The ampacity wire gauge chart dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. For standard residential copper wiring, the baseline answers are straightforward: 14 AWG is rated for 15A, 12 AWG for 20A, and 10 AWG for 30A. These values are derived from the 60°C column of NEC Table 310.16, which governs most branch circuits under 100 amps.
However, simply matching a breaker to the highest number on the chart is a fast track to a failed inspection or a melted termination lug. To use this chart correctly, you must understand temperature columns, termination limits, and derating factors. Below is the complete reference data, followed by the exact rules for applying it on the jobsite.
The NEC Ampacity Wire Gauge Chart (Copper & Aluminum)
How to read this table: This data is sourced directly from NEC Table 310.16 (2020/2023 editions). The values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors bundled in a single raceway or cable. The table is divided by conductor material (Copper vs. Aluminum) and insulation temperature ratings (60°C, 75°C, and 90°C). Aluminum conductors smaller than 8 AWG are rarely used in modern branch wiring and are omitted for clarity.
| AWG / kcmil | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|
| 14 | 15A | 20A | 25A | - | - |
| 12 | 20A | 25A | 30A | - | - |
| 10 | 30A | 35A | 40A | - | - |
| 8 | 40A | 50A | 55A | 40A | 45A |
| 6 | 55A | 65A | 75A | 50A | 55A |
| 4 | 70A | 85A | 95A | 65A | 75A |
| 3 | 85A | 100A | 110A | 75A | 85A |
| 2 | 95A | 115A | 130A | 90A | 100A |
| 1 | 110A | 130A | 145A | 100A | 120A |
| 1/0 | 125A | 150A | 170A | 120A | 135A |
Which Temperature Column Applies to Your Installation?
The most common mistake made by hobbyists and junior electricians is sizing a breaker using the 90°C column because the wire jacket (like THHN) is rated for 90°C. This is almost always a code violation. To determine your actual legal ampacity, you must follow NEC Section 110.14(C), which governs termination temperature limits.
The 60°C Rule (Circuits 100A or Less)
For any circuit rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG, you must use the 60°C column to determine the final breaker size. This is because standard residential breakers, receptacles, and switches are typically only tested and rated for 60°C terminations. Even if you pull 12 AWG THHN (rated 30A at 90°C) to a standard 20A receptacle, the receptacle's termination lug is the weak link. Therefore, the circuit is legally capped at the 60°C rating: 20A.
The 75°C Rule (Circuits Over 100A)
For circuits rated over 100 amps, or conductors larger than 1 AWG, you may use the 75°C column, provided the equipment terminations are explicitly marked as 75°C rated. This is standard for modern main service panels, large subpanel lugs, and heavy-duty HVAC disconnects.
The NM-B (Romex) Exception
If you are using nonmetallic-sheathed cable (NM-B, commonly known as Romex), NEC 334.80 strictly mandates that the ampacity be determined by the 60°C column, regardless of the fact that the individual conductors inside the jacket may have 90°C insulation. You can never use the 75°C or 90°C columns for NM-B cable.
When Can You Use the 90°C Column?
The 90°C column is not useless; it is your baseline for derating calculations (explained below). You start your math at the 90°C column, apply your derating factors, and then compare the result to the 60°C or 75°C termination limits. The final breaker size can never exceed the lowest of these calculated values.
Derating Factors and What the Chart Cannot Tell You
The ampacity wire gauge chart assumes ideal conditions: a cool 30°C (86°F) environment and plenty of physical space for heat dissipation. Real-world installations rarely match this baseline.
How Derating Modifies the Base Value
When you bundle more than three current-carrying conductors in a single conduit, the trapped heat forces you to reduce (derate) the wire's ampacity. Similarly, if your attic or rooftop conduit exceeds 30°C, you must apply ambient temperature correction factors.
You are pulling four 12 AWG THHN current-carrying conductors through a conduit in a standard basement.
1. Base Ampacity: 12 AWG THHN at 90°C is 30A.
2. Bundling Derate: 4 conductors requires an 80% multiplier (NEC Table 310.15(C)(1)).
3. Math: 30A × 0.80 = 24A.
4. Termination Check: Your breaker and device terminations are rated 60°C. The 60°C limit for 12 AWG is 20A.
Verdict: Because 20A is lower than 24A, your maximum breaker size remains 20A. The 90°C insulation bought you thermal headroom, but it did not increase your legal breaker size.
What the Ampacity Chart Cannot Tell You
While Table 310.16 prevents wires from melting, it completely ignores voltage drop. A 12 AWG copper wire is legally permitted to carry 20A indefinitely, but if that wire run is 150 feet long to a shed, the resistance will cause a voltage drop of roughly 11 volts (nearly 10% on a 120V circuit). This will cause motors to overheat and lights to dim.
To calculate voltage drop, you must cross-reference the ampacity chart with NEC Chapter 9, Table 8, which provides the exact circular mil area and DC resistance per 1,000 feet for each gauge. As a rule of thumb, branch circuits should be designed for a maximum 3% voltage drop, and the total feeder-plus-branch drop should not exceed 5%. If your voltage drop calculation dictates a larger wire than the ampacity chart requires, the larger wire wins.
Finally, the chart cannot tell you if a wire will physically fit into a specific lug. Always check the manufacturer's datasheet for your breakers and lugs to verify the maximum and minimum AWG sizes they are mechanically rated to accept. Forcing a 4 AWG wire into a lug designed for 14-10 AWG will result in a loose connection, high resistance, and an eventual arc fault.
