Ampacity is the maximum continuous current a wire can carry before its insulation begins to thermally degrade. If you push 40 amps through a 12 AWG copper wire, the breaker might not trip immediately, but the PVC or XLPE insulation will slowly melt, leading to a short circuit or fire. The AWG chart ampacity values are not arbitrary suggestions; they are the hard thermal limits defined by the National Electrical Code (NEC) to keep your wiring intact.
For residential and commercial wiring in the US, the definitive source for these limits is NFPA 70 (NEC) Table 310.16 (formerly known as 310.15(B)(16)). This guide gives you the exact numbers, explains which temperature column you must legally use, and shows you how to adjust those numbers when real-world conditions change.
The Master AWG Chart Ampacity Reference (NEC Table 310.16)
Before you size a breaker or pull wire, you need to know how to read the master table. The ampacity of a conductor depends on three variables: the wire material (copper vs. aluminum), the physical cross-section (AWG or kcmil), and the temperature rating of the insulation (60°C, 75°C, or 90°C).
If you are wiring standard residential branch circuits, you will almost exclusively use the Copper 60°C column for these sizes:
• 14 AWG: 15 Amps (Lighting/outlets)
• 12 AWG: 20 Amps (Kitchen/bath outlets)
• 10 AWG: 30 Amps (Dryers/water heaters)
For feeders and subpanels, you will likely use the Copper 75°C column:
• 6 AWG: 65 Amps (Often used for 60A subpanels)
• 2 AWG: 115 Amps (Often used for 100A subpanels)
• 4/0 AWG: 230 Amps (Often used for 200A service entrances)
Source: NEC Table 310.16. Values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Size (AWG/kcmil) | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 60°C (140°F) | Aluminum 75°C (167°F) | Aluminum 90°C (194°F) |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | - | - | - |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior apprentices make is looking at a wire's insulation jacket, seeing '90°C' printed on it (like THHN or XHHW-2), and using the 90°C column to size their breaker. This is a code violation.
NEC Section 110.14(C) dictates that the ampacity of a circuit is determined by the lowest temperature rating of any connected component, termination, or conductor. This is the 'weakest link' rule. Even if your wire is rated for 90°C, if it terminates on a standard residential circuit breaker rated for 75°C, you must use the 75°C column. If it terminates on a older disconnect or a small 15A/20A breaker rated for 60°C, you must use the 60°C column.
| Temperature Column | When to Use It (Termination Rating) | Common Applications |
|---|---|---|
| 60°C (140°F) | Equipment rated 60°C, or 14-10 AWG circuits (per NEC 240.4(D)) | Standard 15A, 20A, and 30A residential branch circuits (NM-B cable, THHN in standard breakers). |
| 75°C (167°F) | Equipment explicitly rated 75°C or higher. | Feeders, subpanels, large appliances (ranges, dryers), and commercial breakers sized 4 AWG and larger. |
| 90°C (194°F) | Never for final termination ampacity (unless equipment is explicitly 90°C rated, which is extremely rare). | Used only as the starting baseline for calculating derating factors (ambient temperature or conduit bundling). |
Derating Factors: When Base Ampacity Drops
The AWG chart ampacity values assume you are working in a 30°C (86°F) environment and have no more than three current-carrying conductors bundled together in a single raceway. When you exceed these conditions, the wires cannot dissipate heat effectively, and you must 'derate' (reduce) their allowable ampacity.
This is the one scenario where the 90°C column is your best friend. NEC 310.15(C)(1) allows you to apply derating factors to the 90°C ampacity of the wire, provided the final derated ampacity does not exceed the termination limits we discussed above.
Worked Example: Bundling in Conduit
Imagine you are pulling four current-carrying conductors (two hots, one neutral, one ground doesn't count) through a single EMT conduit to feed a multi-wire branch circuit. You are using 10 AWG THHN copper wire.
- Find the base 90°C ampacity: Looking at the table, 10 AWG Copper at 90°C is 40 Amps.
- Apply the bundling adjustment factor: According to NEC Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% adjustment factor.
- Calculate the derated ampacity: 40A × 0.80 = 32 Amps.
- Check the termination limit: Your breaker terminals are rated 60°C. The 60°C column for 10 AWG is 30 Amps.
- Final Verdict: You must use the lower of the two numbers (32A vs 30A). The final allowable ampacity is 30 Amps. You can protect this circuit with a standard 30A breaker.
If you had instead run ten* current-carrying conductors in that same conduit, the adjustment factor drops to 50%. Your 90°C derated value would be 20A (40A × 0.50). Because 20A is lower than the 60°C termination limit of 30A, your final allowable ampacity drops to 20A, and you must step up to 8 AWG wire to maintain a 30A circuit.
What the Ampacity Table Cannot Tell You
While the AWG chart ampacity table is the bible for thermal limits and breaker sizing, it is completely blind to three critical real-world engineering constraints. If you ignore these, your wire might not melt, but your equipment will fail to operate correctly.
1. Voltage Drop
Ampacity only tells you if the wire will catch fire; it doesn't tell you if the voltage at the end of the run will be sufficient to start a motor or run a compressor. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% overall. If you are running a 120V, 15A circuit to a shed 200 feet away, 14 AWG wire has plenty of ampacity (15A), but the resistance of that much thin wire will cause the voltage to drop below 114V under load. You must consult NEC Chapter 9, Table 8 for DC resistance values, or use a tool like the Southwire Voltage Drop Calculator, and you will likely need to upsize to 10 AWG or 8 AWG purely to maintain voltage, even though the breaker is only 15A or 20A.
2. Conduit Fill Capacity
Table 310.16 assumes the wires physically fit in the pipe. Chapter 9, Table 1 dictates that you cannot fill a conduit with more than 40% of its cross-sectional area when pulling three or more wires. If you try to jam six 6 AWG THHN wires into a 1/2-inch EMT conduit, you will exceed the 40% fill limit, making the pull impossible and trapping heat. You must calculate the physical cross-sectional area of the wires and the conduit before buying materials.
3. Short-Circuit Withstand Ratings
Ampacity deals with continuous, steady-state current. It does not account for the massive, instantaneous magnetic and thermal forces generated during a dead short. In commercial or industrial settings with high available fault currents (e.g., 65kA at the service entrance), standard wire might physically tear itself apart before the breaker clears the fault. In these environments, engineers must calculate short-circuit withstand ratings, which often mandate larger wire sizes or specific cable constructions regardless of the continuous ampacity requirements.
Always start with the AWG chart ampacity to satisfy the thermal and breaker-sizing requirements, then verify your choice against voltage drop and physical fill constraints before cutting your first length of wire.
