Wire Amp Rating Chart: NEC Table 310.16 Copper & Aluminum Sizing

If you need a fast answer for standard residential branch circuits, here are the baseline copper wire ampacities based on the 60°C column: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, and 6 AWG = 55A. These values assume copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in a raceway or cable.

However, picking the right wire for a subpanel feeder, an EV charger, or a long underground run requires looking past the quick-reference basics. The definitive wire amp rating chart used by electricians in the United States is derived from National Electrical Code (NEC) Table 310.16. This reference dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade.

The Master Wire Amp Rating Chart (NEC Table 310.16)

Before scrolling to the numbers, you need to understand how to read this table. The rows represent the American Wire Gauge (AWG) or circular mil (kcmil) cross-sectional area. The columns represent the temperature rating of the wire's insulation—most commonly 60°C, 75°C, and 90°C. The values inside the cells are the maximum allowable ampacities. The data below is extracted directly from the NFPA 70 (NEC) standard for copper conductors.

*Note: While standard NM-B (Romex) cable contains internal conductors with 90°C insulation, NEC Article 334.80 mandates that its final ampacity must be calculated using the 60°C column. The 75°C column above is listed for NM-B in some manufacturer literature for derating purposes only, but the final overcurrent protection must not exceed the 60°C value.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make when using a wire amp rating chart is defaulting to the 90°C column because THHN wire is cheap, ubiquitous, and stamped with "90°C" on the jacket. In practice, you almost never get to use the 90°C ampacity for your final breaker sizing.

NEC Section 110.14(C) establishes the termination temperature rule. The allowable ampacity of a circuit is limited by the lowest temperature rating of any connected component. This includes the wire, the breaker lug, the receptacle, and the terminal block on the appliance.

Most modern breakers, lugs, and receptacles are rated for 75°C. However, you must verify the equipment markings. Furthermore, NEC 240.4(D) introduces the "Small Conductor Rule," which places hard limits on overcurrent protection for the three most common residential wire sizes, regardless of the termination temperature rating.

How Derating Rows Modify the Base Ampacity Value

The base values in the wire amp rating chart assume ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a raceway or cable. When your installation deviates from these baselines, you must apply derating multipliers found in NEC 310.15(B)(1) for temperature and NEC 310.15(C)(1) for bundling.

Here is the exact algorithm an electrician uses to size a wire in a non-standard environment:

1. Start with the 90°C column ampacity (this is the only time you use the 90°C column).
2. Multiply by the Ambient Temperature Correction Factor if the environment is hotter than 30°C.
3. Multiply by the Adjustment Factor for Bundled Conductors if you have 4 or more CCCs in a single conduit.
4. Compare the resulting Derated Ampacity to the Termination Ampacity (from the 60°C or 75°C column). The lower of the two numbers is your final allowable ampacity.

Worked Example: You are pulling four 12 AWG THHN circuits (8 total CCCs) through a single conduit in an attic that reaches 40°C (104°F). You need to supply a 20A receptacle.

• Base (90°C column): 12 AWG THHN = 30A.
• Temperature Derating (40°C ambient, 90°C insulation): Multiplier is 0.87. (30A × 0.87 = 26.1A).
• Bundling Derating (8 CCCs): Multiplier is 0.70. (26.1A × 0.70 = 18.27A).
• Termination Limit (75°C column): 25A.

The final allowable ampacity is the lower of 18.27A and 25A, which is 18.27A. Because 18.27A is less than the 20A load required, 12 AWG is insufficient. You must step up to 10 AWG or pull a separate conduit to reduce the bundling penalty. For deeper reference on bundling penalties, consult the Southwire Ampacity Resources which provide excellent visual matrices for these multipliers.

What the Ampacity Chart Cannot Tell You

While NEC Table 310.16 is the gold standard for preventing wires from melting or starting fires, it completely ignores voltage drop. A wire might be perfectly safe from a thermal perspective while delivering unacceptably low voltage to the load at the end of a long run.

The NEC does not strictly enforce voltage drop for most residential applications (it is covered in Informational Notes, such as in NEC 310.14, recommending a maximum 3% drop on branch circuits and 5% total on feeders). However, motors, compressors, and sensitive electronics will overheat or fail to start if the voltage sags below 114V on a 120V nominal system.

To calculate voltage drop, use this formula for single-phase AC/DC circuits:

VD = (2 × K × I × D) / CM

• K = 12.9 (ohms-cmil/ft for copper at 75°C)
• I = Load current in amps
• D = One-way distance in feet
• CM = Circular mil area of the wire (e.g., 10 AWG = 10,380 CM)

Real-World Scenario: You are wiring a 120V, 15A table saw in a detached garage 120 feet from the panel using 12 AWG copper. The wire amp rating chart says 12 AWG is perfectly safe for 20A thermally. But running the voltage drop math: (2 × 12.9 × 15 × 120) / 6,530 = 7.1V drop. That is a 5.9% drop, which exceeds the 3% recommendation and will cause the saw motor to lug and overheat. The chart says 12 AWG is fine; physics says you need to pull 8 AWG to maintain proper voltage. Always run the voltage drop math for any circuit exceeding 75 feet.