When you are pulling wire for a new branch circuit or sizing a feeder for a subpanel, guessing the physical thickness or current-carrying capacity of a conductor is not an option. The American Wire Gauge (AWG) system is logarithmic and counterintuitive: the smaller the AWG number, the thicker the wire. A 10 AWG wire is significantly thicker than a 14 AWG wire, and mixing them up on a jobsite can lead to overheated terminations, melted insulation, or a failed inspection.
This reference provides the exact physical dimensions and thermal limits for standard building wire. Bookmark this page and use the quick-jump highlights below to find the most common residential and commercial sizes instantly.
The Master Wire Gauge and Diameter Chart
How to read this table: The physical diameters are sourced from ASTM B258 standards for solid and stranded copper. The ampacity (current-carrying capacity) columns are derived directly from NFPA 70 (National Electrical Code) Table 310.16. These ampacity values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. The bolded rows are the most frequently queried sizes for standard residential branch circuits and feeders.
| AWG Size | Diameter (Inches) | Diameter (mm) | Copper 60°C (Amps) | Copper 75°C (Amps) | Copper 90°C (Amps) | Aluminum 75°C (Amps) |
|---|---|---|---|---|---|---|
| 14 AWG | 0.0641 | 1.628 | 15 | 20 | 25 | -- |
| 12 AWG | 0.0808 | 2.053 | 20 | 25 | 30 | -- |
| 10 AWG | 0.1019 | 2.588 | 30 | 35 | 40 | -- |
| 8 AWG | 0.1285 | 3.264 | 40 | 50 | 55 | 40 |
| 6 AWG | 0.1620 | 4.115 | 55 | 65 | 75 | 50 |
| 4 AWG | 0.2043 | 5.189 | 70 | 85 | 95 | 65 |
| 3 AWG | 0.2294 | 5.827 | 85 | 100 | 110 | 75 |
| 2 AWG | 0.2576 | 6.544 | 95 | 115 | 130 | 90 |
| 1 AWG | 0.2893 | 7.348 | 110 | 130 | 145 | 100 |
| 1/0 AWG | 0.3249 | 8.252 | 125 | 150 | 170 | 120 |
| 2/0 AWG | 0.3648 | 9.266 | 145 | 175 | 195 | 135 |
| 3/0 AWG | 0.4096 | 10.404 | 165 | 200 | 225 | 155 |
| 4/0 AWG | 0.4600 | 11.684 | 195 | 230 | 260 | 180 |
Which Column Applies and How Derating Modifies the Base Value
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because it offers the highest ampacity, and then sizing their breaker based on that number. This will result in a failed inspection and a potential fire hazard. Here is how to determine which column actually governs your installation.
Selecting the Correct Temperature Column
- 60°C Column: Use this for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) and any circuit rated 100 Amps or less where the termination temperature rating is unknown. Most standard residential receptacles and switches are only rated for 60°C terminations.
- 75°C Column: Use this for individual THHN/THWN conductors pulled through conduit, provided the lugs on your breakers, panels, and devices are explicitly stamped with a 75°C rating. This is the standard for most commercial work and residential feeders over 100A.
- 90°C Column: This column is almost never used for final overcurrent protection sizing. It exists primarily as a starting point for calculating derating adjustments when you bundle multiple wires together.
How Derating Rows Modify the Base Value
When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Section 310.15(C)(1) requires you to apply an adjustment factor (derating) to the 90°C column ampacity.
Worked Example: You are pulling four current-carrying 12 AWG THHN wires through an EMT conduit to feed two 20A circuits.
- Look at the 90°C column for 12 AWG: 30 Amps.
- Check the derating table for 4-6 conductors: The adjustment factor is 80%.
- Multiply: 30A × 0.80 = 24 Amps.
- Compare to termination limits: Your 20A breaker lugs are rated 75°C (max 25A for 12 AWG) or 60°C (max 20A). Since 24A is greater than the 20A termination limit, the wire is still legally permitted to be protected by a 20A breaker. However, if you had nine conductors (derated to 70%, or 21A), you would still be fine for a 20A breaker, but you are getting dangerously close to the thermal limit of the wire itself.
For a deeper dive into specific bundling scenarios, the Cerrowire Ampacity Charts provide excellent visual matrices for these adjustment factors.
What This Chart Cannot Tell You (And How to Fix It)
A wire gauge and diameter chart is a thermal limit reference, not a complete design tool. It assumes a perfect, short-run installation. If you rely solely on this table, you will run into two major physical limitations on the jobsite.
1. Voltage Drop Over Distance
This chart assumes the wire run is short enough that resistance is negligible. Copper has resistance, and over long distances, that resistance steals voltage from your load. The NEC recommends (via informational notes) a maximum voltage drop of 3% for branch circuits.
The Math: Suppose you are wiring a 120V receptacle in a detached garage, 150 feet from the panel, and you plan to run a 15A space heater. You choose 14 AWG wire because the chart says it is rated for 15A.
The resistance of 14 AWG copper is roughly 2.525 ohms per 1,000 feet. Because current must travel to the garage and back, your total wire length is 300 feet.
- Total Resistance = (300 / 1000) × 2.525 = 0.7575 ohms.
- Voltage Drop = 15A × 0.7575 ohms = 11.36 Volts.
- Percentage Drop = (11.36 / 120) × 100 = 9.4%.
Your space heater is only receiving 108V, which will cause it to underperform and draw excess current to compensate, potentially overheating the motor. The Fix: You must upsize the wire. Moving to 10 AWG drops the resistance to 0.999 ohms/1000ft, resulting in a 4.5V drop (3.7%)—still slightly high, so a short-run 8 AWG or a dedicated 240V circuit is the proper engineering solution. Tools like the Engineering Toolbox Wire Gauges calculator can automate this math.
2. Conduit Fill Capacity
The chart tells you how thick the wire is, but it does not tell you how many of those wires will physically fit inside a conduit. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more conductors to prevent jamming during the pull and to allow for heat dissipation.
For example, you cannot stuff five 10 AWG THHN wires into a 1/2-inch EMT conduit, even if the ampacity math works out. The physical diameter of the insulation (which is thicker than the bare copper diameter listed in the chart above) will exceed the 40% fill limit, and the wires will bind and tear when you try to pull them. Always cross-reference your wire diameter with a conduit fill table before buying your raceway.
3. Terminal Physical Constraints
Finally, the chart cannot tell you if the wire will physically fit under the lug of your breaker or device. While 4 AWG copper is rated for 85A at 75°C, many standard 90A breakers only accept up to 6 AWG or 4 AWG stranded, and will reject 4 AWG solid wire. Always check the manufacturer's datasheet for the specific termination torque and wire-type limitations before cutting your conductors.
