amps | 75°C Amps | 90°C Amps |
|---|---|---|---|---|---|---|
| 14 | 0.0641 | 1.628 | 0.110 | 15 | 20 | 25 |
| 12 | 0.0808 | 2.053 | 0.125 | 20 | 25 | 30 |
| 10 | 0.1019 | 2.588 | 0.165 | 30 | 35 | 40 |
| 8 | 0.1285 | 3.264 | 0.210 | 40 | 50 | 55 |
| 6 | 0.1620 | 4.115 | 0.270 | 55 | 65 | 75 |
| 4 | 0.2043 | 5.189 | 0.330 | 70 | 85 | 95 |
| 3 | 0.2294 | 5.827 | 0.360 | 85 | 100 | 110 |
| 2 | 0.2576 | 6.543 | 0.400 | 95 | 115 | 130 |
| 1 | 0.2893 | 7.348 | 0.450 | 110 | 130 | 145 |
| 1/0 | 0.3249 | 8.252 | 0.500 | 125 | 150 | 170 |
| 2/0 | 0.3648 | 9.266 | 0.550 | 145 | 175 | 195 |
| 3/0 | 0.4096 | 10.404 | 0.600 | 165 | 200 | 225 |
| 4/0 | 0.4600 | 11.684 | 0.675 | 195 | 230 | 260 |
Which Column Applies to Your Installation (and What the Table Hides)
A common trap for DIYers and junior apprentices is looking at a spool of 12 AWG THHN wire, seeing the '90°C' printed on the jacket, and assuming they can run 30 amps through it based on the 90°C column. This is a code violation and a fire hazard. Here is how to correctly apply the ampacity columns and understand the physical limits of the data.
Which Temperature Column Applies?
Under NEC 110.14(C), your allowable ampacity is limited by the lowest temperature rating of any connected component in the circuit. Standard residential breakers, receptacles, and switches are typically rated for 60°C (for 14-10 AWG) or 75°C (for 8 AWG and larger). Even if your THHN wire is rated for 90°C, the termination hardware acts as a bottleneck. Therefore, a 12 AWG wire on a standard 20A residential breaker is limited to the 60°C column (20A). The 75°C column is standard for commercial panels and larger feeder lugs, while the 90°C column is almost exclusively used as the baseline for derating calculations.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to apply a derating factor. You always start your derating math using the 90°C column, because the insulation itself can handle the heat; it is the terminations that cannot.
Worked Example: You are pulling 8 current-carrying 12 AWG THHN conductors in one EMT conduit. The derating factor for 7-9 conductors is 70%.
- Base 90°C ampacity for 12 AWG = 30A.
- 30A × 0.70 = 21A derated ampacity.
- Because 21A is greater than the 60°C termination limit (20A), you must still protect the circuit with a 20A breaker. If you had 10 conductors (50% derating), 30A × 0.50 = 15A. You would then be forced to drop to a 15A breaker or upsize to 10 AWG wire.
What This Table Cannot Tell You
While this chart covers physical diameter and thermal limits, it omits three critical engineering variables:
- Voltage Drop: The diameter alone does not tell you the DC resistance per 1,000 feet. For long runs (over 100 feet), you must consult NEC Chapter 9, Table 8 to calculate voltage drop. A 12 AWG wire might be thermally safe at 20A, but it will suffer unacceptable voltage drop on a 150-foot run to a shed.
- Exact Conduit Fill: The 'Approx. THHN OD' is an industry average. Specific manufacturer spec sheets (like Cerrowire or Southwire) vary by fractions of an inch due to nylon coating thickness. Always check the manufacturer's exact OD when calculating conduit fill to avoid jamming wires in a pull box.
- Stranded vs. Solid: The bare diameters listed above are for solid round wire (ASTM B258). Stranded wire of the same AWG has a slightly larger bare diameter due to the air gaps and the helical lay of the copper strands.
Frequently Asked Questions About AWG Dimensions
Is 12 AWG diameter the same for solid and stranded wire?
No. While the total cross-sectional area of the copper is mathematically identical (approx. 6,530 circular mils for 12 AWG), the physical outside diameter of the bare stranded bundle is larger. A solid 12 AWG wire measures exactly 0.0808 inches. A standard Class B stranded 12 AWG wire measures roughly 0.092 inches across the bare strands. This larger bare diameter means stranded wire takes up slightly more physical space inside a conduit and requires different crimp lugs to avoid splaying the strands.
How do I calculate the exact AWG diameter for sizes not on this chart?
The American Wire Gauge system is logarithmic. The formula to find the diameter of any AWG size in inches is: Diameter = 0.005 × 92^((36 - AWG) / 39). For example, if you need the diameter of an unusually small 22 AWG hookup wire for an Arduino project, the formula yields 0.0253 inches. Note that this formula only applies to solid round wire; it breaks down for stranded, compacted, or compressed conductors used in heavy utility feeders.
Why does my stripped 10 AWG wire measure larger than 0.1019 inches on my calipers?
If your digital calipers read 0.105 inches or higher when measuring the bare copper of a 10 AWG wire, you are likely measuring stranded wire, or your caliper is not zeroed correctly. Another common bench error is measuring the wire with a burr left by a cheap wire stripper. Always strip a clean section, gently wipe away any residual dielectric grease or nylon coating, and measure the bare copper at a 90-degree angle to the wire's axis. If it is solid wire and consistently reads over 0.104 inches, you likely have a 9 AWG or an out-of-spec counterfeit spool.
Does the AWG diameter change if I use aluminum instead of copper?
The physical AWG diameter does not change; a 4 AWG aluminum wire has the exact same bare diameter (0.2043 inches) as a 4 AWG copper wire. The AWG standard defines the physical geometry, not the material. However, because aluminum has roughly 61% of the conductivity of copper, a 4 AWG aluminum wire will have a significantly lower ampacity and higher voltage drop than its copper counterpart. To carry the same current, you must upsize aluminum wire by one or two AWG steps compared to copper.
