Amperage to Wire Size Chart: NEC 310.16 Copper & Aluminum Ampacity Guide

The correct wire gauge for a specific breaker is not a guessing game; it is strictly governed by the National Electrical Code (NEC). This amperage to wire size chart is based directly on NEC Table 310.16 (2023/2026 editions). The baseline assumptions for the chart below are: copper or aluminum conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a raceway or cable.

The Master Amperage to Wire Size Chart (NEC Table 310.16)

How to read this table: The NEC publishes ampacity across three temperature columns (60°C, 75°C, and 90°C) because different insulation types and termination points handle heat differently. The 90°C column represents the raw thermal limit of modern THHN/THWN-2 wire. The 75°C column represents the standard termination rating for most modern breakers and lugs. The 60°C column is a mandatory conservative baseline for smaller circuits. Bookmark the quick-jump rows below for the most common residential and light-commercial branch circuits.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they bought 90°C-rated THHN wire. You cannot use the 90°C column for sizing your breaker. According to NEC 110.14(C) termination rules, the ampacity of a circuit is limited by the lowest temperature rating of any connected component, which includes the breaker terminals, lugs, and splices.

Here is the exact breakdown of which column to use for your final ampacity pick:

• 60°C Column: Mandatory for 14 AWG, 12 AWG, and 10 AWG circuits (up to 100A nominal, but practically restricted to 30A max due to wire size). Even if you pull 90°C THHN for a 20A receptacle circuit, NEC 240.4(D) strictly limits 12 AWG copper to a 20A breaker and 10 AWG to a 30A breaker.
• 75°C Column: The standard for 8 AWG and larger conductors. Almost all modern breakers, panel lugs, and heavy-duty receptacles (like NEMA 14-50) are rated for 75°C. Use this column to size feeders, subpanels, and large appliance circuits.
• 90°C Column: Used exclusively as the starting point for derating calculations and for equipment specifically listed and identified for 90°C terminations (which is exceptionally rare in residential and light-commercial work).

Derating: When the Chart's Base Values Drop

The values in the amperage to wire size chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a single conduit. When real-world conditions deviate, you must apply derating factors outlined in NEC 310.15(C)(1) and 310.15(B).

Ambient Temperature Derating: If you are routing wire through an attic in a hot climate where temperatures regularly exceed 86°F, the wire's ability to shed heat drops. For example, if the ambient temperature is 113°F (45°C), you must multiply the 90°C base ampacity by 0.71. A 6 AWG THHN copper wire (base 75A at 90°C) derates to 53.25A. It can no longer safely be placed on a 60A breaker.

Conduit Fill (Conductor Bundling) Derating: When you pull more than three current-carrying conductors in a single raceway, the wires heat each other up.

• 4 to 6 conductors: Derate to 80% of the 90°C base value.
• 7 to 9 conductors: Derate to 70%.
• 10 to 20 conductors: Derate to 50%.

Worked Example: You are pulling four current-carrying 10 AWG THHN copper wires through a single EMT conduit to feed two 240V circuits. The base 90°C ampacity for 10 AWG is 40A. Because you have four conductors, you multiply 40A by 80%, yielding 32A. Because 32A is greater than the standard 30A breaker size, you are legally permitted to use 10 AWG wire on a 30A breaker for this run. (Note: Grounding conductors do not count as current-carrying for derating purposes).

Decision Path: Picking Your Exact Wire and Breaker

Use this decision tree to terminate your planning phase and select your materials. This path assumes standard copper THHN/THWN-2 wire in a normal 30°C ambient environment with standard 75°C terminations.

What This Chart Cannot Tell You (Voltage Drop & Local Code)

The NEC 310.16 amperage to wire size chart dictates the minimum wire size required to prevent the insulation from melting and to protect against short-term overcurrent events. It does not account for voltage drop over long distances.

If you are running a 50A circuit to a detached garage 150 feet away, 6 AWG copper will safely carry the 50A load without tripping the breaker or overheating. However, under a full 40A continuous load, the voltage at the far end will drop by nearly 6 volts (roughly 5%). While the NEC treats voltage drop as an informational recommendation rather than a strict violation for most branch circuits, a 5% drop will cause motors to run hot, lights to dim, and sensitive electronics to brown out.

The Voltage Drop Fix: For any run exceeding 50 feet, calculate your voltage drop. A standard rule of thumb is to limit voltage drop to 3% for branch circuits and 5% for the combined feeder and branch circuit. If your calculation exceeds 3%, bump your wire size up by one or two AWG steps, even if the chart says the smaller wire is sufficient for the amperage.

Furthermore, this chart cannot override local Authority Having Jurisdiction (AHJ) amendments. Some municipalities require 12 AWG as the absolute minimum for all interior branch circuits, effectively banning 14 AWG regardless of the 15A breaker allowance.