When sizing conductors for a branch circuit or feeder, the amp and wire gauge chart is your primary reference for determining how much current a wire can safely carry without overheating. For a standard 20-amp residential branch circuit, you need a minimum of 12 AWG copper wire. For a 50-amp EV charger or range circuit, you need 6 AWG copper or 4 AWG aluminum. However, simply matching the breaker size to the base ampacity is only the first step. The actual allowable current depends heavily on the insulation temperature rating, termination limits, and installation conditions.
The data below is extracted from the National Electrical Code (NEC) Table 310.16, the definitive standard for wire ampacity in the United States. Bookmark this guide for quick jobsite or bench reference.
The NEC Amp and Wire Gauge Chart (Copper & Aluminum)
Before using this table, you must understand how to read the columns. The ampacity of a wire is dictated by its insulation type (e.g., THHN, THWN-2, XHHW) and the temperature rating of the terminals it connects to. The table below assumes an ambient air temperature of 30°C (86°F) and not more than three current-carrying conductors bundled in a raceway or cable.
| Wire Size (AWG/kcmil) | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 75°C (167°F) |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | N/A |
| 12 | 20A | 25A | 30A | N/A |
| 10 | 30A | 35A | 40A | N/A |
| 8 | 40A | 50A | 55A | 40A |
| 6 | 55A | 65A | 75A | 50A |
| 4 | 70A | 85A | 95A | 65A |
| 3 | 85A | 100A | 115A | 75A |
| 2 | 95A | 115A | 130A | 90A |
| 1 | 110A | 130A | 145A | 100A |
| 1/0 | 125A | 150A | 170A | 120A |
| 2/0 | 145A | 175A | 195A | 135A |
| 3/0 | 165A | 200A | 225A | 155A |
| 4/0 | 195A | 230A | 260A | 180A |
Source: NFPA 70 National Electrical Code, Table 310.16. Aluminum sizes smaller than 8 AWG are not permitted for building wiring under NEC 310.14.
Which Temperature Column Applies to Your Panel?
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because modern THHN wire is rated for 90°C. You almost never use the 90°C column for your final breaker sizing. The correct column is dictated by NEC 110.14(C) terminal temperature limitations.
- Use the 60°C Column: For circuits rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG. Most standard residential branch circuit breakers (15A to 60A) have terminals rated for 60°C or 75°C, but the NEC mandates using the 60°C column as the conservative baseline unless the breaker is explicitly marked otherwise.
- Use the 75°C Column: For circuits rated over 100 amps, or for conductors sized larger than 1 AWG. Most modern main service panels and large subpanel feeders use 75°C rated lugs.
How Derating Factors Modify the Base Ampacity
The base values in the amp and wire gauge chart assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a conduit. When real-world conditions deviate, you must apply derating factors to the 90°C column (for copper THHN/THWN-2), and then verify the final derated number does not exceed the 60°C or 75°C termination limit.
Bundling Derating (NEC Table 310.15(C)(1)):
When you pull 4 to 6 current-carrying conductors through a single conduit, the heat generated by adjacent wires cannot dissipate. You must multiply the 90°C ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.
Worked Example: You are pulling four 10 AWG THHN copper wires in a conduit for a 240V circuit (two hots, one neutral, one ground; the ground does not count as current-carrying, leaving 3 current-carrying conductors—no derating needed). But if you add a second circuit (4 hots, 2 neutrals = 6 current-carrying conductors), you must derate.
Base 90°C ampacity for 10 AWG = 40A.
Derating factor for 6 conductors = 80%.
40A × 0.80 = 32A.
Since 32A is still higher than the 30A termination limit (60°C column), you can still protect this wire with a 30A breaker. If the math had dropped the ampacity below 30A, you would be forced to upsize to 8 AWG wire.
Ambient Temperature Derating:
If you are routing conduit through an attic that reaches 120°F (49°C) in the summer, you must apply a temperature correction factor from NEC Table 310.15(B)(1). At 46-50°C ambient, the 90°C column multiplier is 0.82. A 6 AWG THHN wire (90°C rating = 75A) drops to 61.5A. It can still safely carry a 60A load, but it is operating much closer to its thermal limit than the base chart implies.
What the Amp and Wire Gauge Chart Cannot Tell You
While the NEC table is the law of the land for thermal safety, it is entirely blind to three critical real-world engineering constraints. Relying solely on this chart will result in functional failures even if your installation passes a basic code inspection.
1. Voltage Drop Over Distance
The chart assumes the wire is infinitely short. In reality, wire has resistance. If you run a 50-foot 12 AWG copper wire on a 20A, 120V circuit to a table saw in a detached garage, you will experience roughly 3.2 volts of drop (about 2.6%). This is acceptable. However, if that same run is 150 feet long, the voltage drop hits nearly 10 volts (over 8%). Your motor will draw higher amperage to compensate for the low voltage, overheat, and trip the breaker. Rule of thumb: Upsize your wire by one or two gauges for any branch circuit run exceeding 100 feet to keep voltage drop under 3%.
2. Physical Lug Fit and Termination Torque
The chart might tell you that 4/0 Aluminum is perfect for a 200A service entrance (rated 180A at 75°C, but 4/0 is the standard minimum for 200A residential services per local utility requirements). What it won't tell you is that the stranded 4/0 aluminum wire might physically not fit into the lug of a standard 100A subpanel breaker. You may need to use a reducer pin or a larger panelboard. Furthermore, the chart provides no guidance on torque. Aluminum expands and contracts significantly with heat; if you do not torque the lug to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver, the connection will loosen, arc, and cause a fire.
3. Short-Circuit Withstand Ratings
Ampacity measures continuous thermal load. It does not measure the magnetic and thermal stress a wire can survive during a 10,000-amp short circuit fault before the breaker trips. For standard residential work, standard THHN/THWN-2 is sufficient. But in industrial settings with high available fault currents, engineers must calculate short-circuit withstand, which often forces the use of much larger wire than the continuous ampacity chart demands.
