The exact bare diameter of a solid 1 AWG copper wire is 0.2893 inches (7.348 mm). However, in practical electrical installations, you are almost always working with stranded wire and insulation. A standard Class B stranded 1 AWG copper conductor has a bare diameter of roughly 0.305 inches, and when wrapped in standard THHN/THWN-2 insulation, the overall outside diameter increases to 0.446 inches (11.33 mm).

Whether you are sizing a subpanel feeder, calculating conduit fill, or selecting the correct breaker, relying on bare wire dimensions will lead to installation failures. Below is the complete reference data you need to pull, terminate, and protect 1 AWG wire correctly.

How to Read the 1 AWG Wire Specification Table

Before jumping to the ampacity numbers, it is critical to understand how to read the temperature columns in the table below, which is sourced directly from the NFPA 70 National Electrical Code (NEC) Table 310.16 and Chapter 9 Table 5.

Which column applies to your installation? Most modern THHN and XHHW-2 wire features a 90°C insulation rating, but you cannot simply use the 90°C ampacity column to size your breaker. NEC 110.14(C) dictates that for circuits rated 100A or more, the termination points (breaker lugs and panel bus bars) are generally rated for 75°C. Therefore, the 75°C column governs your final overcurrent protection size. The 90°C column is strictly used as your starting baseline for calculating derating adjustments before you apply the 75°C termination limit.

1 AWG Wire Dimensions and Ampacity Reference (NEC 2023)

Material Insulation Type Temp Rating Bare Diameter Insulated Diameter Ampacity
Copper THHN / THWN-2 90°C 0.305 in (Stranded) 0.446 in 145A
Copper THW / THWN 75°C 0.305 in (Stranded) 0.460 in 130A
Copper XHHW-2 90°C 0.305 in (Stranded) 0.415 in 145A
Aluminum THHN / THWN-2 90°C 0.374 in (Stranded) 0.515 in 115A
Aluminum XHHW / THW 75°C 0.374 in (Stranded) 0.490 in 100A

Source: NEC Table 310.16 (Ampacities) and NEC Chapter 9, Table 5 (Dimensions of Insulated Conductors). Bare diameters reference Class B stranding per Engineering Toolbox AWG standards.

Applying Derating and Installation Adjustments

The ampacity values in the table above assume a standard ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a raceway. When your installation deviates from this, you must apply adjustment factors from NEC 310.15.

How derating rows modify the base value: Let us say you are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced load, and a ground—which is not counted) through a conduit in an attic where the ambient temperature reaches 40°C (104°F).

  1. Start with the 90°C column: 1 AWG Copper THHN = 145A.
  2. Apply bundling derating: 4 conductors requires an 80% adjustment factor. (145A × 0.80 = 116A).
  3. Apply temperature derating: 40°C ambient requires a 0.91 correction factor. (116A × 0.91 = 105.5A).
  4. Check termination limits: Your calculated derated ampacity is 105.5A. Because your breaker lugs are rated 75°C, the maximum allowable ampacity from the 75°C column is 130A. Since 105.5A is lower than 130A, the derated value governs.
  5. Final Breaker Sizing: You must protect this wire with a breaker rated at or below 105.5A. The next standard breaker size down is 100A.
⚠️ What This Table Cannot Tell You: Ampacity tables only address thermal limits (preventing the insulation from melting). They do not account for voltage drop. 1 AWG copper has a resistance of roughly 0.154 ohms per 1,000 feet. If you are running a 100A load over 200 feet, you will experience a voltage drop of roughly 3.08V on a 240V circuit (about 1.2%). While acceptable, if you push that same 1 AWG wire to 300 feet for a heavy motor load, the voltage drop will exceed the recommended 3% limit, causing motor overheating and inefficiency. Always run a voltage drop calculation for feeder runs exceeding 100 feet. For detailed voltage drop formulas, consult the Cerrowire Voltage Drop Calculator.

1 AWG Wire Frequently Asked Questions

What is the exact 1 AWG wire diameter in inches and millimeters?

The physical diameter depends entirely on the stranding class and insulation. A solid bare 1 AWG copper wire is exactly 0.2893 inches (7.35 mm). However, solid 1 AWG is exceptionally stiff and rarely used in modern branch or feeder wiring. Standard Class B stranded bare 1 AWG copper measures 0.305 inches (7.75 mm). If you are using highly flexible Class K or Class M stranding (common in welding cable or marine battery banks), the bare diameter can swell to 0.330 inches (8.38 mm) due to the air gaps between the finer strands. Always measure your specific wire with calipers before cutting conduit.

Can I use 1 AWG aluminum wire for a 100-amp subpanel feeder?

Yes, 1 AWG aluminum is the exact minimum size required for a 100-amp subpanel feeder, provided you use insulation rated for 75°C or higher, such as XHHW-2 or THWN-2. According to the 75°C column in NEC Table 310.16, 1 AWG aluminum is rated for exactly 100 amps. When installing aluminum feeders, you must apply an anti-oxidant compound (like Noalox) to the stripped conductor ends before termination to prevent galvanic corrosion, and you must torque the breaker and bus bar lugs to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver or wrench.

How many 1 AWG THHN wires fit in a 1-inch PVC conduit?

According to NEC Chapter 9, Table 1, you can fill a conduit to a maximum of 40% capacity when pulling three or more conductors. A standard 1-inch Schedule 40 PVC conduit has a total internal area of 0.887 square inches, making the 40% fill limit 0.355 square inches. A single 1 AWG THHN wire has a cross-sectional area of 0.1562 square inches. Dividing 0.355 by 0.1562 yields 2.27. Because NEC Chapter 9, Note 6 states that you must drop the decimal and round down, you can fit a maximum of two 1 AWG THHN wires in a 1-inch Schedule 40 PVC conduit. If you need to pull three conductors (e.g., two hots and a neutral), you must step up to a 1.25-inch PVC conduit.

Does the insulation thickness change the 1 AWG wire diameter for conduit fill?

Yes, the insulation type drastically changes the overall diameter and cross-sectional area, which directly impacts conduit fill calculations. While 1 AWG THHN has an overall diameter of 0.446 inches (0.1562 sq in area), 1 AWG XHHW-2 features a thinner, cross-linked polyethylene insulation. The overall diameter of 1 AWG XHHW-2 is only 0.415 inches, yielding a cross-sectional area of 0.1353 square inches. If you are pulling wire through a conduit with multiple tight 90-degree sweeps, swapping from THHN to XHHW-2 reduces the physical footprint by roughly 13%, making the pull significantly easier and occasionally allowing you to stay in a smaller trade-size conduit.