The Complete AC Ampacity Chart: NEC Table 310.16 Wire Sizing Guide

When sizing conductors for alternating current (AC) circuits, the definitive reference is the AC ampacity chart, formally published as NEC Table 310.16 in the National Electrical Code. For the most common residential branch circuits using copper wire, the baseline ampacities (on the 60°C column) are: 14 AWG = 15A, 12 AWG = 20A, and 10 AWG = 30A. However, pulling a single number from this chart without understanding temperature columns, terminal ratings, and derating factors is the leading cause of overheated lugs and failed inspections. This reference guide provides the exact data rows you need, explains which column legally applies to your specific installation, and details how environmental factors modify the base values.

The Master AC Ampacity Chart (NEC Table 310.16)

Before using the table below, you must understand how to read its columns. The chart is divided by conductor material (Copper vs. Aluminum) and by insulation temperature rating (60°C, 75°C, and 90°C). The values represent the maximum continuous current a single, isolated conductor can carry in an ambient temperature of 30°C (86°F) before its insulation begins to degrade. This data is sourced directly from the NFPA National Electrical Code (NEC), specifically Table 310.16 (2020/2023 editions).

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is sizing a breaker using the 90°C column because the wire jacket says "THHN" (which is rated for 90°C). This violates NEC 110.14(C), which governs terminal temperature limitations. The rule dictates that the ampacity of your wire cannot exceed the temperature rating of the equipment terminals it connects to.

Here is how to determine which column governs your final overcurrent protection sizing:

• The 60°C Column: Mandatory for circuits rated 100A or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment is specifically listed and marked for 75°C. Because standard residential receptacles, switches, and smaller breakers are rarely marked with a specific temperature rating, the code defaults you to 60°C for these smaller wires. Furthermore, NM-B cable assemblies are legally capped at 60°C ampacities by NEC 334.80.
• The 75°C Column: Used for circuits over 100A, or conductors larger than 1 AWG, provided the termination equipment (like a main panel lug or a heavy-duty disconnect) is rated for 75°C. Most modern commercial breakers and panelboard lugs carry a 75°C rating.
• The 90°C Column: This column is almost never used to determine the final breaker size. Its primary purpose is to serve as the starting baseline for derating calculations (explained below). You calculate your losses from the 90°C baseline, but your final adjusted ampacity still cannot exceed the 60°C or 75°C limit of the termination point.

How Derating Modifies the Base Ampacity Values

The values in the AC ampacity chart assume ideal conditions: a single conductor in free air or a maximum of three current-carrying conductors in a raceway, at an ambient temperature of 30°C (86°F). Real-world jobsites rarely match these conditions. When conditions worsen, you must apply derating factors found in Electrical Contractor Magazine's code breakdowns and NEC Tables 310.15(B)(1) and 310.15(C)(1).

1. Ambient Temperature Correction

If your conduit runs through an attic in the summer, the ambient temperature might be 50°C (122°F). You must multiply the base ampacity by a correction factor. For a 90°C rated wire at 50°C ambient, the correction factor is 0.82.
Example: A 6 AWG Copper THHN wire has a 90°C base ampacity of 75A. Multiplied by 0.82, the temperature-adjusted ampacity drops to 61.5A.

2. Conductor Bundling (More Than Three Current-Carrying Conductors)

When you pull four or more current-carrying conductors in a single conduit, they heat each other up. If you have 4 to 6 conductors, you must multiply the 90°C base ampacity by 80%. If you have 7 to 9 conductors, the multiplier drops to 70%.

The Derating Decision Path (Worked Example)

Imagine you are pulling four 12 AWG THHN copper conductors through a conduit in a 40°C (104°F) garage to feed a 240V double-pole 20A breaker.

1. Start at 90°C: The base ampacity for 12 AWG at 90°C is 30A.
2. Apply Ambient Correction (40°C): The factor for 90°C wire at 40°C is 0.96. (30A × 0.96 = 28.8A).
3. Apply Bundling Derating (4 conductors): The factor is 80%. (28.8A × 0.80 = 23.04A).
4. Apply NEC 110.14(C) Termination Rule: Your breaker terminals are rated 75°C, but because the wire is 12 AWG (under the 100A/1 AWG threshold), you are forced to cap the final allowable ampacity at the 60°C column value, which is 20A.

Since 20A (the termination limit) is lower than 23.04A (the derated wire capacity), the wire is legally protected by a 20A breaker. If the math had resulted in a derated value of 18A, you would be forced to upsize to 10 AWG wire, because a standard 15A breaker might not accommodate the load requirements of the circuit.

What the AC Ampacity Chart Cannot Tell You

While Table 310.16 is the bible for thermal limits, relying on it exclusively will lead to design failures in three specific areas that the chart completely ignores.

Always use the AC ampacity chart to establish your thermal baseline and ensure overcurrent protection coordination, but verify voltage drop and physical conduit fill before purchasing your wire. Local Authority Having Jurisdiction (AHJ) inspectors will check your termination temperature compliance and derating math, so keep your calculation notes on hand during rough-in inspections.