The physical bare wire diameter for standard American Wire Gauge (AWG) ranges from 0.0641 inches (14 AWG) to 0.4600 inches (4/0 AWG) for solid copper. However, selecting the right wire is not just about physical thickness; matching the physical diameter to the correct ampacity requires reading the 60°C, 75°C, or 90°C columns of the National Electrical Code (NEC) based on your termination ratings, conductor count, and ambient conditions.

This reference guide provides the exact physical dimensions and allowable ampacities for copper conductors, giving you the data needed to size branch circuits, feeders, and equipment grounding conductors without a second trip to the supply house.

The Master Wire Diameter AWG Reference Table

How to read this table: The physical dimensions (Diameter and Area) are sourced from NEC Chapter 9, Table 8. The diameter listed is for the bare solid conductor; stranded wire of the same AWG will have a slightly larger overall diameter due to the air gaps between the strands. The ampacity values are sourced from NEC Table 310.16 (formerly 310.15(B)(16)) and assume copper conductors in a raceway or cable, with a maximum of three current-carrying conductors and an ambient temperature of 30°C (86°F). For a deeper look at the physical properties of copper wire, the Engineering Toolbox AWG reference provides excellent supplemental metric conversions.

Table 1: Copper Wire Diameter AWG and NEC Ampacity (Table 310.16)
AWG Size Bare Diameter (inches) Cross-Section Area (kcmil) 60°C Ampacity (A) 75°C Ampacity (A) 90°C Ampacity (A)
140.06414.11152025
120.08086.53202530
100.101910.4303540
80.128516.5405055
60.162026.2556575
40.204341.7708595
30.229452.685100110
20.257666.495115130
10.289383.7110130145
1/00.3249106125150170
2/00.3648133145175195
3/00.4096168165200225
4/00.4600212195230260
Quick-Jump Bookmarking: For standard residential branch circuits, you will almost exclusively use 14 AWG (15A lighting), 12 AWG (20A receptacles), and 10 AWG (30A dryers/HVAC). For subpanel feeders, 4 AWG, 2 AWG, and 1/0 AWG are the most common copper sizes.

Which Ampacity Column Applies to Your Installation?

A common mistake on the jobsite is looking at the 90°C column because modern THHN/THWN-2 wire is rated for 90°C, and then sizing the breaker based on that higher number. This violates NEC 110.14(C), which dictates that the ampacity of the circuit is limited by the lowest temperature rating of any connected termination, device, or conductor.

Here is how to determine which column governs your installation:

  • The 60°C Column: Use this for circuits rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment is specifically marked otherwise. Standard residential NM-B (Romex) cable is strictly limited to the 60°C column, regardless of the fact that the individual conductors inside might have 90°C insulation. Standard 15A and 20A receptacles also typically default to 60°C terminations.
  • The 75°C Column: Use this for circuits rated over 100 amps, or for conductors sized larger than 1 AWG. Most modern commercial breakers, lugs in load centers, and heavy-duty appliances (like electric ranges and EV chargers) are rated for 75°C terminations.
  • The 90°C Column: You can only use this column for derating purposes (adjusting for ambient temperature or conduit fill). The final derated ampacity must still be compared to the 60°C or 75°C column, and the lower value must be used to size your overcurrent protection.
Safety Caveat: Never size a breaker larger than the 60°C or 75°C ampacity limit of your wire, even if the wire's 90°C insulation can technically handle the heat. The breaker protects the termination lugs from melting, not just the wire insulation.

How Derating Modifies Your Base AWG Values

The ampacities in the table above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled in a single raceway or cable. When you deviate from these baselines, you must apply correction and adjustment factors that effectively reduce the allowable ampacity of your chosen wire diameter.

Conduit Fill Adjustment (NEC Table 310.15(C)(1)):
When you pull multiple circuits through a single conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors in a conduit, you must multiply the base ampacity by 80%. If you have 7 to 9 conductors, you multiply by 70%.

Worked Example: You are pulling four 12 AWG THHN wires (two hot, two neutral for two separate 20A circuits) through a single EMT conduit. Because you have 4 CCCs, you must apply the 80% derating factor. You are allowed to use the 90°C column for this math: 30A × 0.80 = 24A. Since 24A is greater than the 20A breaker protecting the circuit, 12 AWG is perfectly legal. However, if you pulled nine 12 AWG CCCs in that same conduit, the derating drops to 50% (30A × 0.50 = 15A). You would be forced to upsize to 10 AWG wire to maintain a 20A circuit.

Ambient Temperature Correction (NEC Table 310.16 Correction Factors):
If you are routing wire through a hot attic in the summer where temperatures reach 50°C (122°F), you must multiply the 90°C base ampacity by 0.82. A 10 AWG THHN wire (40A at 90°C) derates to 32.8A. It can still safely protect a 30A circuit, but an 8 AWG wire would be required for a 40A circuit in that environment.

What This Wire Diameter AWG Table Cannot Tell You

While this chart provides the physical dimensions and thermal limits of the copper, it does not account for the physical and electrical realities of long-distance runs and hardware constraints. Keep these three blind spots in mind before finalizing your materials list:

1. Voltage Drop Over Distance

The NEC ampacity tables do not penalize you for distance. A 12 AWG wire is rated for 20 amps whether it is 10 feet long or 200 feet long. However, at 200 feet, a 16A continuous load on a 120V circuit will suffer a voltage drop of roughly 5.4%, exceeding the NEC recommended 3% maximum for branch circuits (NEC 310.15(B) Informational Note). For long runs to detached garages, well pumps, or RV pedestals, you must calculate voltage drop using the circular mil area (kcmil) from the table and upsize the wire diameter accordingly, often jumping two or three AWG sizes larger than the ampacity table requires.

2. Physical Lug and Conduit Fit

A 4/0 AWG wire has a bare diameter of 0.46 inches, and with THHN insulation, the overall diameter exceeds 0.5 inches. Standard 100A or 125A residential panelboard main lugs are often physically incapable of accepting a 4/0 conductor. Furthermore, bending radius requirements for large wire diameters mean you cannot stuff 4/0 wire into a shallow 4-inch square junction box. Always check the manufacturer's spec sheet for the terminal lug's maximum wire acceptance range before purchasing large-gauge feeders.

3. Aluminum vs. Copper Equivalencies

This table is strictly for copper. If you are pricing out a 200-amp service feeder and decide to use SER (Service Entrance Rated) aluminum cable to save money, you cannot use this chart. Aluminum has higher resistance and requires a larger physical diameter to carry the same current. For example, while 2/0 AWG copper is rated for 175A (75°C column), you must use 4/0 AWG aluminum to achieve that same 175A rating. Always verify the conductor material printed on the cable jacket and consult the aluminum columns in NEC Table 310.16.