When sizing conductors for a branch circuit or feeder, the physical diameter of the wire dictates its cross-sectional area, which in turn governs both its DC resistance and its thermal ampacity. In North America, we rely on the American Wire Gauge (AWG) system, while most of the world uses metric cross-sectional area (mm²). Confusing the two—or misreading the insulation thickness as the conductor diameter—is a fast track to a failed inspection or a melted terminal lug.
This reference provides the exact bare conductor dimensions, cross-sectional areas, and baseline ampacities you need for copper wiring. Bookmark this page for quick bench and jobsite lookups.
How to Read This Diameter of Wire Chart
Before pulling wire, you need to understand exactly what the columns in the chart below represent, as misapplying them is a common code violation.
- AWG Size: The standard North American gauge. Note the inverse relationship: as the AWG number increases, the physical diameter decreases.
- Diameter (in / mm): This is the bare conductor diameter. It does not include insulation. If you are calculating conduit fill, you must use the overall diameter (insulation included) found in NEC Chapter 9, Table 5.
- Area (kcmil / mm²): The actual cross-sectional area of the copper. This is the metric used for voltage drop calculations and international metric equivalents.
- Ampacity (75°C Copper): This column reflects the 75°C temperature rating from NEC Table 310.16. Which column applies to you? For 95% of residential and light commercial terminations (breakers, receptacles, lugs), the equipment is rated for 75°C. Even if you pull 90°C THHN wire, your final allowable ampacity is capped by the 75°C termination rating of the device it connects to.
The Complete AWG and Metric Diameter of Wire Chart
Source Standard: Physical dimensions sourced from NFPA 70 (NEC) Chapter 9, Table 8. Ampacity sourced from NEC Table 310.16 (75°C column, copper, not more than three current-carrying conductors in a raceway, ambient 30°C).
Quick-Jump Bookmarks: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG | 1/0 AWG | 4/0 AWG
| AWG Size | Diameter (inches) | Diameter (mm) | Area (kcmil) | Area (mm²) | Ampacity (75°C Cu) |
|---|---|---|---|---|---|
| 14 | 0.0641 | 1.628 | 4,110 | 2.08 | 15A * |
| 12 | 0.0808 | 2.052 | 6,530 | 3.31 | 20A |
| 10 | 0.1019 | 2.588 | 10,380 | 5.26 | 30A |
| 8 | 0.1285 | 3.264 | 16,510 | 8.37 | 40A |
| 6 | 0.1620 | 4.115 | 26,240 | 13.30 | 65A |
| 4 | 0.2043 | 5.189 | 41,740 | 21.15 | 85A |
| 3 | 0.2294 | 5.827 | 52,620 | 26.67 | 100A |
| 2 | 0.2576 | 6.543 | 66,360 | 33.62 | 115A |
| 1 | 0.2893 | 7.348 | 83,690 | 42.40 | 130A |
| 1/0 | 0.3249 | 8.252 | 105,600 | 53.49 | 150A |
| 2/0 | 0.3648 | 9.266 | 133,100 | 67.43 | 175A |
| 3/0 | 0.4096 | 10.404 | 167,800 | 85.01 | 200A |
| 4/0 | 0.4596 | 11.674 | 211,600 | 107.20 | 230A |
* Note: While 14 AWG copper has a physical ampacity of 15A, NEC 240.4(D) strictly limits 14 AWG to 15A overcurrent protection, 12 AWG to 20A, and 10 AWG to 30A for specific small conductor applications.
Applying Derating Factors to Your Base Values
The ampacity column above assumes you have no more than three current-carrying conductors in a raceway or cable, and an ambient temperature of 30°C (86°F). When you bundle wires together, they cannot dissipate heat as efficiently. This is where NEC Table 310.15(C)(1) adjustment factors come into play.
How derating modifies the base value: You multiply the wire's base ampacity by the derating percentage. However, there is a critical nuance regarding which temperature column you use for the math.
Worked Example: You are pulling four 10 AWG THHN current-carrying conductors through a single conduit to feed a 240V water heater and a 120V pump.
- Identify the base 90°C ampacity: 10 AWG THHN at 90°C is rated for 40A.
- Apply the derating factor: Four conductors require an 80% adjustment factor (0.80).
- Calculate derated ampacity: 40A × 0.80 = 32A.
- Check termination limits: The breaker lugs are rated 75°C. The 75°C ampacity for 10 AWG is 30A.
- Final Verdict: You must use the lower of the two numbers. The wire is legally limited to 30A. You must protect it with a 30A breaker.
What This Chart Cannot Tell You
A diameter of wire chart is a thermal and physical reference, not a complete design tool. Relying on it blindly will leave you blind to three critical installation variables:
- Voltage Drop Over Distance: A 12 AWG wire is rated for 20A, but if you run it 150 feet to a 16A table saw, the voltage drop will exceed the recommended 3% limit. The motor will run hot and trip its internal thermal overload. For long runs, you must calculate voltage drop using the ohms-per-thousand-feet values found in NEC Chapter 9, Table 8, and likely upsize to 10 AWG or 8 AWG.
- Conduit Fill Capacity: The bare diameters listed above do not account for insulation. If you are pulling multiple wires through EMT or PVC, you must use the overall diameter (including insulation) to ensure you do not exceed the NEC 40% conduit fill limit.
- Short-Circuit Withstand Ratings: The chart tells you what the wire can carry continuously without melting the insulation. It does not tell you how long the wire can survive a massive short-circuit fault before the breaker clears it. That requires coordination with the breaker's let-through current (I²t) ratings.
Frequently Asked Questions
What is the exact diameter of 12 AWG solid copper wire?
The bare conductor diameter of 12 AWG solid copper wire is exactly 0.0808 inches (2.052 mm). If you are measuring a stranded 12 AWG wire, the overall diameter of the bare copper bundle will be slightly larger (typically around 0.090 inches) due to the air gaps between the individual strands, even though the total cross-sectional copper area remains identical.
How do I convert AWG to mm² for European appliance wiring?
Do not attempt to convert the physical diameter directly to mm² using standard geometry (πr²), as stranded wire includes air gaps that will skew your math. Instead, look at the Area (mm²) column in the chart above. For example, 12 AWG is 3.31 mm². In Europe, the closest standard metric equivalent wire size is 4.0 mm², which is physically larger and safely exceeds the current capacity of 12 AWG.
Does the insulation thickness change the AWG diameter of wire chart values?
No. The AWG system and the diameters listed in this chart strictly define the bare copper conductor. Insulation thickness is entirely separate and varies by cable type. For instance, a 12 AWG THHN wire has a much thinner insulation jacket than a 12 AWG UF-B (underground feeder) cable, but the bare copper inside both measures exactly 0.0808 inches across.
Why is my measured wire diameter slightly different from the chart?
If your digital calipers read 0.082 inches instead of the chart's 0.0808 inches for 12 AWG, you are likely measuring a stranded wire rather than a solid wire, or you are measuring slightly off-axis. Additionally, manufacturing tolerances allow for minor deviations. If your measurement is wildly off (e.g., measuring 0.064 inches for what you thought was 12 AWG), you have likely misidentified the wire and are actually holding 14 AWG. Always verify by checking the printed text on the cable jacket.
