Copper Wire Size Amp Chart: NEC Ampacity Table & Sizing Guide

For standard residential branch circuits, the direct wire size to breaker pairing is: 15A requires 14 AWG, 20A requires 12 AWG, 30A requires 10 AWG, 40A requires 8 AWG, and 50A requires 6 AWG. These baseline numbers assume copper conductors, standard 60°C or 75°C terminal ratings, and an ambient temperature of 30°C (86°F) or less.

However, picking the right wire goes beyond matching a breaker size to a gauge. Misreading the temperature columns or ignoring bundling derating factors can lead to overheated terminals, tripped breakers, or melted insulation. This reference guide breaks down the exact ampacity values, the physics behind the temperature columns, and the edge cases that force you to upsize your wire.

How to Read This Wire Size Amp Chart

The most common mistake DIYers and junior electricians make is looking at the highest ampacity number in the table and sizing the breaker to that value. To use this chart correctly, you must understand the three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F).

Which column applies to your installation? According to NEC Section 110.14(C), the ampacity of your wire is limited by the temperature rating of the equipment terminals it connects to. For circuits rated 100A or less, or for wire sizes 14 AWG through 10 AWG, you must default to the 60°C column unless the breaker or equipment is explicitly marked for 75°C. Most modern residential breakers (like Square D Homeline or QO) are rated for 75°C, which allows you to use the 75°C column for 8 AWG and larger. The 90°C column is almost never used for final breaker sizing; it is reserved strictly as a starting point for derating calculations.

NEC Table 310.16: Copper Wire Ampacity Chart

The following data is extracted from NEC NFPA 70, Table 310.16.
Assumptions: Copper conductors, THHN/THWN-2 or XHHW insulation, ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a raceway or cable.

Quick-Jump to Most Queried Sizes: 14 AWG (15A) • 12 AWG (20A) • 10 AWG (30A) • 6 AWG (50A/65A) • 2 AWG (100A/115A)

When the Chart Isn't Enough: Derating and Edge Cases

The ampacity values above represent ideal conditions. In the real world, electrical safety standards and physics require you to modify these base values when installation conditions deviate from the norm.

How Derating Rows Modify the Base Value

Derating reduces the allowable ampacity of a wire to account for excess heat. There are two primary derating triggers:

• Bundling (Adjustment Factors): If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. For 4 to 6 conductors, you must multiply the base ampacity by 80%. For 7 to 9 conductors, multiply by 70%. Pro-tip: You apply this percentage to the 90°C column, not the 60°C or 75°C column. For example, if you have four 12 AWG THHN wires in a conduit, you take the 90°C rating (30A) and multiply by 0.80, yielding 24A. Since 24A is still higher than the 60°C terminal limit (20A), your final circuit limit remains 20A.
• Ambient Temperature (Correction Factors): If your conduit runs through an attic in a hot climate where ambient temperatures exceed 30°C (86°F), you must apply a temperature correction factor. At 41-45°C, you multiply the 90°C ampacity by 0.82.

What the Table Cannot Tell You

This wire size amp chart is strictly a thermal limit guide. It does not account for three critical installation factors:

1. Voltage Drop: The NEC recommends a maximum 3% voltage drop on branch circuits. If you are running a 120V circuit to a shed 150 feet away, 14 AWG will safely carry 15A thermally, but the voltage at the shed will drop below 114V, causing motors to overheat. You must upsize to 10 AWG or 8 AWG strictly for distance.
2. Physical Lug Fit: You cannot physically terminate a 2 AWG wire into the lug of a standard 20A breaker. The mechanical design of the terminal dictates the maximum wire size it can accept.
3. Short-Circuit Withstand: Ampacity charts assume steady-state continuous loads. They do not tell you if the wire can survive the magnetic and thermal forces of a 10,000A short circuit before the breaker trips. (Standard THHN copper handles this fine for residential breaker trip curves, but it matters in industrial settings).

Wire Size Amp Chart FAQ

What size wire do I need for a 50 amp breaker?

For a standard 50A breaker (like an electric range or EV charger), you need 6 AWG copper wire. Looking at the chart, 6 AWG is rated for 55A in the 60°C column and 65A in the 75°C column. Because 50A is less than 55A, 6 AWG is fully compliant. If you are using NM-B (Romex) cable, you must use the 60°C column, making 6 AWG the absolute minimum. If the run is longer than 75 feet, upsize to 4 AWG to mitigate voltage drop.

Can I use the 90°C column to downsize my home wiring?

No. While the insulation on modern THHN wire is rated for 90°C, the breakers, receptacles, and switches you connect it to are almost universally rated for a maximum of 75°C (and often effectively 60°C for smaller gauges per NEC 110.14(C)). You can use the 90°C column to calculate derating penalties, but the final ampacity after derating cannot exceed the ampacity listed in the 60°C or 75°C column for that wire size.

Does this wire size amp chart apply to aluminum wire?

No, the table above is exclusively for copper conductors. Aluminum has higher electrical resistance and requires a larger cross-sectional area to carry the same current safely. For example, while 2 AWG copper is sufficient for a 100A subpanel feeder (at 75°C), you must use 1/0 AWG aluminum (or 2 AWG aluminum if strictly adhering to specific 75°C column allowances depending on the exact termination equipment, though 1/0 is the standard safe practice for 100A aluminum). Always consult the aluminum section of NEC Table 310.16 when using SER or USE-2 aluminum cable.

How does voltage drop change my required wire size?

Voltage drop is a function of wire length, current, and resistance, not just thermal limits. As a practical rule of thumb for 120V residential circuits: if your one-way wire distance exceeds 50 feet, step up one AWG size. If it exceeds 100 feet, step up two AWG sizes. For 240V circuits (like dryers or HVAC), you can double those distance thresholds before upsizing. Always calculate exact voltage drop using the formula: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is current, D is one-way distance, and CM is the circular mil area of the wire.