AWG Wire Diameter Chart: NEC Ampacity & Sizing Guide

When sizing conductors for a branch circuit or feeder, guessing is not an option. The physical diameter of a wire dictates its cross-sectional area, which in turn determines its electrical resistance and heat dissipation capabilities. Below is the definitive AWG wire diameter chart cross-referenced with the ampacity limits established in the National Electrical Code (NEC). This reference provides the exact bare conductor diameters, circular mil areas, and temperature-rated ampacities you need to select the right wire for your panel, subpanel, or branch circuit.

How to Read the AWG Wire Diameter and Ampacity Chart

Before pulling wire, you must understand how to read the intersecting data points in the chart below. The data is sourced directly from NFPA 70 (NEC) Table 310.16 and standard ASTM B8 bare copper specifications.

• AWG Size: American Wire Gauge. Note that as the AWG number decreases, the physical wire diameter and ampacity increase.
• Diameter (Inches): The physical measurement of the bare metal conductor, excluding insulation. This is critical when calculating conduit fill.
• Area (kcmil): Thousands of circular mils. Used for larger feeder calculations and voltage drop formulas.
• Temperature Columns (60°C, 75°C, 90°C): These represent the maximum allowable ampacity based on the insulation type (e.g., TW, THHN, XHHW) and the temperature rating of the equipment terminations.

The Complete NEC Table 310.16 AWG Reference Chart

The following table lists the most commonly queried copper conductor sizes used in residential and light commercial wiring. All ampacity values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.

Which Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is defaulting to the 90°C column because THHN wire is rated for 90°C. According to NEC 110.14(C) termination temperature provisions, you must size the wire based on the lowest temperature rating of any connected device, terminal, or conductor in the circuit.

• Use the 60°C Column When: You are using NM-B (Romex) cable, UF-B cable, or wiring circuits rated 100A or less where the equipment termination temperature is unmarked. Most residential branch circuits fall here.
• Use the 75°C Column When: You are pulling individual THHN/THWN-2 conductors in conduit and landing them on breakers, lugs, or busbars explicitly marked '75°C' (which includes almost all modern commercial breakers and subpanel lugs).
• Use the 90°C Column When: You are performing ambient temperature or bundling derating calculations. The final derated ampacity must still be equal to or greater than the OCPD (breaker) size, but the starting point for the math is the 90°C column.

How Derating Modifies Your Base Ampacity

The ampacities in the chart above assume perfect conditions: 30°C (86°F) ambient air and no more than three current-carrying conductors bundled together. When you exceed these parameters, the wire cannot dissipate heat effectively, and you must apply derating factors.

Worked Example: Bundled Conductors in a Hot Attic
You are pulling four current-carrying 10 AWG THHN conductors through an attic where the ambient temperature reaches 40°C (104°F), feeding a 30A water heater.

1. Base Ampacity: 10 AWG in the 90°C column is 40A.
2. Bundling Derating: 4 to 6 current-carrying conductors requires an 80% multiplier (40A × 0.80 = 32A).
3. Temperature Derating: 40°C ambient requires a 0.94 multiplier for 90°C insulation (32A × 0.94 = 30.08A).
4. Final Result: 30.08A is greater than the 30A breaker. 10 AWG THHN is legally compliant.

If you had used NM-B cable instead of THHN, you would have started with the 60°C column (30A), and the derating would have dropped the ampacity below the breaker size, forcing you to upsize to 8 AWG.

Decision Path: Picking the Exact Wire for Your Circuit

Use this if-then decision tree to terminate your wire sizing process with a concrete purchase order. These picks assume standard 120V/240V residential copper wiring, 30°C ambient, and standard NEC breaker sizing.

What This Chart Cannot Tell You (And How to Fix It)

While this AWG wire diameter chart dictates thermal limits, it ignores two critical physical and electrical realities of your installation. Before finalizing your wire size, check these variables:

1. Voltage Drop Over Distance

The NEC recommends keeping voltage drop under 3% for branch circuits and 5% overall. The chart above does not account for distance. If you are running a 20A circuit 150 feet to a detached garage, 12 AWG wire will suffer a voltage drop exceeding 3%. The Fix: Use a voltage drop calculator. For a 120V, 20A load at 150 feet, you must upsize from 12 AWG to 8 AWG copper to maintain a 2.9% drop.

2. Conduit Fill Capacity

The bare diameter column tells you the size of the metal, but you must account for the insulation thickness when pulling through PVC or EMT. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. The Fix: If you are pulling four 6 AWG THHN wires, a 3/4-inch EMT conduit is only rated for 39% fill. It will physically fit, but pulling it will be a nightmare and risks damaging the insulation. Upsize to 1-inch EMT for a manageable 22% fill.