Ampacity Wire Size Chart: NEC 310.16 Copper and Aluminum

The ampacity wire size is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating. For a standard 15A residential lighting circuit, 14 AWG copper is the absolute minimum; for a 20A receptacle circuit, 12 AWG copper is required. However, simply matching the breaker to the wire's highest temperature rating is a common and dangerous mistake. The correct wire size depends on the termination temperature limits of your devices, the number of conductors in your raceway, and the ambient temperature of the installation environment.

Below is the master reference based on NFPA 70 (National Electrical Code) Table 310.16. Bookmark the quick-jump links below for the most common residential and commercial branch circuits.

The Master Ampacity Wire Size Table (NEC 310.16)

Quick-Jump to Common Residential Sizes: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG

Source: NFPA 70 (NEC) Table 310.16. Values assume not more than three current-carrying conductors in a raceway, cable, or earth, at an ambient temperature of 30°C (86°F).

Which Temperature Column Applies to Your Installation?

The most frequent error in wire sizing is looking at the 90°C column because modern THHN/THWN-2 wire is rated for 90°C, and then sizing the breaker to that higher number. This violates NEC 110.14(C) and creates a fire hazard at the termination points. Here is how to select the correct column:

The 60°C Column (The Default for Small Circuits)

According to NEC 110.14(C)(1)(a), for circuits rated 100A or less, or for conductors sized 14 AWG through 1 AWG, you must use the 60°C column unless the equipment is explicitly marked and listed for 75°C. Most standard residential receptacles, switches, and basic breakers are only rated for 60°C terminations. Therefore, a 12 AWG copper wire on a standard receptacle circuit is limited to 20A, regardless of its 90°C insulation.

The 75°C Column (The Standard for Larger Equipment)

For circuits rated over 100A, or conductors larger than 1 AWG, the 75°C column applies by default. Most modern commercial breakers, panelboard lugs, and heavy-duty disconnects are tested and listed for 75°C terminations. If you are pulling 2/0 AWG copper for a 200A residential service, you use the 75°C column (175A) and rely on the next-standard-size breaker rule to use a 200A main breaker.

The 90°C Column (For Derating Calculations Only)

You almost never use the 90°C column to determine your final breaker size. The 90°C column is used exclusively as the starting point for derating calculations. If you have high ambient temperatures or multiple conductors in a single conduit, you start with the 90°C ampacity, apply your derating multipliers, and then verify that the final calculated ampacity still exceeds the rating of your overcurrent protective device (OCPD) based on the 60°C or 75°C termination limits.

Derating Math and What the Base Table Cannot Tell You

The baseline table assumes ideal conditions: exactly three current-carrying conductors (e.g., two hots and a neutral in a 240V multi-wire branch circuit, or a hot, neutral, and ground in a standard 120V circuit—grounds do not count as current-carrying) in a raceway at 86°F. When real-world conditions change, the ampacity wire size must be adjusted.

Applying Bundling Derating Factors

When you pull more than three current-carrying conductors through a single conduit, they heat each other up. You must apply a derating factor from NEC Table 310.15(C)(1).

Worked Example: You are pulling 8 current-carrying conductors (four 120V circuits) through a single EMT conduit in a garage. You are using 12 AWG THHN copper.

• Base Ampacity (90°C column): 30A
• Derating Factor (7-9 conductors): 70%
• Adjusted Ampacity: 30A × 0.70 = 21A
• Termination Check: 21A is greater than the 20A breaker requirement, and the 12 AWG wire satisfies the 60°C termination limit (20A). The installation is compliant.

Ambient Temperature Corrections

If your conduit runs through an attic in the summer or near a boiler, the ambient temperature exceeds 30°C (86°F). You must apply the correction factors from the bottom of Table 310.16. For instance, if the attic reaches 113°F (45°C), the correction factor for 90°C insulation is 0.82. You multiply the 90°C base ampacity by 0.82 before applying any bundling derating.

What the Ampacity Table Cannot Tell You

While the Copper Development Association and the NEC provide excellent thermal limits, the ampacity table is blind to three critical installation realities:

1. Voltage Drop: The table will tell you that 10 AWG copper is perfectly safe for a 30A load. It will not tell you that running that 10 AWG wire 150 feet to a detached workshop will result in a 5% voltage drop, which can cause motors to overheat and electronics to brown out. For runs over 100 feet, you must calculate voltage drop and typically upsize the wire by one or two AWG sizes.
2. Conduit Fill Capacity: The table assumes the wires physically fit in the raceway. You must cross-reference NEC Chapter 9, Table 1 to ensure your conduit is not overfilled. Overfilled conduits make pulling difficult, risking insulation damage and trapped heat.
3. Short-Circuit Thermal Withstand: Ampacity is about continuous, steady-state heating. It does not account for the massive thermal spike of a short circuit. For services with high available fault currents (e.g., 42kA at the main panel), the wire must be sized to withstand the let-through current of the breaker before it trips, which sometimes requires upsizing the equipment grounding conductor per NEC Table 250.122.

Always verify your final wire size against the specific manufacturer's installation instructions for the equipment being connected, as UL listing requirements can sometimes dictate a larger minimum wire size than the NEC baseline.