For standard US residential and commercial wiring, the definitive gauge current chart is derived from NEC Table 310.16 (formerly 310.15(B)(16)). This chart dictates the maximum continuous current (ampacity) a copper or aluminum conductor can carry before its insulation degrades or creates a fire hazard. The direct answer for most DIYers and journeymen: a 14 AWG copper wire is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps, assuming standard 60°C terminations and an ambient temperature of 30°C (86°F).
How to Read the AWG Gauge Current Chart (NEC Table 310.16)
Before sizing a breaker, you must understand which temperature column applies to your specific installation. Modern wire insulation like THHN is rated for 90°C, but you rarely get to use that full capacity. According to NFPA 70 (NEC) Article 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected component, termination, or conductor. This is known as the weakest link rule.
If you pull 90°C THHN wire but terminate it on a standard residential receptacle or breaker rated for 60°C, you must use the 60°C column to determine your maximum overcurrent protection. The 75°C and 90°C columns are primarily used for derating calculations (which we cover below) or when terminating on industrial equipment explicitly rated for 75°C/90°C.
Copper Conductor Ampacity Chart (60°C to 90°C)
Source: NEC 2023 Table 310.16. Conditions: Copper conductors, not more than three current-carrying conductors in a raceway, cable, or earth, ambient temperature 30°C (86°F).
| AWG / kcmil | 60°C (140°F) - TW, UF | 75°C (167°F) - RHW, THHW | 90°C (194°F) - THHN, XHHW |
|---|---|---|---|
| 14 AWG | 15A * | 20A | 25A |
| 12 AWG | 20A * | 25A | 30A |
| 10 AWG | 30A * | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 110A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Derating Factors: When the Chart Lies to You
The base gauge current chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors bundled together. When you stuff more wires into a single conduit or run them through a hot attic, the wires heat each other up. To prevent insulation meltdown, you must apply derating factors to the 90°C column (yes, even if your terminations are 60°C, you start your derating math from the 90°C base).
1. Bundling Derating (NEC Table 310.15(C)(1))
When you have 4 to 9 current-carrying conductors in a single raceway, you must multiply the base 90°C ampacity by the corresponding percentage.
| Number of Current-Carrying Conductors | Adjustment Factor (Percent) |
|---|---|
| 1 - 3 | 100% |
| 4 - 6 | 80% |
| 7 - 9 | 70% |
| 10 - 20 | 50% |
Worked Example: You are pulling four 10 AWG THHN current-carrying conductors through a single EMT conduit for a multi-wire branch circuit.
- Base 90°C ampacity for 10 AWG = 40A.
- Derating factor for 4 conductors = 80%.
- Adjusted ampacity = 40A × 0.80 = 32A.
- Because 32A exceeds the 240.4(D) limit for 10 AWG, you must still protect this circuit with a 30A breaker. The derating math proves the wire can safely handle the 30A load despite the bundling.
2. Ambient Temperature Correction
If your conduit runs through an environment hotter than 30°C (like an unventilated attic in summer), you must apply a temperature correction factor. According to the Cerrowire Ampacity Charts and NEC Table 310.15(B)(1), a 90°C rated wire in a 50°C (122°F) ambient environment must be derated to 82% of its base value.
If you combine bundling and high ambient heat, you multiply both factors together. A 6 AWG THHN wire (75A base at 90°C) in a 50°C attic (0.82 factor) bundled with 5 other current-carrying wires (0.80 factor) yields: 75 × 0.82 × 0.80 = 49.2A. You would then size your breaker accordingly and verify the final number still satisfies the 60°C termination column (55A).
What the Gauge Current Chart Cannot Tell You
Beginners often treat the gauge current chart as the final word on wire sizing. In reality, ampacity tables only solve for thermal limits (preventing the wire from catching fire). They completely ignore three critical real-world constraints.
1. Voltage Drop
The NEC ampacity chart does not account for distance. A 12 AWG copper wire is legally allowed to carry 20A indefinitely on a 120V circuit. However, if that wire run is 150 feet long, the resistance of the copper will cause a voltage drop of roughly 11.5V (nearly 10%). Your 120V outlet will only deliver 108V under load, which can cause motors to overheat, lights to dim, and sensitive electronics to brown out.
As a rule of thumb, keep voltage drop under 3% for branch circuits and 5% total from the service panel to the furthest outlet. For long runs, you must upsize the wire gauge beyond what the ampacity chart demands. Use a dedicated voltage drop calculator or 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 circular mils of the wire).
2. Physical Conduit Fill Limits
You might calculate that you can safely run nine 10 AWG THHN wires in a 1/2-inch EMT conduit based on derating math. However, NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Nine 10 AWG wires physically will not fit into 1/2-inch conduit without jamming or stripping the insulation during the pull. Always check conduit fill tables after finalizing your gauge and derating calculations.
3. Aluminum vs. Copper Discrepancies
The chart provided above is strictly for copper. If you are sizing aluminum feeders (common for 100A+ subpanels or service entrances due to cost), aluminum has higher resistance and lower thermal mass. A 2 AWG copper wire handles 115A (75°C column), but you must step up to 1/0 AWG aluminum to achieve the same 120A rating. Furthermore, aluminum requires anti-oxidant paste (like Noalox) and specific torque settings on lugs to prevent arcing and fires over time.
