The Ultimate AWG Size Chart PDF Guide: Ampacity, Derating, and NEC Columns

When you search for an AWG size chart PDF, you are almost always looking for the allowable ampacity table defined in NEC Article 310.16. This table dictates the maximum continuous current a copper or aluminum conductor can carry before its insulation degrades or it becomes a fire hazard. However, simply printing the chart and matching your breaker to the highest number on the page is a fast track to a failed inspection or a melted terminal. The correct wire size depends on insulation type, termination temperature ratings, conduit fill, and ambient heat.

How to Read the AWG Size Chart (and Which Column Applies to You)

A standard copper AWG size chart is divided into three primary temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These columns correspond to the thermal rating of the wire's insulation (e.g., TW, THWN, THHN). To use the chart correctly, you must understand which column governs your specific installation.

Which column applies to your installation? The governing rule is found in NEC 110.14(C), which states that the ampacity of a circuit is limited by the lowest temperature rating of any connected device, termination, or conductor.

• The 60°C Column: Use this for circuits rated 100A or less if the termination temperature is unknown, or when using NM-B (Romex) cable. Even though the individual conductors inside NM-B have 90°C insulation, NEC 334.80 strictly limits the assembly's ampacity to the 60°C column.
• The 75°C Column: Use this for most modern residential branch circuits over 100A, and for individual THWN wires in conduit terminating on standard 75°C-rated breakers and receptacles.
• The 90°C Column: You almost never use this column to size your final breaker. Its primary purpose is to serve as the starting point for calculating derating factors in hot environments or packed conduits.

The Complete Copper Ampacity Table (NEC 310.16)

The table below provides the baseline allowable ampacities for copper conductors with up to three current-carrying conductors in a raceway, based on an ambient temperature of 86°F (30°C). This data aligns with standard manufacturer references like the Cerro Wire ampacity tables.

Derating Factors and What the Chart Cannot Tell You

The baseline numbers in the table above assume ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors (CCCs) bundled together. When real-world jobsite conditions deviate from this, you must apply adjustment factors outlined in NEC 310.15.

How Derating Modifies the Base Value

When you bundle 4 to 6 CCCs in a single conduit, you must multiply the base ampacity by 80%. For 7 to 9 CCCs, the factor drops to 70%. Crucially, derating calculations always start with the 90°C column, regardless of your termination limits.

Worked Example: You are pulling four 12 AWG THHN circuits (8 CCCs total) through a single EMT conduit to a subpanel.
1. Find the 90°C base value for 12 AWG: 30A.
2. Apply the 70% derating factor for 8 CCCs: 30A × 0.70 = 21A.
3. Compare to the termination limit (75°C column for 12 AWG is 25A). Because 21A is less than 25A, the derated wire governs. You must protect this wire with a 20A breaker, not a 25A breaker. If the math had dropped the derated ampacity to 18A, you would be forced to upsize to 10 AWG wire to maintain a 20A circuit.

What the AWG Chart Cannot Tell You

Relying solely on an AWG size chart PDF leaves two massive blind spots in your design:

1. Voltage Drop: The NEC ampacity tables do not account for distance. A 12 AWG wire is rated for 20A whether it is 10 feet long or 200 feet long. However, pushing 20A through 200 feet of 12 AWG copper will result in a voltage drop exceeding 8%, causing motors to overheat and lights to dim. For runs over 100 feet, you must calculate voltage drop (target <3% for branch circuits) and upsize the wire accordingly.
2. Conduit Fill Capacity: The chart tells you how much current the wire can handle, but NEC Chapter 9, Table 1 dictates how many wires physically fit inside a conduit. You cannot stuff six 6 AWG THHN wires into a 1/2-inch EMT conduit, even if the derating math technically allows it. Physical jamming and heat dissipation limits require you to check conduit fill percentages (max 40% for 3+ wires).